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BGP community attributes and Confederations

Maybe some BGP guru out there can answer this question definitively:

How does BGP treat the community attribute when used within confederations? Is the community attribute carried to the next AS outside the confederation, treating the conferation as a whole AS, or carried only to the next AS within the confederation.

For example you have several sub-ASes within a confederation, and do not want to advertise a certain route outside the confederation, but want to advertise it to all the other ASes within the confederation. There are of course many ways to do this, but we want to use the "no-export" and community attributes. Setting the particular route to be adertised as a "no-export" community would prevent the route from being advertised. The question is, which routers within the confederation have to have the "send-community" string added to the BGP configuration.

Regards to all,

Joe

2 REPLIES
Gold

Re: BGP community attributes and Confederations

From here:

http://www.cisco.com/en/US/tech/tk365/tk80/technologies_tech_note09186a0080094431.shtml

Prefer the path with the lowest multi-exit discriminator (MED). Note the following:

This comparison is only done if the first (neighboring) AS is the same in the two paths; any confederation sub-ASs are ignored. In other words, MEDs are compared only if the first AS in the AS_SEQUENCE is the same for multiple paths. Any preceding AS_CONFED_SEQUENCE is ignored.

So, MEDs within a confed are compared, as long as the external AS the two routes were received from is the same. And this:

If bgp bestpath med-confed is enabled, MEDs are compared for all paths that consist only of AS_CONFED_SEQUENCE (paths originated within the local confederation).

So there is an option to compare the MEDS only on routes originating within the confederation.

:-)

Russ.W

Cisco Employee

Re: BGP community attributes and Confederations

Two things. First, the send-community has to be used regardless of whether you want to propagate the community to an iBGP or eBGP peer.

Second, Prefixes with no-export community will be propagated to all sub-ASes. If you hypothetically wanted to limit a given prefix to a sub-AS, you would need to use local-as.

Hope this helps

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
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Cuauhtémoc, Juárez
Ciudad de México, 06600
México
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