BGP4 COMMUNITY attribute

John W. Stewart III jstewart at
Thu Mar 27 18:32:38 UTC 1997

many of your questions are for MCI and not nanog

 > 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) ?

the attribute is defined as transitive (i.e., once associated
with a route it *stays* associated with the route).  however, in
practice, many providers are configured to not send communities
to other providers

 > 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 ?

is one of your immediate upstream provider's MCI?  if so, then
you would tag routes with 3561:70 and MCI would immediately see
and react to that.  however, you imply that your "next hop
supplier" isn't MCI, which makes it a bit odd for you to be
sending communities for MCI, but is doable if your "next hop
supplier" agrees to carry your communities all the way to MCI

 > 3. Is COMMUNITY a CISCO only or is it part of RFC ? Has any other router
 > vendor implemented
 > this parameter ?

rfc1997.  i think there's a GateD with communities


 > 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 at

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