RFC 1998 (rfc1998) - Page 2 of 9
An Application of the BGP Community Attribute in Multi-home Routing
Alternative Format: Original Text Document
RFC 1998 Use of Community August 1996
configuration of the BGP "LOCAL_PREF". It essentially removes the
need for customized configuration of the BGP "LOCAL_PREF" attribute
at the provider level while maintaining the same level of routing
functionality and flexibility.
It also represents a paradigm shift in that it gives the potential
for the customer to control its own routing policy with respect to
its service provider, as well as providing the ability for policy
configuration to be done at a prefix based granularity rather than
the more common AS based granularity in use today.
2. AS-based Configuration and its Drawbacks
As discussed in [3], in today's multi-provider Internet, customized
configuration of the BGP "LOCAL_PREF" attribute is often required to
implement common routing strategies such as load-sharing or backup.
There are two main reasons:
o Lack of available implementations and deployment of routing
software that supports the "Destination Preference Attribute"
(DPA) as specified in [4].
DPA allows one to specify a globally transitive preference so
that return traffic favors certain path. As discussed in [3],
the attribute will be very useful in influencing route selection
for routes with identical "LOCAL_PREF" and equal AS-path length.
o In the multi-provider Internet, it is common for a provider
to assign higher BGP "LOCAL_PREF" values for routes from its
customers than from other service providers. This practice
provides some degree of protection for its customer routes,
and it facilitates implementation of certain routing
strategies. It, however, also complicates other routing
implementations such as backup arrangement, thus, requiring
customized "LOCAL_PREF" configuration.
Figure 1 shows a typical case of a backup arrangement in the multi-
provider Internet. In Figure 1, AS1 and AS2 are both providers, and
AS3 and AS4 are customers of AS1 and AS2, respectively. AS3 has
entered a bilateral agreement with AS4 to provide backup to each
other. That is, AS3 would use its direct link to AS4 to reach only
AS4 in the normal circumstance, and for transit in the case of a
failure between AS3 and AS1. To realize this routing agreement, AS3
requests that its provider AS1 adjust its BGP "LOCAL_PREF"
configuration so that AS1 reaches AS4 via AS2.
Chen & Bates Informational