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Keywords: BGP-ASC, Border, Gateway, Protocol
Network Working Group P. Traina
Request for Comments: 1965 cisco Systems
Category: Experimental June 1996
Autonomous System Confederations for BGP
Status of this Memo
This memo defines an Experimental Protocol for the Internet
community. This memo does not specify an Internet standard of any
kind. Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Abstract
Border Gateway Protocol [1] is an inter-autonomous system routing
protocol designed for TCP/IP networks.
This document describes an extension to BGP which may be used to
create a confederation of autonomous systems which is represented as
one single autonomous system to BGP peers external to the
confederation.
The intention of this extension is to aid in policy administration
and reduce the management complexity of maintaining a large
autonomous system.
The extension this document describes is widely deployed in the
Internet today.
Introduction
It may be useful to subdivide autonomous systems with a very large
number of BGP speakers into smaller domains for purposes of
controlling routing policy via information contained in the BGP
AS_PATH attribute. For example, one may chose to consider all BGP
speakers in a geographic region as a single entity.
In addition to improvements in routing policy control, current
techniques for deploying BGP among speakers in the same autonomous
system establish a full mesh of TCP connections among all speakers
for the purpose of exchanging exterior routing information. In
autonomous systems the number of intra-domain connections that need
to be maintained by each border router can become significant.
Subdividing a large autonomous system allows a significant reduction
in the total number of intra-domain BGP connections, as the
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connectivity requirements simplify to the model used for inter-domain
connections.
Unfortunately subdividing an autonomous system may increase the
complexity of policy routing based on AS_PATH information for all
members of the Internet. Additionally, this division increases the
maintenance overhead of coordinating external peering when the
internal topology of this collection of autonomous systems is
modified.
Finally, dividing a large AS may unnecessarily increase the length of
the sequence portions of the AS_PATH attribute. Several common BGP
implementations can use the number of "hops" required to reach a
given destination as part of the path selection criteria. While this
is not an optimal method of determining route preference, given the
lack of other in-band information, it provides a reasonable default
behavior which is widely used across the Internet. Therefore,
division of an autonomous system into separate systems may adversely
affect optimal routing of packets through the Internet.
However, there is usually no need to expose the internal topology of
this divided autonomous system, which means it is possible to regard
a collection of autonomous systems under a common administration as a
single entity or autonomous system when viewed from outside the
confines of the confederation of autonomous systems itself.
Terms and Definitions
AS Confederation
A collection of autonomous systems advertised as a single AS
number to BGP speakers that are not members of the confederation.
AS Confederation Identifier
An externally visible autonomous system number that identifies the
confederation as a whole.
Member-AS
An autonomous system that is contained in a given AS
confederation.
Overview
IDRP[2] has the concept of a routing domain confederation. An IDRP
routing domain confederation appears to IDRP speakers external to the
confederation as a single administrative entity. This extension is
based upon that work.
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In IDRP, routing domain confederations may be nested within each
other or disjoint portions of still larger confederations. The
algorithm BGP defines for additions to the AS_PATH attribute imposes
an additional restriction that AS confederations must be strictly
hierarchical in nature.
AS_CONFED segment type extension
Currently, BGP specifies that the AS_PATH attribute is a well-known
mandatory attribute that is composed of a sequence of AS path
segments. Each AS path segment is represented by a type/length/value
triple.
In [1], the path segment type is a 1-octet long field with the two
following 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
This document reserves two additional segment types:
3 AS_CONFED_SET: unordered set of ASs in the local
confederation that the UPDATE message
has traversed
4 AS_CONFED_SEQUENCE: ordered set of ASs in the
local confederation that the UPDATE
message has traversed
Operation
A member of a BGP confederation will use its confederation identifier
in all transactions with peers that are not members of its
confederation. This confederation identifier is considered to be the
"externally visible" AS number and this number is used in OPEN
messages and advertised in the AS_PATH attribute.
A member of a BGP confederation will use its routing domain
identifier (the internally visible AS number) in all transactions
with peers that are members of the same confederation as the given
router.
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A BGP speaker receiving an AS_PATH attribute containing a
confederation ID matching its own confederation shall treat the path
in the same fashion as if it had received a path containing its own
AS number.
AS_PATH modification rules
Section 5.1.2 of [1] is replaced with the following text.
When a BGP speaker propagates a route which it has learned from
another BGP speaker's UPDATE message, it shall modify 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 another BGP
speaker located in its own autonomous system, the advertising
speaker shall not modify the AS_PATH attribute associated with
the route.
b) When a given BGP speaker advertises the route to a BGP
speaker located in a neighboring autonomous system that is a
member of the local autonomous system confederation, then the
advertising speaker shall update the AS_PATH attribute as
follows:
1) if the first path segment of the AS_PATH is of type
AS_CONFED_SEQUENCE, the local system shall prepend its own AS
number as the last element of the sequence (put it in the
leftmost position).
2) if the first path segment of the AS_PATH is not of type
AS_CONFED_SEQUENCE the local system shall prepend a new path
segment of type AS_CONFED_SEQUENCE to the AS_PATH, including
its own confederation identifier in that segment.
c) When a given BGP speaker advertises the route to a BGP
speaker located in a neighboring autonomous system that is not a
member of the current routing domain confederation, then the
advertising speaker shall update the AS_PATH attribute as
follows:
1) if the first path segment of the AS_PATH is of type
AS_CONFED_SEQUENCE, that segment and any immediately
following segments of the type AS_CONFED_SET are removed from
the AS_PATH attribute, leaving the sanitized AS_PATH
attribute to be operated on by steps 2, or 3.
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2) if the first path segment of the remaining AS_PATH is of
type AS_SEQUENCE, the local system shall prepend its own
confederation identifier as the last element of the sequence
(put it in the leftmost position).
3) if there are no path segments following the removal of the
first AS_CONFED_SET/AS_CONFED_SEQUENCE segments, or if the
first path segment of the remaining AS_PATH is of type AS_SET
the local system shall prepend a new path segment of type
AS_SEQUENCE to the AS_PATH, including its own confederation
identifier in that segment.
When a BGP speaker originates a route:
a) the originating speaker shall include an empty AS_PATH
attribute in all UPDATE messages sent to BGP speakers located in
its own autonomous system. (An empty AS_PATH attribute is one
whose length field contains the value zero).
b) the originating speaker shall include its own AS number in an
AS_CONFED_SEQUENCE segment of the AS_PATH attribute of all
UPDATE messages sent to BGP speakers located in neighboring
autonomous systems that are members of the local confederation.
(In this case, the AS number of the originating speaker's member
autonomous system number will be the only entry in the AS_PATH
attribute).
c) the originating speaker shall include its own confederation
identifier in a AS_SEQUENCE segment of the AS_PATH attribute of
all UPDATE messages sent to BGP speakers located in neighboring
autonomous systems that are not members of the local
confederation. (In this case, the confederation identifier of
the originating speaker's member confederation will be the only
entry in the AS_PATH attribute).
Common Administration Issues
It is reasonable for member ASs of a confederation to share a common
administration and IGP information for the entire confederation.
It shall be legal for a BGP speaker to advertise an unchanged
NEXT_HOP and MULTI_EXIT_DISCRIMINATOR attribute to peers in a
neighboring AS within the same confederation. In addition, the
restriction against sending the LOCAL_PREFERENCE attribute to peers
in a neighboring AS within the same confederation is removed. Path
selection criteria for information received from members inside a
confederation may follow the same rules used for information received
from members inside the same autonomous system.
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Compatibility
All BGP speakers participating in a confederation must recognize the
AS_CONFED_SET and AS_CONFED_SEQUENCE segment type extensions to the
AS_PATH attribute.
Any BGP speaker not supporting these extensions will generate a
notification message specifying an "UPDATE Message Error" and a sub-
code of "Malformed AS_PATH".
This compatibility issue implies that all BGP speakers participating
in a confederation must support BGP confederations, however BGP
speakers outside the confederation need not support these extensions.
Compatibility Discussion
We considered the use of a distinct, optional, transitive attribute
to carry AS confederation information as opposed to specifying new
types in the existing AS path attribute. This would relax the
requirement that all BGP speakers participating in a confederation to
allow the use of legacy units provided they have no external (i.e.
neither inter-AS nor intra-confederation) connectivity.
At the time of this writing, an implementation of this extension as
documented is widely deployed throughout the Internet, therefore the
value of any change that is incompatible with this document must be
weighed against the benefit gained from a relaxation of this
restriction.
References
[1] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4 (BGP-4)",
RFC 1771, March 1995.
[2] Kunzinger, C. Editor, "Inter-Domain Routing Protocol", ISO/IEC
10747, October 1993.
Security Considerations
Security issues are not discussed in this memo.
Acknowledgments
Ravi Chandra and Yakov Rekhter reviewed this document and provided
constructive and valuable comments.
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Author's Address
Paul Traina
cisco Systems, Inc.
170 W. Tasman Dr.
San Jose, CA 95134
EMail: pst@cisco.com
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