Network Working Group Y. Rekhter
Request for Comments: 1268 T.J. Watson Research Center, IBM Corp.
Obsoletes: RFC 1164 P. Gross
ANS
Editors
October 1991
Application of the Border Gateway Protocol in the Internet
Status of this Memo
This protocol is being developed by the Border Gateway Protocol
Working Group (BGP) of the Internet Engineering Task Force (IETF).
This RFC specifies an IAB standards track protocol for the Internet
community, and requests discussion and suggestions for improvements.
Please refer to the current edition of the "IAB Official Protocol
Standards" for the standardization state and status of this protocol.
Distribution of this memo is unlimited.
Abstract
This document, together with its companion document, "A Border
Gateway Protocol (BGP-3)", define an inter-autonomous system routing
protocol for the Internet. "A Border Gateway Protocol (BGP-3)"
defines the BGP protocol specification, and this document describes
the usage of the BGP in the Internet.
Information about the progress of BGP can be monitored and/or
reported on the BGP mailing list (iwg@rice.edu).
Table of Contents
1. Introduction................................................... 2
2. BGP Topological Model.......................................... 3
3. BGP in the Internet............................................ 4
4. Policy Making with BGP......................................... 5
5. Path Selection with BGP........................................ 6
6. Required set of supported routing policies..................... 8
7. Conclusion..................................................... 9
Appendix A. The Interaction of BGP and an IGP..................... 9
References........................................................ 12
Security Considerations........................................... 12
Authors' Addresses................................................ 13
Acknowledgements
This document was original published as RFC 1164 in June 1990,
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jointly authored by Jeffrey C. Honig (Cornell University), Dave Katz
(MERIT), Matt Mathis (PSC), Yakov Rekhter (IBM), and Jessica Yu
(MERIT).
The following also made key contributions to RFC 1164 -- Guy Almes
(ANS, then at Rice University), Kirk Lougheed (cisco Systems), Hans-
Werner Braun (SDSC, then at MERIT), and Sue Hares (MERIT).
This updated version of the document is the product of the IETF BGP
Working Group with Phillip Gross (ANS) and Yakov Rekhter (IBM) as
editors. John Moy (Proteon) contributed Section 6 "Recommended set
of supported routing policies".
We also like to explicitly thank Bob Braden (ISI) for the review of
this document as well as his constructive and valuable comments.
1. Introduction
This memo describes the use of the Border Gateway Protocol (BGP) [1]
in the Internet environment. BGP is an inter-Autonomous System
routing protocol. The network reachability information exchanged via
BGP provides sufficient information to detect routing loops and
enforce routing decisions based on performance preference and policy
constraints as outlined in RFC 1104 [2]. In particular, BGP exchanges
routing information containing full AS paths and enforces routing
policies based on configuration information.
All of the discussions in this paper are based on the assumption that
the Internet is a collection of arbitrarily connected Autonomous
Systems. That is, the Internet will be modeled as a general graph
whose nodes are AS's and whose edges are connections between pairs of
AS's.
The classic definition of an Autonomous System is a set of routers
under a single technical administration, using an interior gateway
protocol and common metrics to route packets within the AS, and using
an exterior gateway protocol to route packets to other AS's. Since
this classic definition was developed, it has become common for a
single AS to use several interior gateway protocols 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 administration of an AS appears to other AS's
to have a single coherent interior routing plan and presents a
consistent picture of which networks are reachable through it. From
the standpoint of exterior routing, an AS can be viewed as
monolithic: networks within an AS must maintain connectivity via
intra-AS paths.
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AS's are assumed to be administered by a single administrative
entity, at least for the purposes of representation of routing
information to systems outside of the AS.
2. BGP Topological Model
When we say that a connection exists between two AS's, we mean two
things:
Physical connection: There is a shared network between the two
AS's, and on this shared network each AS has at least one border
gateway belonging to that AS. Thus the border gateway of each AS
can forward packets to the border gateway of the other AS without
resort to Inter-AS or Intra-AS routing.
BGP connection: There is a BGP session between BGP speakers on
each of the AS's, and this session communicates to each connected
AS those routes through the physically connected border gateways
of the other AS that can be used for specific networks. Throughout
this document we place an additional restriction on the BGP
speakers that form the BGP connection: they must themselves share
the same network that their border gateways share. Thus, a BGP
session between the adjacent AS's requires no support from either
Inter-AS or Intra-AS routing. Cases that do not conform to this
restriction fall outside the scope of this document.
Thus, at each connection, each AS has one or more BGP speakers and
one or more border gateways, and these BGP speakers and border
gateways are all located on a shared network. Note that BGP speakers
do not need to be a border gateway, and vice versa. Paths announced
by a BGP speaker of one AS on a given connection are taken to be
feasible for each of the border gateways of the other AS on the same
connection, i.e. indirect neighbors are allowed.
Much of the traffic carried within an AS either originates or
terminates at that AS (i.e., either the source IP address or the
destination IP address of the IP packet identifies a host on a
network directly connected to that AS). Traffic that fits this
description is called "local traffic". Traffic that does not fit this
description is called "transit traffic". A major goal of BGP usage is
to control the flow of transit traffic.
Based on how a particular AS deals with transit traffic, the AS may
now be placed into one of the following categories:
stub AS: an AS that has only a single connection to one other AS.
Naturally, a stub AS only carries local traffic.
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multihomed AS: an AS that has connections to more than one other
AS, but refuses to carry transit traffic.
transit AS: an AS that has connections to more than one other AS,
and is designed (under certain policy restrictions) to carry both
transit and local traffic.
Since a full AS path provides an efficient and straightforward way of
suppressing routing loops and eliminates the "count-to-infinity"
problem associated with some distance vector algorithms, BGP imposes
no topological restrictions on the interconnection of AS's.
3. BGP in the Internet
3.1 Topology Considerations
The overall Internet topology may be viewed as an arbitrary
interconnection of transit, multihomed, and stub AS's. In order to
minimize the impact on the current Internet infrastructure, stub and
multihomed AS's need not use BGP. These AS's may run other protocols
(e.g., EGP) to exchange reachability information with transit AS's.
Transit AS's using BGP will tag this information as having been
learned by some method other than BGP. The fact that BGP need not run
on stub or multihomed AS's has no negative impact on the overall
quality of inter-AS routing for traffic not local to the stub or
multihomed AS's in question.
However, it is recommended that BGP may be used for stub and
multihomed AS's as well, providing an advantage in bandwidth and
performance over some of the currently used protocols (such as EGP).
In addition, this would result in less need for the use of defaults
and in better choices of Inter-AS routes for multihomed AS's.
3.2 Global Nature of BGP
At a global level, BGP is used to distribute routing information
among multiple Autonomous Systems. The information flows can be
represented as follows:
+-------+ +-------+
BGP | BGP | BGP | BGP | BGP
---------+ +---------+ +---------
| IGP | | IGP |
+-------+ +-------+
<-AS A--> <--AS B->
This diagram points out that, while BGP alone carries information
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between AS's, a combination of BGP and an IGP carries information
across an AS. Ensuring consistency of routing information between
BGP and an IGP within an AS is a significant issue and is discussed
at length later in Appendix A.
3.3 BGP Neighbor Relationships
The Internet is viewed as a set of arbitrarily connected AS's. BGP
speakers in each AS communicate with each other to exchange network
reachability information based on a set of policies established
within each AS. Routers that communicate directly with each other via
BGP are known as BGP neighbors. BGP neighbors can be located within
the same AS or in different AS's. For the sake of discussion, BGP
communications with neighbors in different AS's will be referred to
as External BGP, and with neighbors in the same AS as Internal BGP.
There can be as many BGP speakers as deemed necessary within an AS.
Usually, if an AS has multiple connections to other AS's, multiple
BGP speakers are needed. All BGP speakers representing the same AS
must give a consistent image of the AS to the outside. This requires
that the BGP speakers have consistent routing information among them.
These gateways can communicate with each other via BGP or by other
means. The policy constraints applied to all BGP speakers within an
AS must be consistent. Techniques such as using tagged IGP (see
A.2.2) may be employed to detect possible inconsistencies.
In the case of External BGP, the BGP neighbors must belong to
different AS's, but share a common network. This common network
should be used to carry the BGP messages between them. The use of BGP
across an intervening AS invalidates the AS path information. An
Autonomous System number must be used with BGP to specify which
Autonomous System the BGP speaker belongs to.
4. Policy Making with BGP
BGP provides the capability for enforcing policies based on various
routing preferences and constraints. Policies are not directly
encoded in the protocol. Rather, policies are provided to BGP in the
form of configuration information.
BGP enforces policies by affecting the selection of paths from
multiple alternatives, and by controlling the redistribution of
routing information. Policies are determined by the AS
administration.
Routing policies are related to political, security, or economic
considerations. For example, if an AS is unwilling to carry traffic
to another AS, it can enforce a policy prohibiting this. The
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following are examples of routing policies that can be enforced with
the use of BGP:
1. A multihomed AS can refuse to act as a transit AS for other
AS's. (It does so by not advertising routes to networks other
than those directly connected to it.)
2. A multihomed AS can become a transit AS for a restricted set of
adjacent AS's, i.e., some, but not all, AS's can use multihomed
AS as a transit AS. (It does so by advertising its routing
information to this set of AS's.)
3. An AS can favor or disfavor the use of certain AS's for
carrying transit traffic from itself.
A number of performance-related criteria can be controlled with the
use of BGP:
1. An AS can minimize the number of transit AS's. (Shorter AS
paths can be preferred over longer ones.)
2. The quality of transit AS's. If an AS determines that two or
more AS paths can be used to reach a given destination, that
AS can use a variety of means to decide which of the candidate
AS paths it will use. The quality of an AS can be measured by
such things as diameter, link speed, capacity, tendency to
become congested, and quality of operation. Information about
these qualities might be determined by means other than BGP.
3. Preference of internal routes over external routes.
For consistency within an AS, equal cost paths, resulting from
combinations of policies and/or normal route selection procedures,
must be resolved in a consistent fashion.
Fundamental to BGP is the rule that an AS advertises to its
neighboring AS's only those routes that it uses. This rule reflects
the "hop-by-hop" routing paradigm generally used by the current
Internet.
5. Path Selection with BGP
One of the major tasks of a BGP speaker is to evaluate different
paths to a destination network from its border gateways at that
connection, select the best one, apply applicable policy constraints,
and then advertise it to all of its BGP neighbors at that same
connection. The key issue is how different paths are evaluated and
compared.
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In traditional distance vector protocols (e.g., RIP) there is only
one metric (e.g., hop count) associated with a path. As such,
comparison of different paths is reduced to simply comparing two
numbers. A complication in Inter-AS routing arises from the lack of a
universally agreed-upon metric among AS's that can be used to
evaluate external paths. Rather, each AS may have its own set of
criteria for path evaluation.
A BGP speaker builds a routing database consisting of the set of all
feasible paths and the list of networks reachable through each path.
For purposes of precise discussion, it's useful to consider the set
of feasible paths for a given destination network. In most cases, we
would expect to find only one feasible path. However, when this is
not the case, all feasible paths should be maintained, and their
maintenance speeds adaptation to the loss of the primary path. Only
the primary path at any given time will ever be advertised.
The path selection process can be formalized by defining a partial
order over the set of all feasible paths to a given destination
network. One way to define this partial order is to define a function
that maps each full AS path to a non-negative integer that denotes
the path's degree of preference. Path selection is then reduced to
applying this function to all feasible paths and choosing the one
with the highest degree of preference.
In actual BGP implementations, criteria for assigning degree of
preferences to a path are specified in configuration information.
The process of assigning a degree of preference to a path can be
based on several sources of information:
1. Information explicitly present in the full AS path.
2. A combination of information that can be derived from the full
AS path and information outside the scope of BGP (e.g., policy
routing constraints provided at configuration).
Possible criteria for assigning a degree of preference to a path are:
- AS count. Paths with a smaller AS count are generally better.
- Policy consideration. BGP supports policy-based routing based
on the controlled distribution of routing information. A BGP
speaker may be aware of some policy constraints (both within
and outside of its own AS) and do appropriate path selection.
Paths that do not comply with policy requirements are not
considered further.
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- Presence or absence of a certain AS or AS's in the path. By
means of information outside the scope of BGP, an AS may know
some performance characteristics (e.g., bandwidth, MTU, intra-AS
diameter) of certain AS's and may try to avoid or prefer them.
- Path origin. A path learned entirely from BGP (i.e., whose
endpoint is internal to the last AS on the path is generally
better than one for which part of the path was learned via EGP
or some other means.
- AS path subsets. An AS path that is a subset of a longer AS
path to the same destination should be preferred over the longer
path. Any problem in the shorter path (such as an outage) will
also be a problem in the longer path.
- Link dynamics. Stable paths should be preferred over unstable
ones. Note that this criterion must be used in a very careful
way to avoid causing unnecessary route fluctuation. Generally,
any criteria that depend on dynamic information might cause
routing instability and should be treated very carefully.
6. Required set of supported routing policies.
Policies are provided to BGP in the form of configuration
information. This information is not directly encoded in the
protocol. Therefore, BGP can provides support for quite complex
routing policies. However, it is not required for all BGP
implementations to support such policies.
We are not attempting to standardize the routing policies that must
be supported in every BGP implementation, we strongly encourage all
implementors to support the following set of routing policies:
1. BGP implementations should allow an AS to control announcements
of BGP-learned routes to adjacent AS's. Implementations should
also support such control with at least the granularity of
a single network. Implementations should also support such
control with the granularity of an autonomous system, where
the autonomous system may be either the autonomous system that
originated the route, or the autonomous system that advertised
the route to the local system (adjacent autonomous system).
2. BGP implementations should allow an AS to prefer a particular
path to a destination (when more than one path is available).
This function should be implemented by allowing system
administrators to assign "weights" to AS's, and making route
selection process to select a route with the lowest "weight"
(where "weight" of a route is defined as a sum of "weights" of
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all AS's in the AS_PATH path attribute associated with that
route).
3. BGP implementations should allow an AS to ignore routes with
certain AS's in the AS_PATH path attribute. Such function can
be implemented by using technique outlined in (2), and by
assigning "infinity" as "weights" for such AS's. The route
selection process must ignore routes that have "weight" equal
to "infinity".
7. Conclusion
The BGP protocol provides a high degree of control and flexibility
for doing interdomain routing while enforcing policy and performance
constraints and avoiding routing loops. The guidelines presented here
will provide a starting point for using BGP to provide more
sophisticated and manageable routing in the Internet as it grows.
Appendix A. The Interaction of BGP and an IGP
This section outlines methods by which BGP can exchange routing
information with an IGP. The methods outlined here are not proposed
as part of the standard BGP usage at this time. These methods are
outlined for information purposes only. Implementors may want to
consider these methods when importing IGP information.
This is general information that applies to any generic IGP.
Interaction between BGP and any specific IGP is outside the scope of
this section. Methods for specific IGP's should be proposed in
separate documents. Methods for specific IGP's could be proposed for
standard usage in the future.
Overview
By definition, all transit AS's must be able to carry traffic which
originates from and/or is destined to locations outside of that AS.
This requires a certain degree of interaction and coordination
between BGP and the Interior Gateway Protocol (IGP) used by that
particular AS. In general, traffic originating outside of a given AS
is going to pass through both interior gateways (gateways that
support the IGP only) and border gateways (gateways that support both
the IGP and BGP). All interior gateways receive information about
external routes from one or more of the border gateways of the AS via
the IGP.
Depending on the mechanism used to propagate BGP information within a
given AS, special care must be taken to ensure consistency between
BGP and the IGP, since changes in state are likely to propagate at
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different rates across the AS. There may be a time window between the
moment when some border gateway (A) receives new BGP routing
information which was originated from another border gateway (B)
within the same AS, and the moment the IGP within this AS is capable
of routing transit traffic to that border gateway (B). During that
time window, either incorrect routing or "black holes" can occur.
In order to minimize such routing problems, border gateway (A) should
not advertise a route to some exterior network X via border gateway
(B) to all of its BGP neighbors in other AS's until all the interior
gateways within the AS are ready to route traffic destined to X via
the correct exit border gateway (B). In other words, interior routing
should converge on the proper exit gateway before/advertising routes
via that exit gateway to other AS's.
A.2 Methods for Achieving Stable Interactions
The following discussion outlines several techniques capable of
achieving stable interactions between BGP and the IGP within an
Autonomous System.
A.2.1 Propagation of BGP Information via the IGP
While BGP can provide its own mechanism for carrying BGP information
within an AS, one can also use an IGP to transport this information,
as long as the IGP supports complete flooding of routing information
(providing the mechanism to distribute the BGP information) and
onepass convergence (making the mechanism effectively atomic). If an
IGP is used to carry BGP information, then the period of
desynchronization described earlier does not occur at all, since BGP
information propagates within the AS synchronously with the IGP, and
the IGP converges more or less simultaneously with the arrival of the
new routing information. Note that the IGP only carries BGP
information and should not interpret or process this information.
A.2.2 Tagged Interior Gateway Protocol
Certain IGPs can tag routes exterior to an AS with the identity of
their exit points while propagating them within the AS. Each border
gateway should use identical tags for announcing exterior routing
information (received via BGP) both into the IGP and into Internal
BGP when propagating this information to other border gateways within
the same AS. Tags generated by a border gateway must uniquely
identify that particular border gateway--different border gateways
must use different tags.
All Border Gateways within a single AS must observe the following two
rules:
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1. Information received via Internal BGP by a border gateway A
declaring a network to be unreachable must immediately be
propagated to all of the External BGP neighbors of A.
2. Information received via Internal BGP by a border gateway A
about a reachable network X cannot be propagated to any of
the External BGP neighbors of A unless/until A has an IGP
route to X and both the IGP and the BGP routing information
have identical tags.
These rules guarantee that no routing information is announced
externally unless the IGP is capable of correctly supporting it. It
also avoids some causes of "black holes".
One possible method for tagging BGP and IGP routes within an AS is to
use the IP address of the exit border gateway announcing the exterior
route into the AS. In this case the "gateway" field in the BGP UPDATE
message is used as the tag.
A.2.3 Encapsulation
Encapsulation provides the simplest (in terms of the interaction
between the IGP and BGP) mechanism for carrying transit traffic
across the AS. In this approach, transit traffic is encapsulated
within an IP datagram addressed to the exit gateway. The only
requirement imposed on the IGP by this approach is that it should be
capable of supporting routing between border gateways within the same
AS.
The address of the exit gateway A for some exterior network X is
specified in the BGP identifier field of the BGP OPEN message
received from gateway A via Internal BGP by all other border gateways
within the same AS. In order to route traffic to network X, each
border gateway within the AS encapsulates it in datagrams addressed
to gateway A. Gateway A then performs decapsulation and forwards the
original packet to the proper gateway in another AS
Since encapsulation does not rely on the IGP to carry exterior
routing information, no synchronization between BGP and the IGP is
required.
Some means of identifying datagrams containing encapsulated IP, such
as an IP protocol type code, must be defined if this method is to be
used.
Note, that if a packet to be encapsulated has length that is very
close to the MTU, that packet would be fragmented at the gateway that
performs encapsulation.
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A.2.4 Other Cases
There may be AS's with IGPs which can neither carry BGP information
nor tag exterior routes (e.g., RIP). In addition, encapsulation may
be either infeasible or undesirable. In such situations, the
following two rules must be observed:
1. Information received via Internal BGP by a border gateway A
declaring a network to be unreachable must immediately be
propagated to all of the External BGP neighbors of A.
2. Information received via Internal BGP by a border gateway A
about a reachable network X cannot be propagated to any of
the External BGP neighbors of A unless A has an IGP route to
X and sufficient time (holddown) has passed for the IGP routes
to have converged.
The above rules present necessary (but not sufficient) conditions for
propagating BGP routing information to other AS's. In contrast to
tagged IGPs, these rules cannot ensure that interior routes to the
proper exit gateways are in place before propagating the routes other
AS's.
If the convergence time of an IGP is less than some small value X,
then the time window during which the IGP and BGP are unsynchronized
is less than X as well, and the whole issue can be ignored at the
cost of transient periods (of less than length X) of routing
instability. A reasonable value for X is a matter for further study,
but X should probably be less than one second.
If the convergence time of an IGP cannot be ignored, a different
approach is needed. Mechanisms and techniques which might be
appropriate in this situation are subjects for further study.
References
[1] Lougheed, K., and Y. Rekhter, "A Border Gateway Protocol 3 (BGP-
3)", RFC 1267, cisco Systems, T.J. Watson Research Center, IBM
Corp., October 1991.
[2] Braun, H-W., "Models of Policy Based Routing", RFC 1104,
Merit/NSFNET, June 1989.
Security Considerations
Security issues are not discussed in this memo.
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Authors' Addresses
Yakov Rekhter
T.J. Watson Research Center IBM Corporation
P.O. Box 218
Yorktown Heights, NY 10598
Phone: (914) 945-3896
EMail: yakov@watson.ibm.com
Phill Gross
Advanced Network and Services (ANS)
100 Clearbrook Road
Elmsford, NY 10523
Phone: (914) 789-5300
Email: pgross@NIS.ANS.NET
IETF BGP WG mailing list: iwg@rice.edu
To be added: iwg-request@rice.edu
BGP Working Group [Page 13]
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