Re: BGP4 COMMUNITY attribute

[Paul Knight <pknight () BayNetworks COM>]

Walter Towbin wrote:
> In the preparation for upcoming 2nd (multihomed) connection to Internet
> I am looking into
> COMMUNITY attribute of BGP4 as a way (in conjunction with my own
> LOCAL_PREF) of load balancing
> traffic between my two internet links (both T3). After reading CISCO
> documentation I am still not
> clear on some issues:
>
> 1. Is COMMUNITY a transitive attribute only between me and my immediate
> upstream supplier or
> is it being propagated further into Internet (so I can influence how
> somebody ,say, 5 AS hops
> away from me sees my routes) ?

It is propagated further into the Internet.  Note that only BGP routers
which are configured with specific policies will pay any attention to
any communities you set, other than the well-known communities.  Unless
an ISP decides to set up its policies to handle communities, you can't
use communities to influence how it sees your routes.
> 2. If COMMUNITY propagates into big I, and I set COMMUNITY to 3561:70
> (for MCI to set
> LOCAL_PREF on my routes to 70) and my next hop supplier sets COMMUNITY
> of my routes to
> 3561:80, what happens ? what MCI is going to do ?

MCI will receive and act on 3561:80
> 3. Is COMMUNITY a CISCO only or is it part of RFC ? Has any other router
> vendor implemented
> this parameter ?

Bay Networks also supports Communities, including the well-known
communities:
      NO_EXPORT (0xFFFFFF01)
         All routes received carrying a communities attribute containing
         this value MUST NOT be advertised outside a BGP confederation
         boundary (a stand-alone autonomous system that is not part of a
         confederation should be considered a confederation itself).
      NO_ADVERTISE (0xFFFFFF02)
         All routes received carrying a communities attribute containing
         this value MUST NOT be advertised to other BGP peers.
      NO_EXPORT_SUBCONFED (0xFFFFFF03)
         All routes received carrying a communities attribute containing
         this value MUST NOT be advertised to external BGP peers (this

> 4. Which major Internet providers have COMMUNITY/LOCAL_PREF implemented
>  (I am aware of MCI and Sprint) ?

> 5. Your experiences, comments, suggestions .....
>
> Thanks in advance, Walter
>
> Walter Towbin
> Telus Advanced Communications
> phone: 403-543-2032, fax: 403-543-2030, cell: 403-620-0019
> walter.towbin () telus com

-- 
Paul Knight                          mailto:pknight () BayNetworks com
IP Engineering, Systems Test         Office: (508) 916-7087
Bay Networks, Inc.      M/S BL2-02      Lab: (508) 670-8888, x-65404
2 Federal St., Billerica, MA 01821      Fax: (508) 670-4004
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