INTERNET-DRAFT Enke Chen
<draft-ietf-idr-bgp-dpa-04.txt> Tony Bates
Expires in six months MCI
January 1996
Destination Preference Attribute for BGP
<draft-ietf-idr-bgp-dpa-04.txt>
Status of this Memo
This document is an Internet Draft. Internet Drafts are working
documents of the Internet Engineering Task Force (IETF), its Areas,
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Please check the I-D abstract listing contained in each Internet
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Abstract
The Border Gateway Protocol [1] is an inter-autonomous system routing
protocol designed for TCP/IP internets.
This document describes a new BGP path attribute termed "Destination
Preference Attribute" (DPA) which can be used by a single autonomous
system (AS) to specify globally transitive metrics in its routing
announcement via BGP. The metric can then be used by upstream BGP
speakers to favor certain path for return traffic. The application
of this attribute includes facilitating the implementation of
symmetric routing and load sharing in the multi-provider Internet.
Introduction
In certain cases there is a need for an autonomous system (AS) to
specify a globally transitive preference in its routing announcement
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via BGP so that the upstream BGP speakers can use the preference to
favor certain path for return traffic. For instance, as discussed in
[3], currently it is difficult to implement symmetric routing and
load sharing in the multi-provider Internet due to the lack of this
preference in BGP.
In this paper, we propose a new BGP attribute termed "Destination
Preference Attribute" (DPA) to address such a need. More
specifically, the DPA is a globally transitive metric that can be
used by an AS to specify preference in its routing announcement so
that the return traffic favors certain path. As illustrated in [4]
through several examples, this metric, combined with AS-based
"LOCAL_PREF" offers much greater flexibility and manageability in
implementing symmetric inter-domain routing and load sharing in the
multi-provider Internet.
Destination Preference Attribute (DPA)
This document proposes the DPA path attribute, which is an optional
transitive attribute of fixed length. The attribute is represented
by a pair <AS#, DPA value>. The AS# is a two octet non-negative
integer, which denotes the AS that specifies the preference. The DPA
value is a four octet non-negative integer.
The DPA attribute has Type Code 11.
Route Selection Process
The DPA attributes shall be used as a route selection criteria, after
the "LOCAL_PREF" attribute is evaluated, and before the evaluation of
the AS path length and the multi-exit-discriminator (MED) attribute.
However, if a route contains both MED and DPA attributes from the
same neighboring AS, the MED values shall be favored over DPA values
for route selection.
The higher the DPA attribute value, the more preferred the route.
A route with missing DPA attribute must be treated as having an DPA
attribute with value zero.
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Operation
The AS that sets this attribute must include its AS number in the
attribute. A BGP speaker may use the "LOCAL_PREF" attribute to
select a different path other than the one specified by the DPA
attribute value. This does not preclude an AS from re-setting this
attribute. However, coordination with the upstream and/or downstream
neighbors is strongly recommended.
To make sure that the MED attribute and not the DPA attribute is used
in the selection of routes from multiple peers of the same
neighboring AS, the DPA value, if set, must be identical for all
peers with the same neighboring AS. It is an operational matter to
ensure the correct setting of the DPA value for multiple peers to the
same neighboring AS.
Aggregation
If aggregation is done, the resultant aggregate shall be treated as a
new NLRI. No DPA attribute shall be derived from more specific NLRIs
which formed the aggregate. The resultant aggregate is free to have
the DPA attribute set if so desired.
Remarks
It is noted that this new BGP attribute is simple and requires little
change to the current practice and operation of BGP4. Nevertheless,
the new attribute would offer the flexibility of shifting more
influence on route selection to where the route originates, which has
become increasingly meaningful as the Internet becomes more complex
and dynamic. At the same time, the autonomy of an AS is preserved as
the "LOCAL_PREF" feature remains unchanged. A typical application of
this attribute is illustrated in [4] where the DPA attribute is used
to simplify the implementation of symmetric inter-domain routing and
load-sharing.
Applicability
The DPA path attribute may be used with BGP version 4 and all
subsequent versions of BGP unless specifically noted otherwise.
Security Considerations
Security considerations are not discussed in this memo.
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Acknowledgments
The authors would like to thank Yakov Rekhter of cisco for his
insightful comments and suggestions. We also acknowledge Ramesh
Govindan (ISI) and Ravi Chandra (cisco) for their helpful comments.
References
[1] Rekhter, Y., and Li, T., "A Border Gateway Protocol 4 (BGP-4)",
RFC1771, March 1995.
[2] Y. Rekhter, and P. Gross, "Application of the Border Gateway
Protocol in the Internet", RFC1772, March 1995.
[3] Chen, E., and Bates, T., "Current Practice of Implementing
Symmetric Routing and Load Sharing in the Multi-Provider Internet",
INTERNET-DRAFT, <draft-ietf-idr-symm-multi-prov-02.txt>, January
1996.
[4] Chen, E., and Bates, T., "Application of the BGP Destination
Preference Attribute in Implementing Symmetric Routing", INTERNET-
DRAFT, <draft-ietf-idr-dpa-application-02.txt>, January 1996.
[5] Antonov, V., "BGP AS Path Metrics", INTERNET DRAFT, <draft-ietf-
idr-bgp-metrics-00.txt>, March 1995.
[6] Rekhter, Y., "Routing in a Multi-provider Internet", RFC1787,
April 1995.
Author's Addresses
Enke Chen
MCI
2100 Reston Parkway
Reston, VA 22091
phone: +1 703 715 7087
email: enke@mci.net
Tony Bates
MCI
2100 Reston Parkway
Reston, VA 22091
phone: +1 703 715 7521
email: Tony.Bates@mci.net
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