Internet DRAFT - draft-ietf-idr-bgp4
draft-ietf-idr-bgp4
Network Working Group Y. Rekhter
INTERNET DRAFT T.Li
Obsoletes: RFC1771 S. Hares
Editors
A Border Gateway Protocol 4 (BGP-4)
<draft-ietf-idr-bgp4-26.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
The Border Gateway Protocol (BGP) is an inter-Autonomous System
routing protocol.
The primary function of a BGP speaking system is to exchange network
reachability information with other BGP systems. This network
reachability information includes information on the list of
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Autonomous Systems (ASs) that reachability information traverses.
This information is sufficient to construct a graph of AS
connectivity for this reachability from which routing loops may be
pruned and some policy decisions at the AS level may be enforced.
BGP-4 provides a set of mechanisms for supporting Classless Inter-
Domain Routing (CIDR) [RFC1518, RFC1519]. These mechanisms include
support for advertising a set of destinations as an IP prefix, and
eliminating the concept of network "class" within BGP. BGP-4 also
introduces mechanisms which allow aggregation of routes, including
aggregation of AS paths.
Routing information exchanged via BGP supports only the destination-
based forwarding paradigm, which assumes that a router forwards a
packet based solely on the destination address carried in the IP
header of the packet. This, in turn, reflects the set of policy
decisions that can (and can not) be enforced using BGP. BGP can
support only the policies conforming to the destination-based
forwarding paradigm.
This specification covers only the exchange of IP version 4 network
reachability information.
This document obsoletes RFC1771.
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Table of Contents
1. Definition of commonly used terms . . . . . . . . . . . . . . 5
2. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7
Specification of Requirements . . . . . . . . . . . . . . . . . . 8
3. Summary of Operation . . . . . . . . . . . . . . . . . . . . . 8
3.1 Routes: Advertisement and Storage . . . . . . . . . . . . . . 9
3.2 Routing Information Bases . . . . . . . . . . . . . . . . . . 10
4. Message Formats . . . . . . . . . . . . . . . . . . . . . . . 12
4.1 Message Header Format . . . . . . . . . . . . . . . . . . . . 12
4.2 OPEN Message Format . . . . . . . . . . . . . . . . . . . . . 13
4.3 UPDATE Message Format . . . . . . . . . . . . . . . . . . . . 15
4.4 KEEPALIVE Message Format . . . . . . . . . . . . . . . . . . 22
4.5 NOTIFICATION Message Format . . . . . . . . . . . . . . . . . 22
5. Path Attributes . . . . . . . . . . . . . . . . . . . . . . . 24
5.1 Path Attribute Usage . . . . . . . . . . . . . . . . . . . . 26
5.1.1 ORIGIN . . . . . . . . . . . . . . . . . . . . . . . . . . 26
5.1.2 AS_PATH . . . . . . . . . . . . . . . . . . . . . . . . . . 26
5.1.3 NEXT_HOP . . . . . . . . . . . . . . . . . . . . . . . . . 27
5.1.4 MULTI_EXIT_DISC . . . . . . . . . . . . . . . . . . . . . . 29
5.1.5 LOCAL_PREF . . . . . . . . . . . . . . . . . . . . . . . . 30
5.1.6 ATOMIC_AGGREGATE . . . . . . . . . . . . . . . . . . . . . 30
5.1.7 AGGREGATOR . . . . . . . . . . . . . . . . . . . . . . . . 31
6. BGP Error Handling . . . . . . . . . . . . . . . . . . . . . . 31
6.1 Message Header error handling . . . . . . . . . . . . . . . . 31
6.2 OPEN message error handling . . . . . . . . . . . . . . . . . 32
6.3 UPDATE message error handling . . . . . . . . . . . . . . . . 33
6.4 NOTIFICATION message error handling . . . . . . . . . . . . . 35
6.5 Hold Timer Expired error handling . . . . . . . . . . . . . . 35
6.6 Finite State Machine error handling . . . . . . . . . . . . . 35
6.7 Cease . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
6.8 BGP connection collision detection . . . . . . . . . . . . . 36
7. BGP Version Negotiation . . . . . . . . . . . . . . . . . . . 37
8. BGP Finite State machine . . . . . . . . . . . . . . . . . . . 38
8.1 Events for the BGP FSM . . . . . . . . . . . . . . . . . . . 39
8.1.1 Optional Events linked to Optional Session attributes
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
8.1.2 Administrative Events . . . . . . . . . . . . . . . . . . 44
8.1.3 Timer Events . . . . . . . . . . . . . . . . . . . . . . . 47
8.1.4 TCP connection based Events . . . . . . . . . . . . . . . . 49
8.1.5 BGP Messages based Events . . . . . . . . . . . . . . . . . 51
8.2 Description of FSM . . . . . . . . . . . . . . . . . . . . . 53
8.2.1 FSM Definition . . . . . . . . . . . . . . . . . . . . . . 53
8.2.1.1 Terms "active" and "passive" . . . . . . . . . . . . . . 54
8.2.1.2 FSM and collision detection . . . . . . . . . . . . . . . 54
8.2.1.3 FSM and Optional Attributes . . . . . . . . . . . . . . 55
8.2.1.4 FSM Event numbers . . . . . . . . . . . . . . . . . . . . 55
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8.2.1.5 FSM actions that are implementation dependent . . . . . . 56
8.2.2 Finite State Machine . . . . . . . . . . . . . . . . . . . 56
9. UPDATE Message Handling . . . . . . . . . . . . . . . . . . . 72
9.1 Decision Process . . . . . . . . . . . . . . . . . . . . . . 73
9.1.1 Phase 1: Calculation of Degree of Preference . . . . . . . 74
9.1.2 Phase 2: Route Selection . . . . . . . . . . . . . . . . . 74
9.1.2.1 Route Resolvability Condition . . . . . . . . . . . . . . 76
9.1.2.2 Breaking Ties (Phase 2) . . . . . . . . . . . . . . . . . 77
9.1.3 Phase 3: Route Dissemination . . . . . . . . . . . . . . . 79
9.1.4 Overlapping Routes . . . . . . . . . . . . . . . . . . . . 80
9.2 Update-Send Process . . . . . . . . . . . . . . . . . . . . . 81
9.2.1 Controlling Routing Traffic Overhead . . . . . . . . . . . 82
9.2.1.1 Frequency of Route Advertisement . . . . . . . . . . . . 82
9.2.1.2 Frequency of Route Origination . . . . . . . . . . . . . 83
9.2.2 Efficient Organization of Routing Information . . . . . . . 83
9.2.2.1 Information Reduction . . . . . . . . . . . . . . . . . . 83
9.2.2.2 Aggregating Routing Information . . . . . . . . . . . . . 84
9.3 Route Selection Criteria . . . . . . . . . . . . . . . . . . 86
9.4 Originating BGP routes . . . . . . . . . . . . . . . . . . . 87
10. BGP Timers . . . . . . . . . . . . . . . . . . . . . . . . . 87
Appendix A. Comparison with RFC1771 . . . . . . . . . . . . . . . 88
Appendix B. Comparison with RFC1267 . . . . . . . . . . . . . . . 89
Appendix C. Comparison with RFC 1163 . . . . . . . . . . . . . . 90
Appendix D. Comparison with RFC 1105 . . . . . . . . . . . . . . 90
Appendix E. TCP options that may be used with BGP . . . . . . . . 91
Appendix F. Implementation Recommendations . . . . . . . . . . . 91
Appendix F.1 Multiple Networks Per Message . . . . . . . . . . . 91
Appendix F.2 Reducing route flapping . . . . . . . . . . . . . . 92
Appendix F.3 Path attribute ordering . . . . . . . . . . . . . . 92
Appendix F.4 AS_SET sorting . . . . . . . . . . . . . . . . . . . 92
Appendix F.5 Control over version negotiation . . . . . . . . . . 93
Appendix F.6 Complex AS_PATH aggregation . . . . . . . . . . . . 93
Security Considerations . . . . . . . . . . . . . . . . . . . . . 94
IANA Considerations . . . . . . . . . . . . . . . . . . . . . . . 95
IPR Disclosure Acknowledgement . . . . . . . . . . . . . . . . . 97
Copyright Notice . . . . . . . . . . . . . . . . . . . . . . . . 98
Normative References . . . . . . . . . . . . . . . . . . . . . . 98
Non-normative References . . . . . . . . . . . . . . . . . . . . 99
Authors Information . . . . . . . . . . . . . . . . . . . . . . . 100
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Abstract
The Border Gateway Protocol (BGP) is an inter-Autonomous System rout-
ing protocol.
The primary function of a BGP speaking system is to exchange network
reachability information with other BGP systems. This network reacha-
bility information includes information on the list of Autonomous
Systems (ASs) that reachability information traverses. This informa-
tion is sufficient to construct a graph of AS connectivity for this
reachability from which routing loops may be pruned and some policy
decisions at the AS level may be enforced.
BGP-4 provides a set of mechanisms for supporting Classless Inter-
Domain Routing (CIDR) [RFC1518, RFC1519]. These mechanisms include
support for advertising a set of destinations as an IP prefix and
eliminating the concept of network "class" within BGP. BGP-4 also
introduces mechanisms which allow aggregation of routes, including
aggregation of AS paths.
Routing information exchanged via BGP supports only the destination-
based forwarding paradigm, which assumes that a router forwards a
packet based solely on the destination address carried in the IP
header of the packet. This, in turn, reflects the set of policy deci-
sions that can (and can not) be enforced using BGP. BGP can support
only the policies conforming to the destination-based forwarding par-
adigm.
1. Definition of commonly used terms
This section provides definition for terms that have a specific mean-
ing to the BGP protocol and that are used throughout the text.
Adj-RIB-In
The Adj-RIBs-In contain unprocessed routing information that has
been advertised to the local BGP speaker by its peers.
Adj-RIB-Out
The Adj-RIBs-Out contains the routes for advertisement to specific
peers by means of the local speaker's UPDATE messages.
Autonomous System (AS)
The classic definition of an Autonomous System is a set of routers
under a single technical administration, using an interior gateway
protocol (IGP) and common metrics to determine how to route pack-
ets within the AS, and using an inter-AS routing protocol to
determine how to route packets to other ASs. Since this classic
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definition was developed, it has become common for a single AS to
use several IGPs and sometimes several sets of metrics within an
AS. The use of the term Autonomous System here stresses the fact
that, even when multiple IGPs and metrics are used, the adminis-
tration of an AS appears to other ASs to have a single coherent
interior routing plan and presents a consistent picture of what
destinations are reachable through it.
BGP Identifier
A 4-octet unsigned integer indicating the BGP Identifier of the
sender of BGP messages. A given BGP speaker sets the value of its
BGP Identifier to an IP address assigned to that BGP speaker. The
value of the BGP Identifier is determined on startup and is the
same for every local interface and every BGP peer.
BGP speaker
A router that implements BGP.
EBGP
External BGP (BGP connection between external peers).
External peer
Peer that is in a different Autonomous System than the local sys-
tem.
Feasible route
An advertised route that is available for use by the recipient.
IBGP
Internal BGP (BGP connection between internal peers).
Internal peer
Peer that is in the same Autonomous System as the local system.
IGP
Interior Gateway Protocol - a routing protocol used to exchange
routing information among routers within a single Autonomous Sys-
tem.
Loc-RIB
The Loc-RIB contains the routes that have been selected by the
local BGP speaker's Decision Process.
NLRI
Network Layer Reachability Information.
Route
A unit of information that pairs a set of destinations with the
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attributes of a path to those destinations. The set of destina-
tions are systems whose IP addresses are contained in one IP
address prefix carried in the Network Layer Reachability Informa-
tion (NLRI) field of an UPDATE message. The path is the informa-
tion reported in the path attributes field of the same UPDATE mes-
sage.
RIB
Routing Information Base.
Unfeasible route
A previously advertised feasible route that is no longer available
for use.
2. Acknowledgments
This document was originally published as RFC 1267 in October 1991,
jointly authored by Kirk Lougheed and Yakov Rekhter.
We would like to express our thanks to Guy Almes, Len Bosack, and
Jeffrey C. Honig for their contributions to the earlier version
(BGP-1) of this document.
We would like to specially acknowledge numerous contributions by Den-
nis Ferguson to the earlier version of this document.
We like to explicitly thank Bob Braden for the review of the earlier
version (BGP-2) of this document as well as his constructive and
valuable comments.
We would also like to thank Bob Hinden, Director for Routing of the
Internet Engineering Steering Group, and the team of reviewers he
assembled to review the earlier version (BGP-2) of this document.
This team, consisting of Deborah Estrin, Milo Medin, John Moy, Radia
Perlman, Martha Steenstrup, Mike St. Johns, and Paul Tsuchiya, acted
with a strong combination of toughness, professionalism, and cour-
tesy.
Certain sections of the document borrowed heavily from IDRP
[IS10747], which is the OSI counterpart of BGP. For this credit
should be given to the ANSI X3S3.3 group chaired by Lyman Chapin and
to Charles Kunzinger who was the IDRP editor within that group.
We would also like to thank Benjamin Abarbanel, Enke Chen, Edward
Crabbe, Mike Craren, Vincent Gillet, Eric Gray, Jeffrey Haas, Dimitry
Haskin, Stephen Kent, John Krawczyk, David LeRoy, Dan Massey,
Jonathan Natale, Dan Pei, Mathew Richardson, John Scudder, John
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Stewart III, Dave Thaler, Paul Traina, Russ White, Curtis Villamizar,
and Alex Zinin for their comments.
We would like to specially acknowledge Andrew Lange for his help in
preparing the final version of this document.
Finally, we would like to thank all the members of the IDR Working
Group for their ideas and support they have given to this document.
Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [RFC2119].
3. Summary of Operation
The Border Gateway Protocol (BGP) is an inter-Autonomous System rout-
ing protocol. It is built on experience gained with EGP as defined in
[RFC904] and EGP usage in the NSFNET Backbone as described in
[RFC1092] and [RFC1093].
The primary function of a BGP speaking system is to exchange network
reachability information with other BGP systems. This network reacha-
bility information includes information on the list of Autonomous
Systems (ASs) that reachability information traverses. This informa-
tion is sufficient to construct a graph of AS connectivity from which
routing loops may be pruned and some policy decisions at the AS level
may be enforced.
In the context of this document we assume that a BGP speaker adver-
tises to its peers only those routes that it itself uses (in this
context a BGP speaker is said to "use" a BGP route if it is the most
preferred BGP route and is used in forwarding). All other cases are
outside the scope of this document.
In the context of this document the term "IP address" refers to an IP
Version 4 address [RFC791].
Routing information exchanged via BGP supports only the destination-
based forwarding paradigm, which assumes that a router forwards a
packet based solely on the destination address carried in the IP
header of the packet. This, in turn, reflects the set of policy deci-
sions that can (and can not) be enforced using BGP. Note that some
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policies can not be supported by the destination-based forwarding
paradigm, and thus require techniques such as source routing (aka
explicit routing) to be enforced. Such policies can not be enforced
using BGP either. For example, BGP does not enable one AS to send
traffic to a neighboring AS for forwarding to some destination
(reachable through but) beyond that neighboring AS intending that the
traffic take a different route to that taken by the traffic originat-
ing in the neighboring AS (for that same destination). On the other
hand, BGP can support any policy conforming to the destination-based
forwarding paradigm.
BGP-4 provides a new set of mechanisms for supporting Classless
Inter-Domain Routing (CIDR) [RFC1518, RFC1519]. These mechanisms
include support for advertising a set of destinations as an IP prefix
and eliminating the concept of network "class" within BGP. BGP-4
also introduces mechanisms which allow aggregation of routes, includ-
ing aggregation of AS paths.
This document uses the term `Autonomous System' (AS) throughout. The
classic definition of an Autonomous System is a set of routers under
a single technical administration, using an interior gateway protocol
(IGP) and common metrics to determine how to route packets within the
AS, and using an inter-AS routing protocol to determine how to route
packets to other ASs. Since this classic definition was developed, it
has become common for a single AS to use several IGPs and sometimes
several sets of metrics within an AS. The use of the term Autonomous
System here stresses the fact that, even when multiple IGPs and met-
rics are used, the administration of an AS appears to other ASs to
have a single coherent interior routing plan and presents a consis-
tent picture of what destinations are reachable through it.
BGP uses TCP [RFC793] as its transport protocol. This eliminates the
need to implement explicit update fragmentation, retransmission,
acknowledgment, and sequencing. BGP listens on TCP port 179. The
error notification mechanism used in BGP assumes that TCP supports a
"graceful" close, i.e., that all outstanding data will be delivered
before the connection is closed.
Two systems form a TCP connection between one another. They exchange
messages to open and confirm the connection parameters.
The initial data flow is the portion of the BGP routing table that is
allowed by the export policy, called the Adj-Ribs-Out (see 3.2).
Incremental updates are sent as the routing tables change. BGP does
not require periodic refresh of the routing table. To allow local
policy changes to have the correct effect without resetting any BGP
connections, a BGP speaker SHOULD either (a) retain the current ver-
sion of the routes advertised to it by all of its peers for the
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duration of the connection, or (b) make use of the Route Refresh
extension [RFC2918].
KEEPALIVE messages may be sent periodically to ensure the liveness of
the connection. NOTIFICATION messages are sent in response to errors
or special conditions. If a connection encounters an error condition,
a NOTIFICATION message is sent and the connection is closed.
A peer in a different AS is referred to as an external peer, while a
peer in the same AS is referred to as an internal peer. Internal BGP
and external BGP are commonly abbreviated IBGP and EBGP.
If a particular AS has multiple BGP speakers and is providing transit
service for other ASs, then care must be taken to ensure a consistent
view of routing within the AS. A consistent view of the interior
routes of the AS is provided by the IGP used within the AS. For the
purpose of this document, it is assumed that a consistent view of the
routes exterior to the AS is provided by having all BGP speakers
within the AS maintain IBGP with each other.
This document specifies the base behavior of the BGP protocol. This
behavior can and is modified by extension specifications. When the
protocol is extended the new behavior is fully documented in the
extension specifications.
3.1 Routes: Advertisement and Storage
For the purpose of this protocol, a route is defined as a unit of
information that pairs a set of destinations with the attributes of a
path to those destinations. The set of destinations are systems whose
IP addresses are contained in one IP address prefix carried in the
Network Layer Reachability Information (NLRI) field of an UPDATE mes-
sage, and the path is the information reported in the path attributes
field of the same UPDATE message.
Routes are advertised between BGP speakers in UPDATE messages. Mul-
tiple routes that have the same path attributes can be advertised in
a single UPDATE message by including multiple prefixes in the NLRI
field of the UPDATE message.
Routes are stored in the Routing Information Bases (RIBs): namely,
the Adj-RIBs-In, the Loc-RIB, and the Adj-RIBs-Out, as described in
Section 3.2.
If a BGP speaker chooses to advertise a previously received route, it
MAY add to or modify the path attributes of the route before adver-
tising it to a peer.
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BGP provides mechanisms by which a BGP speaker can inform its peer
that a previously advertised route is no longer available for use.
There are three methods by which a given BGP speaker can indicate
that a route has been withdrawn from service:
a) the IP prefix that expresses the destination for a previously
advertised route can be advertised in the WITHDRAWN ROUTES field
in the UPDATE message, thus marking the associated route as being
no longer available for use
b) a replacement route with the same NLRI can be advertised, or
c) the BGP speaker - BGP speaker connection can be closed, which
implicitly removes from service all routes which the pair of
speakers had advertised to each other.
Changing attribute(s) of a route is accomplished by advertising a
replacement route. The replacement route carries new (changed)
attributes and has the same address prefix as the original route.
3.2 Routing Information Base
The Routing Information Base (RIB) within a BGP speaker consists of
three distinct parts:
a) Adj-RIBs-In: The Adj-RIBs-In store routing information that has
been learned from inbound UPDATE messages received from other BGP
speakers. Their contents represent routes that are available as an
input to the Decision Process.
b) Loc-RIB: The Loc-RIB contains the local routing information
that the BGP speaker has selected by applying its local policies
to the routing information contained in its Adj-RIBs-In. These are
the routes that will be used by the local BGP speaker. The next
hop for each of these routes MUST be resolvable via the local BGP
speaker's Routing Table.
c) Adj-RIBs-Out: The Adj-RIBs-Out store the information that the
local BGP speaker has selected for advertisement to its peers. The
routing information stored in the Adj-RIBs-Out will be carried in
the local BGP speaker's UPDATE messages and advertised to its
peers.
In summary, the Adj-RIBs-In contain unprocessed routing information
that has been advertised to the local BGP speaker by its peers; the
Loc-RIB contains the routes that have been selected by the local BGP
speaker's Decision Process; and the Adj-RIBs-Out organize the routes
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for advertisement to specific peers by means of the local speaker's
UPDATE messages.
Although the conceptual model distinguishes between Adj-RIBs-In, Loc-
RIB, and Adj-RIBs-Out, this neither implies nor requires that an
implementation must maintain three separate copies of the routing
information. The choice of implementation (for example, 3 copies of
the information vs 1 copy with pointers) is not constrained by the
protocol.
Routing information that the BGP speaker uses to forward packets (or
to construct the forwarding table that is used for packet forwarding)
is maintained in the Routing Table. The Routing Table accumulates
routes to directly connected networks, static routes, routes learned
from the IGP protocols, and routes learned from BGP. Whether or not
a specific BGP route should be installed in the Routing Table, and
whether a BGP route should override a route to the same destination
installed by another source is a local policy decision, not specified
in this document. Besides actual packet forwarding, the Routing Table
is used for resolution of the next-hop addresses specified in BGP
updates (see Section 5.1.3).
4. Message Formats
This section describes message formats used by BGP.
BGP messages are sent over a TCP connection. A message is processed
only after it is entirely received. The maximum message size is 4096
octets. All implementations are required to support this maximum mes-
sage size. The smallest message that may be sent consists of a BGP
header without a data portion, or 19 octets.
All multi-octet fields are in network byte order.
4.1 Message Header Format
Each message has a fixed-size header. There may or may not be a data
portion following the header, depending on the message type. The lay-
out of these fields is shown below:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ +
| Marker |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Marker:
This 16-octet field is included for compatibility; it MUST be
set to all ones.
Length:
This 2-octet unsigned integer indicates the total length of the
message, including the header, in octets. Thus, e.g., it allows
one to locate in the TCP stream the (Marker field of the) next
message. The value of the Length field MUST always be at least
19 and no greater than 4096, and MAY be further constrained,
depending on the message type. No "padding" of extra data after
the message is allowed, so the Length field MUST have the
smallest value required given the rest of the message.
Type:
This 1-octet unsigned integer indicates the type code of the
message. This document defines the following type codes:
1 - OPEN
2 - UPDATE
3 - NOTIFICATION
4 - KEEPALIVE
[RFC2918] defines one more type code.
4.2 OPEN Message Format
After a TCP connection is established, the first message sent by each
side is an OPEN message. If the OPEN message is acceptable, a
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KEEPALIVE message confirming the OPEN is sent back.
In addition to the fixed-size BGP header, the OPEN message contains
the following fields:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+
| Version |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| My Autonomous System |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Hold Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BGP Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opt Parm Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Optional Parameters (variable) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Version:
This 1-octet unsigned integer indicates the protocol version
number of the message. The current BGP version number is 4.
My Autonomous System:
This 2-octet unsigned integer indicates the Autonomous System
number of the sender.
Hold Time:
This 2-octet unsigned integer indicates the number of seconds
that the sender proposes for the value of the Hold Timer. Upon
receipt of an OPEN message, a BGP speaker MUST calculate the
value of the Hold Timer by using the smaller of its configured
Hold Time and the Hold Time received in the OPEN message. The
Hold Time MUST be either zero or at least three seconds. An
implementation MAY reject connections on the basis of the Hold
Time. The calculated value indicates the maximum number of
seconds that may elapse between the receipt of successive
KEEPALIVE, and/or UPDATE messages by the sender.
BGP Identifier:
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This 4-octet unsigned integer indicates the BGP Identifier of
the sender. A given BGP speaker sets the value of its BGP Iden-
tifier to an IP address assigned to that BGP speaker. The
value of the BGP Identifier is determined on startup and is the
same for every local interface and every BGP peer.
Optional Parameters Length:
This 1-octet unsigned integer indicates the total length of the
Optional Parameters field in octets. If the value of this field
is zero, no Optional Parameters are present.
Optional Parameters:
This field contains a list of optional parameters, where each
parameter is encoded as a <Parameter Type, Parameter Length,
Parameter Value> triplet.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...
| Parm. Type | Parm. Length | Parameter Value (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...
Parameter Type is a one octet field that unambiguously identi-
fies individual parameters. Parameter Length is a one octet
field that contains the length of the Parameter Value field in
octets. Parameter Value is a variable length field that is
interpreted according to the value of the Parameter Type field.
[RFC3392] defines the Capabilities Optional Parameter.
The minimum length of the OPEN message is 29 octets (including mes-
sage header).
4.3 UPDATE Message Format
UPDATE messages are used to transfer routing information between BGP
peers. The information in the UPDATE message can be used to construct
a graph describing the relationships of the various Autonomous Sys-
tems. By applying rules to be discussed, routing information loops
and some other anomalies may be detected and removed from inter-AS
routing.
An UPDATE message is used to advertise feasible routes sharing common
path attributes to a peer, or to withdraw multiple unfeasible routes
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from service (see 3.1). An UPDATE message MAY simultaneously adver-
tise a feasible route and withdraw multiple unfeasible routes from
service. The UPDATE message always includes the fixed-size BGP
header, and also includes the other fields as shown below (note, some
of the shown fields may not be present in every UPDATE message):
+-----------------------------------------------------+
| Withdrawn Routes Length (2 octets) |
+-----------------------------------------------------+
| Withdrawn Routes (variable) |
+-----------------------------------------------------+
| Total Path Attribute Length (2 octets) |
+-----------------------------------------------------+
| Path Attributes (variable) |
+-----------------------------------------------------+
| Network Layer Reachability Information (variable) |
+-----------------------------------------------------+
Withdrawn Routes Length:
This 2-octets unsigned integer indicates the total length of
the Withdrawn Routes field in octets. Its value allows the
length of the Network Layer Reachability Information field to
be determined as specified below.
A value of 0 indicates that no routes are being withdrawn from
service, and that the WITHDRAWN ROUTES field is not present in
this UPDATE message.
Withdrawn Routes:
This is a variable length field that contains a list of IP
address prefixes for the routes that are being withdrawn from
service. Each IP address prefix is encoded as a 2-tuple of the
form <length, prefix>, whose fields are described below:
+---------------------------+
| Length (1 octet) |
+---------------------------+
| Prefix (variable) |
+---------------------------+
The use and the meaning of these fields are as follows:
a) Length:
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The Length field indicates the length in bits of the IP
address prefix. A length of zero indicates a prefix that
matches all IP addresses (with prefix, itself, of zero
octets).
b) Prefix:
The Prefix field contains an IP address prefix followed by
the minimum number of trailing bits needed to make the end
of the field fall on an octet boundary. Note that the value
of trailing bits is irrelevant.
Total Path Attribute Length:
This 2-octet unsigned integer indicates the total length of the
Path Attributes field in octets. Its value allows the length of
the Network Layer Reachability field to be determined as speci-
fied below.
A value of 0 indicates that neither the Network Layer Reacha-
bility Information field, nor the Path Attribute field is
present in this UPDATE message.
Path Attributes:
A variable length sequence of path attributes is present in
every UPDATE message, except for an UPDATE message that carries
only the withdrawn routes. Each path attribute is a triple
<attribute type, attribute length, attribute value> of variable
length.
Attribute Type is a two-octet field that consists of the
Attribute Flags octet followed by the Attribute Type Code
octet.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attr. Flags |Attr. Type Code|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The high-order bit (bit 0) of the Attribute Flags octet is the
Optional bit. It defines whether the attribute is optional (if
set to 1) or well-known (if set to 0).
The second high-order bit (bit 1) of the Attribute Flags octet
is the Transitive bit. It defines whether an optional attribute
is transitive (if set to 1) or non-transitive (if set to 0).
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For well-known attributes, the Transitive bit MUST be set to 1.
(See Section 5 for a discussion of transitive attributes.)
The third high-order bit (bit 2) of the Attribute Flags octet
is the Partial bit. It defines whether the information con-
tained in the optional transitive attribute is partial (if set
to 1) or complete (if set to 0). For well-known attributes and
for optional non-transitive attributes the Partial bit MUST be
set to 0.
The fourth high-order bit (bit 3) of the Attribute Flags octet
is the Extended Length bit. It defines whether the Attribute
Length is one octet (if set to 0) or two octets (if set to 1).
The lower-order four bits of the Attribute Flags octet are
unused. They MUST be zero when sent and MUST be ignored when
received.
The Attribute Type Code octet contains the Attribute Type Code.
Currently defined Attribute Type Codes are discussed in Section
5.
If the Extended Length bit of the Attribute Flags octet is set
to 0, the third octet of the Path Attribute contains the length
of the attribute data in octets.
If the Extended Length bit of the Attribute Flags octet is set
to 1, then the third and the fourth octets of the path
attribute contain the length of the attribute data in octets.
The remaining octets of the Path Attribute represent the
attribute value and are interpreted according to the Attribute
Flags and the Attribute Type Code. The supported Attribute Type
Codes, their attribute values and uses are the following:
a) ORIGIN (Type Code 1):
ORIGIN is a well-known mandatory attribute that defines the
origin of the path information. The data octet can assume
the following values:
Value Meaning
0 IGP - Network Layer Reachability Information
is interior to the originating AS
1 EGP - Network Layer Reachability Information
learned via the EGP protocol [RFC904]
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2 INCOMPLETE - Network Layer Reachability
Information learned by some other means
Usage of this attribute is defined in 5.1.1.
b) AS_PATH (Type Code 2):
AS_PATH is a well-known mandatory attribute that is composed
of a sequence of AS path segments. Each AS path segment is
represented by a triple <path segment type, path segment
length, path segment value>.
The path segment type is a 1-octet long field with the fol-
lowing values defined:
Value Segment Type
1 AS_SET: unordered set of ASs a route in the
UPDATE message has traversed
2 AS_SEQUENCE: ordered set of ASs a route in
the UPDATE message has traversed
The path segment length is a 1-octet long field containing
the number of ASs (not the number of octets) in the path
segment value field.
The path segment value field contains one or more AS num-
bers, each encoded as a 2-octets long field.
Usage of this attribute is defined in 5.1.2.
c) NEXT_HOP (Type Code 3):
This is a well-known mandatory attribute that defines the
(unicast) IP address of the router that SHOULD be used as
the next hop to the destinations listed in the Network Layer
Reachability Information field of the UPDATE message.
Usage of this attribute is defined in 5.1.3.
d) MULTI_EXIT_DISC (Type Code 4):
This is an optional non-transitive attribute that is a four
octet unsigned integer. The value of this attribute MAY be
used by a BGP speaker's Decision Process to discriminate
among multiple entry points to a neighboring autonomous
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system.
Usage of this attribute is defined in 5.1.4.
e) LOCAL_PREF (Type Code 5):
LOCAL_PREF is a well-known attribute that is a four octet
unsigned integer. A BGP speaker uses it to inform its other
internal peers of the advertising speaker's degree of pref-
erence for an advertised route.
Usage of this attribute is defined in 5.1.5.
f) ATOMIC_AGGREGATE (Type Code 6)
ATOMIC_AGGREGATE is a well-known discretionary attribute of
length 0.
Usage of this attribute is defined in 5.1.6.
g) AGGREGATOR (Type Code 7)
AGGREGATOR is an optional transitive attribute of length 6.
The attribute contains the last AS number that formed the
aggregate route (encoded as 2 octets), followed by the IP
address of the BGP speaker that formed the aggregate route
(encoded as 4 octets). This SHOULD be the same address as
the one used for the BGP Identifier of the speaker.
Usage of this attribute is defined in 5.1.7.
Network Layer Reachability Information:
This variable length field contains a list of IP address pre-
fixes. The length in octets of the Network Layer Reachability
Information is not encoded explicitly, but can be calculated
as:
UPDATE message Length - 23 - Total Path Attributes Length -
Withdrawn Routes Length
where UPDATE message Length is the value encoded in the fixed-
size BGP header, Total Path Attribute Length and Withdrawn
Routes Length are the values encoded in the variable part of
the UPDATE message, and 23 is a combined length of the fixed-
size BGP header, the Total Path Attribute Length field and the
Withdrawn Routes Length field.
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Reachability information is encoded as one or more 2-tuples of
the form <length, prefix>, whose fields are described below:
+---------------------------+
| Length (1 octet) |
+---------------------------+
| Prefix (variable) |
+---------------------------+
The use and the meaning of these fields are as follows:
a) Length:
The Length field indicates the length in bits of the IP
address prefix. A length of zero indicates a prefix that
matches all IP addresses (with prefix, itself, of zero
octets).
b) Prefix:
The Prefix field contains an IP address prefix followed by
enough trailing bits to make the end of the field fall on an
octet boundary. Note that the value of the trailing bits is
irrelevant.
The minimum length of the UPDATE message is 23 octets -- 19 octets
for the fixed header + 2 octets for the Withdrawn Routes Length + 2
octets for the Total Path Attribute Length (the value of Withdrawn
Routes Length is 0 and the value of Total Path Attribute Length is
0).
An UPDATE message can advertise at most one set of path attributes,
but multiple destinations, provided that the destinations share these
attributes. All path attributes contained in a given UPDATE message
apply to all destinations carried in the NLRI field of the UPDATE
message.
An UPDATE message can list multiple routes to be withdrawn from ser-
vice. Each such route is identified by its destination (expressed as
an IP prefix), which unambiguously identifies the route in the con-
text of the BGP speaker - BGP speaker connection to which it has been
previously advertised.
An UPDATE message might advertise only routes to be withdrawn from
service, in which case it will not include path attributes or Network
Layer Reachability Information. Conversely, it may advertise only a
feasible route, in which case the WITHDRAWN ROUTES field need not be
present.
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An UPDATE message SHOULD NOT include the same address prefix in the
WITHDRAWN ROUTES and Network Layer Reachability Information fields,
however a BGP speaker MUST be able to process UPDATE messages in this
form. A BGP speaker SHOULD treat an UPDATE message of this form as if
the WITHDRAWN ROUTES doesn't contain the address prefix.
4.4 KEEPALIVE Message Format
BGP does not use any TCP-based keep-alive mechanism to determine if
peers are reachable. Instead, KEEPALIVE messages are exchanged
between peers often enough as not to cause the Hold Timer to expire.
A reasonable maximum time between KEEPALIVE messages would be one
third of the Hold Time interval. KEEPALIVE messages MUST NOT be sent
more frequently than one per second. An implementation MAY adjust the
rate at which it sends KEEPALIVE messages as a function of the Hold
Time interval.
If the negotiated Hold Time interval is zero, then periodic KEEPALIVE
messages MUST NOT be sent.
A KEEPALIVE message consists of only message header and has a length
of 19 octets.
4.5 NOTIFICATION Message Format
A NOTIFICATION message is sent when an error condition is detected.
The BGP connection is closed immediately after sending it.
In addition to the fixed-size BGP header, the NOTIFICATION message
contains the following fields:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error code | Error subcode | Data (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Error Code:
This 1-octet unsigned integer indicates the type of NOTIFICA-
TION. The following Error Codes have been defined:
Error Code Symbolic Name Reference
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1 Message Header Error Section 6.1
2 OPEN Message Error Section 6.2
3 UPDATE Message Error Section 6.3
4 Hold Timer Expired Section 6.5
5 Finite State Machine Error Section 6.6
6 Cease Section 6.7
Error subcode:
This 1-octet unsigned integer provides more specific informa-
tion about the nature of the reported error. Each Error Code
may have one or more Error Subcodes associated with it. If no
appropriate Error Subcode is defined, then a zero (Unspecific)
value is used for the Error Subcode field.
Message Header Error subcodes:
1 - Connection Not Synchronized.
2 - Bad Message Length.
3 - Bad Message Type.
OPEN Message Error subcodes:
1 - Unsupported Version Number.
2 - Bad Peer AS.
3 - Bad BGP Identifier.
4 - Unsupported Optional Parameter.
5 - [Deprecated - see Appendix A].
6 - Unacceptable Hold Time.
UPDATE Message Error subcodes:
1 - Malformed Attribute List.
2 - Unrecognized Well-known Attribute.
3 - Missing Well-known Attribute.
4 - Attribute Flags Error.
5 - Attribute Length Error.
6 - Invalid ORIGIN Attribute.
7 - [Deprecated - see Appendix A].
8 - Invalid NEXT_HOP Attribute.
9 - Optional Attribute Error.
10 - Invalid Network Field.
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11 - Malformed AS_PATH.
Data:
This variable-length field is used to diagnose the reason for
the NOTIFICATION. The contents of the Data field depend upon
the Error Code and Error Subcode. See Section 6 below for more
details.
Note that the length of the Data field can be determined from
the message Length field by the formula:
Message Length = 21 + Data Length
The minimum length of the NOTIFICATION message is 21 octets (includ-
ing message header).
5. Path Attributes
This section discusses the path attributes of the UPDATE message.
Path attributes fall into four separate categories:
1. Well-known mandatory.
2. Well-known discretionary.
3. Optional transitive.
4. Optional non-transitive.
BGP implementations MUST recognize all well-known attributes. Some
of these attributes are mandatory and MUST be included in every
UPDATE message that contains NLRI. Others are discretionary and MAY
or MAY NOT be sent in a particular UPDATE message.
Once a BGP peer has updated any well-known attributes, it MUST pass
these attributes in any updates it transmits to its peers.
In addition to well-known attributes, each path MAY contain one or
more optional attributes. It is not required or expected that all BGP
implementations support all optional attributes. The handling of an
unrecognized optional attribute is determined by the setting of the
Transitive bit in the attribute flags octet. Paths with unrecognized
transitive optional attributes SHOULD be accepted. If a path with
unrecognized transitive optional attribute is accepted and passed
along to other BGP peers, then the unrecognized transitive optional
attribute of that path MUST be passed along with the path to other
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BGP peers with the Partial bit in the Attribute Flags octet set to 1.
If a path with recognized transitive optional attribute is accepted
and passed along to other BGP peers and the Partial bit in the
Attribute Flags octet is set to 1 by some previous AS, it MUST NOT be
set back to 0 by the current AS. Unrecognized non-transitive optional
attributes MUST be quietly ignored and not passed along to other BGP
peers.
New transitive optional attributes MAY be attached to the path by the
originator or by any other BGP speaker in the path. If they are not
attached by the originator, the Partial bit in the Attribute Flags
octet is set to 1. The rules for attaching new non-transitive
optional attributes will depend on the nature of the specific
attribute. The documentation of each new non-transitive optional
attribute will be expected to include such rules. (The description of
the MULTI_EXIT_DISC attribute gives an example.) All optional
attributes (both transitive and non-transitive) MAY be updated (if
appropriate) by BGP speakers in the path.
The sender of an UPDATE message SHOULD order path attributes within
the UPDATE message in ascending order of attribute type. The receiver
of an UPDATE message MUST be prepared to handle path attributes
within the UPDATE message that are out of order.
The same attribute (attribute with the same type) can not appear more
than once within the Path Attributes field of a particular UPDATE
message.
The mandatory category refers to an attribute which MUST be present
in both IBGP and EBGP exchanges if NLRI are contained in the UPDATE
message. Attributes classified as optional for the purpose of the
protocol extension mechanism may be purely discretionary, or discre-
tionary, required, or disallowed in certain contexts.
attribute EBGP IBGP
ORIGIN mandatory mandatory
AS_PATH mandatory mandatory
NEXT_HOP mandatory mandatory
MULTI_EXIT_DISC discretionary discretionary
LOCAL_PREF see Section 5.1.5 required
ATOMIC_AGGREGATE see Section 5.1.6 and 9.1.4
AGGREGATOR discretionary discretionary
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5.1 Path Attribute Usage
The usage of each BGP path attribute is described in the following
clauses.
5.1.1 ORIGIN
ORIGIN is a well-known mandatory attribute. The ORIGIN attribute is
generated by the speaker that originates the associated routing
information. Its value SHOULD NOT be changed by any other speaker.
5.1.2 AS_PATH
AS_PATH is a well-known mandatory attribute. This attribute identi-
fies the autonomous systems through which routing information carried
in this UPDATE message has passed. The components of this list can be
AS_SETs or AS_SEQUENCEs.
When a BGP speaker propagates a route which it has learned from
another BGP speaker's UPDATE message, it modifies the route's AS_PATH
attribute based on the location of the BGP speaker to which the route
will be sent:
a) When a given BGP speaker advertises the route to an internal
peer, the advertising speaker SHALL NOT modify the AS_PATH
attribute associated with the route.
b) When a given BGP speaker advertises the route to an external
peer, then the advertising speaker updates the AS_PATH attribute
as follows:
1) if the first path segment of the AS_PATH is of type
AS_SEQUENCE, the local system prepends its own AS number as the
last element of the sequence (put it in the leftmost position
with respect to the position of octets in the protocol mes-
sage). If the act of prepending will cause an overflow in the
AS_PATH segment, i.e. more than 255 ASs, it SHOULD prepend a
new segment of type AS_SEQUENCE and prepend its own AS number
to this new segment.
2) if the first path segment of the AS_PATH is of type AS_SET,
the local system prepends a new path segment of type
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AS_SEQUENCE to the AS_PATH, including its own AS number in that
segment.
3) if the AS_PATH is empty, the local system creates a path
segment of type AS_SEQUENCE, places its own AS into that seg-
ment, and places that segment into the AS_PATH.
When a BGP speaker originates a route then:
a) the originating speaker includes its own AS number in a path
segment of type AS_SEQUENCE in the AS_PATH attribute of all UPDATE
messages sent to an external peer. (In this case, the AS number of
the originating speaker's autonomous system will be the only entry
the path segment, and this path segment will be the only segment
in the AS_PATH attribute).
b) the originating speaker includes an empty AS_PATH attribute in
all UPDATE messages sent to internal peers. (An empty AS_PATH
attribute is one whose length field contains the value zero).
Whenever the modification of the AS_PATH attribute calls for includ-
ing or prepending the AS number of the local system, the local system
MAY include/prepend more than one instance of its own AS number in
the AS_PATH attribute. This is controlled via local configuration.
5.1.3 NEXT_HOP
The NEXT_HOP is a well-known mandatory attribute that defines the IP
address of the router that SHOULD be used as the next hop to the des-
tinations listed in the UPDATE message. The NEXT_HOP attribute is
calculated as follows.
1) When sending a message to an internal peer, if the route is not
locally originated the BGP speaker SHOULD NOT modify the NEXT_HOP
attribute, unless it has been explicitly configured to announce
its own IP address as the NEXT_HOP. When announcing a locally
originated route to an internal peer, the BGP speaker SHOULD use
as the NEXT_HOP the interface address of the router through which
the announced network is reachable for the speaker; if the route
is directly connected to the speaker, or the interface address of
the router through which the announced network is reachable for
the speaker is the internal peer's address, then the BGP speaker
SHOULD use for the NEXT_HOP attribute its own IP address (the
address of the interface that is used to reach the peer).
2) When sending a message to an external peer X, and the peer is
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one IP hop away from the speaker:
- If the route being announced was learned from an internal
peer or is locally originated, the BGP speaker can use for the
NEXT_HOP attribute an interface address of the internal peer
router (or the internal router) through which the announced
network is reachable for the speaker, provided that peer X
shares a common subnet with this address. This is a form of
"third party" NEXT_HOP attribute.
- Otherwise, if the route being announced was learned from an
external peer, the speaker can use in the NEXT_HOP attribute an
IP address of any adjacent router (known from the received
NEXT_HOP attribute) that the speaker itself uses for local
route calculation, provided that peer X shares a common subnet
with this address. This is a second form of "third party"
NEXT_HOP attribute.
- Otherwise, if the external peer to which the route is being
advertised shares a common subnet with one of the interfaces of
the announcing BGP speaker, the speaker MAY use the IP address
associated with such an interface in the NEXT_HOP attribute.
This is known as a "first party" NEXT_HOP attribute.
- By default (if none of the above conditions apply), the BGP
speaker SHOULD use in the NEXT_HOP attribute the IP address of
the interface that the speaker uses to establish the BGP con-
nection to peer X.
3) When sending a message to an external peer X, and the peer is
multiple IP hops away from the speaker (aka "multihop EBGP"):
- The speaker MAY be configured to propagate the NEXT_HOP
attribute. In this case when advertising a route that the
speaker learned from one of its peers, the NEXT_HOP attribute
of the advertised route is exactly the same as the NEXT_HOP
attribute of the learned route (the speaker just doesn't modify
the NEXT_HOP attribute).
- By default, the BGP speaker SHOULD use in the NEXT_HOP
attribute the IP address of the interface that the speaker uses
to establish the BGP connection to peer X.
Normally the NEXT_HOP attribute is chosen such that the shortest
available path will be taken. A BGP speaker MUST be able to support
disabling advertisement of third party NEXT_HOP attributes to handle
imperfectly bridged media.
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A route originated by a BGP speaker SHALL NOT be advertised to a peer
using an address of that peer as NEXT_HOP. A BGP speaker SHALL NOT
install a route with itself as the next hop.
The NEXT_HOP attribute is used by the BGP speaker to determine the
actual outbound interface and immediate next-hop address that SHOULD
be used to forward transit packets to the associated destinations.
The immediate next-hop address is determined by performing a recur-
sive route lookup operation for the IP address in the NEXT_HOP
attribute using the contents of the Routing Table, selecting one
entry if multiple entries of equal cost exist. The Routing Table
entry which resolves the IP address in the NEXT_HOP attribute will
always specify the outbound interface. If the entry specifies an
attached subnet, but does not specify a next-hop address, then the
address in the NEXT_HOP attribute SHOULD be used as the immediate
next-hop address. If the entry also specifies the next-hop address,
this address SHOULD be used as the immediate next-hop address for
packet forwarding.
5.1.4 MULTI_EXIT_DISC
The MULTI_EXIT_DISC is an optional non-transitive attribute which is
intended to be used on external (inter-AS) links to discriminate
among multiple exit or entry points to the same neighboring AS. The
value of the MULTI_EXIT_DISC attribute is a four octet unsigned num-
ber which is called a metric. All other factors being equal, the exit
point with lower metric SHOULD be preferred. If received over EBGP,
the MULTI_EXIT_DISC attribute MAY be propagated over IBGP to other
BGP speakers within the same AS (see also 9.1.2.2). The
MULTI_EXIT_DISC attribute received from a neighboring AS MUST NOT be
propagated to other neighboring ASs.
A BGP speaker MUST implement a mechanism based on local configuration
which allows the MULTI_EXIT_DISC attribute to be removed from a
route. If a BGP speaker is configured to remove the MULTI_EXIT_DISC
attribute from a route, then this removal MUST be done prior to
determining the degree of preference of the route and performing
route selection (Decision Process phases 1 and 2).
An implementation MAY also (based on local configuration) alter the
value of the MULTI_EXIT_DISC attribute received over EBGP. If a BGP
speaker is configured to alter the value of the MULTI_EXIT_DISC
attribute received over EBGP, then altering the value MUST be done
prior to determining the degree of preference of the route and per-
forming route selection (Decision Process phases 1 and 2). See
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Section 9.1.2.2 for necessary restrictions on this.
5.1.5 LOCAL_PREF
LOCAL_PREF is a well-known attribute that SHALL be included in all
UPDATE messages that a given BGP speaker sends to the other internal
peers. A BGP speaker SHALL calculate the degree of preference for
each external route based on the locally configured policy, and
include the degree of preference when advertising a route to its
internal peers. The higher degree of preference MUST be preferred. A
BGP speaker uses the degree of preference learned via LOCAL_PREF in
its Decision Process (see Section 9.1.1).
A BGP speaker MUST NOT include this attribute in UPDATE messages that
it sends to external peers, except for the case of BGP Confederations
[RFC3065]. If it is contained in an UPDATE message that is received
from an external peer, then this attribute MUST be ignored by the
receiving speaker, except for the case of BGP Confederations
[RF3065].
5.1.6 ATOMIC_AGGREGATE
ATOMIC_AGGREGATE is a well-known discretionary attribute.
When a BGP speaker aggregates several routes for the purpose of
advertisement to a particular peer, the AS_PATH of the aggregated
route normally includes an AS_SET formed from the set of ASs from
which the aggregate was formed. In many cases the network adminis-
trator can determine that the aggregate can safely be advertised
without the AS_SET and not form route loops.
If an aggregate excludes at least some of the AS numbers present in
the AS_PATH of the routes that are aggregated as a result of dropping
the AS_SET, the aggregated route, when advertised to the peer, SHOULD
include the ATOMIC_AGGREGATE attribute.
A BGP speaker that receives a route with the ATOMIC_AGGREGATE
attribute SHOULD NOT remove the attribute from the route when propa-
gating it to other speakers.
A BGP speaker that receives a route with the ATOMIC_AGGREGATE
attribute MUST NOT make any NLRI of that route more specific (as
defined in 9.1.4) when advertising this route to other BGP speakers.
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A BGP speaker that receives a route with the ATOMIC_AGGREGATE
attribute needs to be aware of the fact that the actual path to des-
tinations, as specified in the NLRI of the route, while having the
loop-free property, may not be the path specified in the AS_PATH
attribute of the route.
5.1.7 AGGREGATOR
AGGREGATOR is an optional transitive attribute which MAY be included
in updates which are formed by aggregation (see Section 9.2.2.2). A
BGP speaker which performs route aggregation MAY add the AGGREGATOR
attribute which SHALL contain its own AS number and IP address. The
IP address SHOULD be the same as the BGP Identifier of the speaker.
6. BGP Error Handling.
This section describes actions to be taken when errors are detected
while processing BGP messages.
When any of the conditions described here are detected, a NOTIFICA-
TION message with the indicated Error Code, Error Subcode, and Data
fields is sent, and the BGP connection is closed, unless it is
explicitly stated that no NOTIFICATION message is to be sent and the
BGP connection is not to be closed. If no Error Subcode is specified,
then a zero MUST be used.
The phrase "the BGP connection is closed" means that the TCP connec-
tion has been closed, the associated Adj-RIB-In has been cleared, and
that all resources for that BGP connection have been deallocated.
Entries in the Loc-RIB associated with the remote peer are marked as
invalid. The local system recalculates its best routes for the des-
tinations of the routes marked as invalid, and before the invalid
routes are deleted from the system advertises to its peers either
withdraws for the routes marked as invalid, or the new best routes
before the invalid routes are deleted from the system.
Unless specified explicitly, the Data field of the NOTIFICATION mes-
sage that is sent to indicate an error is empty.
6.1 Message Header error handling.
All errors detected while processing the Message Header MUST be
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indicated by sending the NOTIFICATION message with Error Code Message
Header Error. The Error Subcode elaborates on the specific nature of
the error.
The expected value of the Marker field of the message header is all
ones. If the Marker field of the message header is not as expected,
then a synchronization error has occurred and the Error Subcode MUST
be set to Connection Not Synchronized.
If at least one of the following is true:
- if the Length field of the message header is less than 19 or
greater than 4096, or
- if the Length field of an OPEN message is less than the minimum
length of the OPEN message, or
- if the Length field of an UPDATE message is less than the mini-
mum length of the UPDATE message, or
- if the Length field of a KEEPALIVE message is not equal to 19,
or
- if the Length field of a NOTIFICATION message is less than the
minimum length of the NOTIFICATION message,
then the Error Subcode MUST be set to Bad Message Length. The Data
field MUST contain the erroneous Length field.
If the Type field of the message header is not recognized, then the
Error Subcode MUST be set to Bad Message Type. The Data field MUST
contain the erroneous Type field.
6.2 OPEN message error handling.
All errors detected while processing the OPEN message MUST be indi-
cated by sending the NOTIFICATION message with Error Code OPEN Mes-
sage Error. The Error Subcode elaborates on the specific nature of
the error.
If the version number contained in the Version field of the received
OPEN message is not supported, then the Error Subcode MUST be set to
Unsupported Version Number. The Data field is a 2-octets unsigned
integer, which indicates the largest locally supported version number
less than the version the remote BGP peer bid (as indicated in the
received OPEN message), or if the smallest locally supported version
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number is greater than the version the remote BGP peer bid, then the
smallest locally supported version number.
If the Autonomous System field of the OPEN message is unacceptable,
then the Error Subcode MUST be set to Bad Peer AS. The determination
of acceptable Autonomous System numbers is outside the scope of this
protocol.
If the Hold Time field of the OPEN message is unacceptable, then the
Error Subcode MUST be set to Unacceptable Hold Time. An implementa-
tion MUST reject Hold Time values of one or two seconds. An imple-
mentation MAY reject any proposed Hold Time. An implementation which
accepts a Hold Time MUST use the negotiated value for the Hold Time.
If the BGP Identifier field of the OPEN message is syntactically
incorrect, then the Error Subcode MUST be set to Bad BGP Identifier.
Syntactic correctness means that the BGP Identifier field represents
a valid unicast IP host address.
If one of the Optional Parameters in the OPEN message is not recog-
nized, then the Error Subcode MUST be set to Unsupported Optional
Parameters.
If one of the Optional Parameters in the OPEN message is recognized,
but is malformed, then the Error Subcode MUST be set to 0 (Unspe-
cific).
6.3 UPDATE message error handling.
All errors detected while processing the UPDATE message MUST be indi-
cated by sending the NOTIFICATION message with Error Code UPDATE Mes-
sage Error. The error subcode elaborates on the specific nature of
the error.
Error checking of an UPDATE message begins by examining the path
attributes. If the Withdrawn Routes Length or Total Attribute Length
is too large (i.e., if Withdrawn Routes Length + Total Attribute
Length + 23 exceeds the message Length), then the Error Subcode MUST
be set to Malformed Attribute List.
If any recognized attribute has Attribute Flags that conflict with
the Attribute Type Code, then the Error Subcode MUST be set to
Attribute Flags Error. The Data field MUST contain the erroneous
attribute (type, length and value).
If any recognized attribute has Attribute Length that conflicts with
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the expected length (based on the attribute type code), then the
Error Subcode MUST be set to Attribute Length Error. The Data field
MUST contain the erroneous attribute (type, length and value).
If any of the mandatory well-known attributes are not present, then
the Error Subcode MUST be set to Missing Well-known Attribute. The
Data field MUST contain the Attribute Type Code of the missing well-
known attribute.
If any of the mandatory well-known attributes are not recognized,
then the Error Subcode MUST be set to Unrecognized Well-known
Attribute. The Data field MUST contain the unrecognized attribute
(type, length and value).
If the ORIGIN attribute has an undefined value, then the Error Sub-
code MUST be set to Invalid Origin Attribute. The Data field MUST
contain the unrecognized attribute (type, length and value).
If the NEXT_HOP attribute field is syntactically incorrect, then the
Error Subcode MUST be set to Invalid NEXT_HOP Attribute. The Data
field MUST contain the incorrect attribute (type, length and value).
Syntactic correctness means that the NEXT_HOP attribute represents a
valid IP host address.
The IP address in the NEXT_HOP MUST meet the following criteria to be
considered semantically correct:
a) It MUST NOT be the IP address of the receiving speaker
b) In the case of an EBGP where the sender and receiver are one IP
hop away from each other, either the IP address in the NEXT_HOP
MUST be the sender's IP address (that is used to establish the BGP
connection), or the interface associated with the NEXT_HOP IP
address MUST share a common subnet with the receiving BGP speaker.
If the NEXT_HOP attribute is semantically incorrect, the error SHOULD
be logged, and the route SHOULD be ignored. In this case, a NOTIFICA-
TION message SHOULD NOT be sent, and connection SHOULD NOT be closed.
The AS_PATH attribute is checked for syntactic correctness. If the
path is syntactically incorrect, then the Error Subcode MUST be set
to Malformed AS_PATH.
If the UPDATE message is received from an external peer, the local
system MAY check whether the leftmost (with respect to the position
of octets in the protocol message) AS in the AS_PATH attribute is
equal to the autonomous system number of the peer that sent the mes-
sage. If the check determines that this is not the case, the Error
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Subcode MUST be set to Malformed AS_PATH.
If an optional attribute is recognized, then the value of this
attribute MUST be checked. If an error is detected, the attribute
MUST be discarded, and the Error Subcode MUST be set to Optional
Attribute Error. The Data field MUST contain the attribute (type,
length and value).
If any attribute appears more than once in the UPDATE message, then
the Error Subcode MUST be set to Malformed Attribute List.
The NLRI field in the UPDATE message is checked for syntactic valid-
ity. If the field is syntactically incorrect, then the Error Subcode
MUST be set to Invalid Network Field.
If a prefix in the NLRI field is semantically incorrect (e.g., an
unexpected multicast IP address), an error SHOULD be logged locally,
and the prefix SHOULD be ignored.
An UPDATE message that contains correct path attributes, but no NLRI,
SHALL be treated as a valid UPDATE message.
6.4 NOTIFICATION message error handling.
If a peer sends a NOTIFICATION message, and the receiver of the mes-
sage detects an error in that message, the receiver can not use a
NOTIFICATION message to report this error back to the peer. Any such
error, such as an unrecognized Error Code or Error Subcode, SHOULD be
noticed, logged locally, and brought to the attention of the adminis-
tration of the peer. The means to do this, however, lies outside the
scope of this document.
6.5 Hold Timer Expired error handling.
If a system does not receive successive KEEPALIVE and/or UPDATE
and/or NOTIFICATION messages within the period specified in the Hold
Time field of the OPEN message, then the NOTIFICATION message with
Hold Timer Expired Error Code is sent and the BGP connection is
closed.
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6.6 Finite State Machine error handling.
Any error detected by the BGP Finite State Machine (e.g., receipt of
an unexpected event) is indicated by sending the NOTIFICATION message
with Error Code Finite State Machine Error.
6.7 Cease.
In absence of any fatal errors (that are indicated in this section),
a BGP peer MAY choose at any given time to close its BGP connection
by sending the NOTIFICATION message with Error Code Cease. However,
the Cease NOTIFICATION message MUST NOT be used when a fatal error
indicated by this section does exist.
A BGP speaker MAY support the ability to impose an (locally config-
ured) upper bound on the number of address prefixes the speaker is
willing to accept from a neighbor. When the upper bound is reached,
the speaker (under control of local configuration) either (a) dis-
cards new address prefixes from the neighbor (while maintaining BGP
connection with the neighbor), or (b) terminates the BGP connection
with the neighbor. If the BGP speaker decides to terminate its BGP
connection with a neighbor because the number of address prefixes
received from the neighbor exceeds the locally configured upper
bound, then the speaker MUST send to the neighbor a NOTIFICATION mes-
sage with the Error Code Cease. The speaker MAY also log this
locally.
6.8 BGP connection collision detection.
If a pair of BGP speakers try simultaneously to establish a BGP con-
nection to each other, then two parallel connections between this
pair of speakers might well be formed. If the source IP address used
by one of these connections is the same as the destination IP address
used by the other, and the destination IP address used by the first
connection is the same as the source IP address used by the other, we
refer to this situation as connection collision. Clearly in the
presence of connection collision, one of these connections MUST be
closed.
Based on the value of the BGP Identifier a convention is established
for detecting which BGP connection is to be preserved when a colli-
sion does occur. The convention is to compare the BGP Identifiers of
the peers involved in the collision and to retain only the connection
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initiated by the BGP speaker with the higher-valued BGP Identifier.
Upon receipt of an OPEN message, the local system MUST examine all of
its connections that are in the OpenConfirm state. A BGP speaker MAY
also examine connections in an OpenSent state if it knows the BGP
Identifier of the peer by means outside of the protocol. If among
these connections there is a connection to a remote BGP speaker whose
BGP Identifier equals the one in the OPEN message, and this connec-
tion collides with the connection over which the OPEN message is
received then the local system performs the following collision reso-
lution procedure:
1. The BGP Identifier of the local system is compared to the BGP
Identifier of the remote system (as specified in the OPEN mes-
sage). Comparing BGP Identifiers is done by converting them to
host byte order and treating them as (4-octet long) unsigned inte-
gers.
2. If the value of the local BGP Identifier is less than the
remote one, the local system closes the BGP connection that
already exists (the one that is already in the OpenConfirm state),
and accepts the BGP connection initiated by the remote system.
3. Otherwise, the local system closes newly created BGP connection
(the one associated with the newly received OPEN message), and
continues to use the existing one (the one that is already in the
OpenConfirm state).
Unless allowed via configuration, a connection collision with an
existing BGP connection that is in Established state causes closing
of the newly created connection.
Note that a connection collision can not be detected with connections
that are in Idle, or Connect, or Active states.
Closing the BGP connection (that results from the collision resolu-
tion procedure) is accomplished by sending the NOTIFICATION message
with the Error Code Cease.
7. BGP Version Negotiation
BGP speakers MAY negotiate the version of the protocol by making mul-
tiple attempts to open a BGP connection, starting with the highest
version number each supports. If an open attempt fails with an Error
Code OPEN Message Error, and an Error Subcode Unsupported Version
Number, then the BGP speaker has available the version number it
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tried, the version number its peer tried, the version number passed
by its peer in the NOTIFICATION message, and the version numbers that
it supports. If the two peers do support one or more common versions,
then this will allow them to rapidly determine the highest common
version. In order to support BGP version negotiation, future versions
of BGP MUST retain the format of the OPEN and NOTIFICATION messages.
8. BGP Finite State machine (FSM)
The data structures and FSM described in this document are
conceptual and do not have to be implemented precisely as described
here, as long as the implementations support the described
functionality and their externally visible behavior is the same.
This section specifies the BGP operation in terms of a Finite State
Machine (FSM). The section falls into 2 parts:
1) Description of Events for the State machine (Section 8.1)
2) Description of the FSM (Section 8.2)
Session attributes required (mandatory) for each connection are:
1) State
2) ConnectRetryCounter
3) ConnectRetryTimer
4) ConnectRetryTime
5) HoldTimer
6) HoldTime
7) KeepaliveTimer
8) KeepaliveTime
The state session attribute indicates what state the BGP FSM
is in. The ConnectRetryCounter indicates the number of times
a BGP peer has tried to establish a peer session.
The mandatory attributes related to timers are described in
section 10. Each timer has a "timer" and a "time" (the initial
value).
The optional Session attributes are listed below. These optional
attributes may be supported either per connection or per local sys-
tem:
1) AcceptConnectionsUnconfiguredPeers
2) AllowAutomaticStart
3) AllowAutomaticStop
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4) CollisionDetectEstablishedState
5) DampPeerOscillations
6) DelayOpen
7) DelayOpenTime
8) DelayOpenTimer
9) IdleHoldTime
10) IdleHoldTimer
11) PassiveTcpEstablishment
12) SendNOTIFICATIONwithoutOPEN
13) TrackTcpState
The optional session attributes support different features of the BGP
functionality that have implications for the BGP FSM state
transitions. Two groups of the attributes which relate to timers are:
group 1: DelayOpen, DelayOpenTime, DelayOpenTimer
group 2: DampPeerOscillations, IdleHoldTime, IdleHoldTimer
The first parameter (DelayOpen, DampPeerOscillations) is an
optional attribute that indicates that the Timer function is
active. The "Time" value specifies the initial value for "Timer"
(DelayOpenTime, IdleHoldTime). The "Timer" specifies the actual timer.
Please refer to section 8.1.1 for an explanation
of the interaction between these optional attributes and the events
signaled to the state machine. Section 8.2.1.3 also provides
a short overview of the different types of optional attributes
(flags or timers).
8.1 Events for the BGP FSM
8.1.1 Optional Events linked to Optional Session attributes
The Inputs to the BGP FSM are events. Events can either be
mandatory or optional. Some optional events are linked to
optional session attributes. Optional session attributes enable
several groups of FSM functionality.
The description below describes the linkage between FSM
functionality, events and the optional session attributes.
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Group 1: Automatic Administrative Events (Start/Stop)
Optional Session Attributes: AllowAutomaticStart, AllowAutomaticStop,
DampPeerOscillations, IdleHoldTime,
IdleHoldTimer
Option 1: AllowAutomaticStart
Description: A BGP peer connection can be started and stopped
by administrative control. This administrative
control can either be manual, based on
operator intervention, or under the control
of logic specific to a BGP implementation.
The term "automatic" refers to a start being
issued to the BGP peer connection FSM when
such logic determines that the BGP peer
connection should be restarted.
The AllowAutomaticStart attribute specifies
that this BGP connection supports automatic
starting of the BGP connection.
If the BGP implementation supports
AllowAutomaticStart, the peer may be
repeatedly restarted. Three other options
control the rate at which the automatic
restart occurs: DampPeerOscillations,
IdleHoldTime, and the IdleHoldTimer.
The DampPeerOscillations option specifies
that the implementation engages additional
logic to damp the oscillations of BGP peers
in the face of sequences of automatic start
and automatic stop. IdleHoldTime specifies
how long the BGP peer is held in the Idle
state prior to allowing the next automatic
restart. The IdleHoldTimer is the timer
that runs to hold the peer in Idle state.
An example of DampPeerOscillations logic
is an increase of the IdleHoldTime value
if a BGP peer oscillates connectivity
(connected/disconnected) repeatedly
within a time period. To engage this
logic, a peer could connect and disconnect
10 times within 5 minutes. The IdleHoldTime
value would be reset from 0 to 120 seconds.
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Values: TRUE or FALSE
Option 2: AllowAutomaticStop
Description: This BGP peer session optional attribute
indicates that the BGP connection allows
"automatic" stopping of the BGP connection.
An "automatic" stop is defined as a stop under
the control of implementation specific logic.
The implementation specific logic is outside
the scope of this specification.
Values: TRUE or FALSE
Option 3: DampPeerOscillations
Description: The DampPeerOscillations optional session
attribute indicates that this BGP connection
is using logic that damps BGP peer oscillations
in the Idle State.
Value: TRUE or FALSE
Option 4: IdleHoldTime
Description: The IdleHoldTime is the value
that is set in the IdleHoldTimer.
Values: Time in seconds
Option 5: IdleHoldTimer
Description: The IdleHoldTimer aids in controlling BGP peer
oscillation. The IdleHoldTimer is used to keep
the BGP peer in Idle for a particular duration.
The IdleHoldTimer_Expires event is described
in section 8.1.3.
Values: Time in seconds
Group 2: Unconfigured Peers
Optional Session Attributes: AcceptConnectionsUnconfiguredPeers
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Option 1: AcceptConnectionsUnconfiguredPeers
Description: The BGP FSM optionally allows the acceptance of BGP
peer connections from neighbors that are not
pre-configured. The
"AcceptConnectionsUnconfiguredPeers" optional
session attribute allows the FSM to support
the state transitions that allow the
implementation to accept or reject these
unconfigured peers.
The AcceptConnectionsUnconfiguredPeers has
security implications. Please refer to the
BGP Vulnerabilities document[BGP_VULN] for
details.
Value: True or False
Group 3: TCP processing
Optional Session Attributes: PassiveTcpEstablishment, TrackTcpState
Option 1: PassiveTcpEstablishment
Description: This option indicates that the BGP FSM will passively
wait for the remote BGP peer to establish the BGP
TCP connection.
value: TRUE or FALSE
Option 2: TrackTcpState
Description: The BGP FSM normally tracks the end result of a TCP
connection attempt rather than individual TCP messages.
Optionally, the BGP FSM can support additional
interaction with the TCP connection negotiation. The
interaction with the TCP events may increase the
amount of logging the BGP peer connection
requires and the number of BGP FSM changes.
Value: TRUE or FALSE
Group 4: BGP Message Processing
Optional Session Attributes: DelayOpen, DelayOpenTime,
DelayOpenTimer,
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SendNOTIFICATIONwithoutOPEN,
CollisionDetectEstablishedState
Option 1: DelayOpen
Description: The DelayOpen optional session attribute allows
implementations to be configured to delay
sending an OPEN message for a specific time
period (DelayOpenTime). The delay allows
the remote BGP Peer time to send the first
OPEN message.
Value: TRUE or FALSE
Option 2: DelayOpenTime
Description: The DelayOpenTime is the initial value that is
set in the DelayOpenTimer.
Value: Time in seconds
Option 3: DelayOpenTimer
Description: The DelayOpenTimer optional session attribute
is used to delay the sending of an OPEN message
on a connection. The DelayOpenTimer_Expires event
(Event 12) is described in section 8.1.3.
Value: Time in seconds
Option 4: SendNOTIFICATIONwithoutOPEN
Description: The SendNOTIFICATIONwithoutOPEN allows a peer to
send a NOTIFICATION without first sending an
OPEN message. Without this optional session
attribute, the BGP connection assumes that an
OPEN message must be sent by a peer prior
to the peer sending a NOTIFICATION message.
Value: True or False
Option 5: CollisionDetectEstablishedState
Description: Normally, a Detect Collision (6.8) will
be ignored in the Established state. This
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optional session attribute indicates that
this BGP connection processes
collisions in the Established state.
Value: True or False
Note: The optional session attributes clarify the BGP FSM description
for existing features of BGP implementations. The optional
session attributes may be pre-defined for an implementation
and not readable via management interfaces for existing
correct implementations. As newer BGP MIBs (version 2
and beyond) are supported, these fields will be accessible
via a management interface.
8.1.2 Administrative Events
An administrative event is an event in which the operator interface
and BGP Policy engine signal the BGP finite state machine to start or
stop the BGP state machine. The basic start and stop indication are
augmented by optional connection attributes to signal a certain type
of start or stop mechanism to the BGP FSM. An example of this combi-
nation is Event 5, AutomaticStart_with_PassiveTcpEstablishment. With
this event, the BGP implementation signals to the BGP FSM that the
implementation is using an Automatic Start with option to use a Pas-
sive TCP Establishment. The Passive TCP establishment signals that
this BGP FSM will wait for the remote side to start the TCP estab-
lishment.
Please note that only Event 1 (ManualStart) and Event 2 (ManualStop)
are mandatory administrative events. All other administrative events
are optional (Events 3-8). Each event below has a name, definition,
status (mandatory or optional), and what optional session attributes
SHOULD be set at each stage. When generating Event 1 through Event 8
for the BGP FSM, the conditions specified in the "Optional Attribute
Status" section are verified. If any of these conditions are not
satisfied, then the local system should log a FSM error.
The settings of optional session attributes may be implicit in some
implementations and therefore may not be set explicitly by an exter-
nal operator action. Section 8.2.1.5 describes these implicit set-
tings of the optional session attributes. The administrative states
described below may also be implicit in some implementations and not
directly configurable by an external operator.
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Event 1: ManualStart
Definition: Local system administrator manually starts peer
connection.
Status: Mandatory
Optional
Attribute
Status: The PassiveTcpEstablishment attribute SHOULD be
set to FALSE.
Event 2: ManualStop
Definition: Local system administrator manually
stops the peer connection.
Status: Mandatory
Optional
Attribute
Status: No interaction with any optional attributes.
Event 3: AutomaticStart
Definition: Local system automatically starts the
BGP connection.
Status: Optional, depending on local system
Optional
Attribute
Status: 1) The AllowAutomaticStart attribute SHOULD be set
to TRUE if this event occurs.
2) If the PassiveTcpEstablishment optional session
attribute is supported, it SHOULD be set to FALSE.
3) If the DampPeerOscillations is supported, it
SHOULD be set to FALSE when this event occurs.
Event 4: ManualStart_with_PassiveTcpEstablishment
Definition: Local system administrator manually starts peer
connection, but has PassiveTcpEstablishment
enabled. The PassiveTcpEstablishment optional
attribute indicates that the peer will listen prior
to establishing the connection.
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Status: Optional, depending on local system
Optional
Attribute
Status: 1) The PassiveTcpEstablishment attribute SHOULD
be set to TRUE if this event occurs.
2) The DampPeerOscillations attribute SHOULD be
set to FALSE when this event occurs.
Event 5: AutomaticStart_with_PassiveTcpEstablishment
Definition: Local system automatically starts the
BGP connection with the PassiveTcpEstablishment
enabled. The PassiveTcpEstablishment
optional attribute indicates
that the peer will listen prior to
establishing a connection.
Status: Optional, depending on local system
Optional
Attribute
Status: 1) The AllowAutomaticStart attribute SHOULD
be set to TRUE.
2) The PassiveTcpEstablishment attribute SHOULD
be set to TRUE
3) If the DampPeerOscillations attribute is
supported, the DampPeerOscillations SHOULD
be set to FALSE.
Event 6: AutomaticStart_with_DampPeerOscillations
Definition: Local system automatically starts the
BGP peer connection with peer oscillation
damping enabled. The exact method of damping
persistent peer oscillations is left up to the
implementation and is outside the scope of
this document.
Status: Optional, depending on local system.
Optional
Attribute
Status: 1) The AllowAutomaticStart attribute SHOULD
be set to TRUE.
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2) The DampPeerOscillations attribute SHOULD
be set to TRUE.
3) The PassiveTcpEstablishment attribute
SHOULD be set to FALSE.
Event 7: AutomaticStart_with_DampPeerOscillations_and_
PassiveTcpEstablishment
Definition: Local system automatically starts the
BGP peer connection with peer oscillation
damping enabled and PassiveTcpEstablishment
enabled. The exact method of damping
persistent peer oscillations is left up to the
implementation and is outside the scope of
this document.
Status: Optional, depending on local system
Optional
Attributes
Status: 1) The AllowAutomaticStart attribute
SHOULD be set to TRUE.
2) The DampPeerOscillations attribute SHOULD
be set to TRUE.
3) The PassiveTcpEstablishment attribute
SHOULD be set to TRUE.
Event 8: AutomaticStop
Definition: Local system automatically stops the
BGP connection.
An example of an automatic stop event is
exceeding the number of prefixes for a given
peer and the local system automatically
disconnecting the peer.
Status: Optional, depending on local system
Optional
Attribute
Status: 1) The AllowAutomaticStop attribute
SHOULD be TRUE
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8.1.3 Timer Events
Event 9: ConnectRetryTimer_Expires
Definition: An event generated when the ConnectRetryTimer
expires.
Status: Mandatory
Event 10: HoldTimer_Expires
Definition: An event generated when the HoldTimer expires.
Status: Mandatory
Event 11: KeepaliveTimer_Expires
Definition: An event generated when the KeepaliveTimer expires.
Status: Mandatory
Event 12: DelayOpenTimer_Expires
Definition: An event generated when the DelayOpenTimer expires.
Status: Optional
Optional
Attribute
Status: If this event occurs,
1) DelayOpen attribute SHOULD be set to TRUE,
2) DelayOpenTime attribute SHOULD be supported,
3) DelayOpenTimer SHOULD be supported,
Event 13: IdleHoldTimer_Expires
Definition: An event generated when the IdleHoldTimer
expires indicating that the BGP connection has
completed waiting for the back-off period
to prevent BGP peer oscillation.
The IdleHoldTimer is only used when the
persistent peer oscillation damping
function is enabled by setting the
DampPeerOscillations optional attribute
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to TRUE.
Implementations not implementing the
persistent peer oscillation damping
function may not have the IdleHoldTimer.
Status: Optional
Optional
Attribute
Status: If this event occurs:
1) DampPeerOscillations attribute SHOULD be set
to TRUE.
2) IdleHoldTimer SHOULD have just expired.
8.1.4 TCP Connection based Events
Event 14: TcpConnection_Valid
Definition: Event indicating the local system reception of
a TCP connection request with a valid
source IP address and TCP port and a valid
destination IP address and TCP Port. The
definition of invalid source and invalid
destination IP address is left to the
implementation.
BGP's destination port SHOULD be port 179
as defined by IANA.
TCP connection request is denoted by the
local system receiving a TCP SYN.
Status: Optional
Optional
Attribute
Status: 1) The TrackTcpState attribute SHOULD be set to
TRUE if this event occurs.
Event 15: Tcp_CR_Invalid
Definition: Event indicating the local system reception
of a TCP connection request with either
Expiration Date April 2005 [Page 49]
RFC DRAFT October 2004
an invalid source address or port
