BGP Communities - almost an operational question

Alex Bligh amb at
Sat Aug 29 11:43:06 UTC 1998

I've been trying to find out any logical reason why Ciscos limit me
to 16 communities per route. I'm not asking on cisco-nsp as I want
to know whether other routers suffer this limitation.

Having read and reread rfc1771 and rfc1997, I think the community
attribute list is limited by (a) the message length (not really
relevant) and (b) the attribute length. The latter appears to be
mainly an issue of whether one can be bothered to use extended-length
in lieu of non-extended length. Even without using extended-length
you have 255 bytes to play with which my calculation gives you 61 or
62 community attributes to play with.

a) Have I just read the RFC wrong?
b) Does other s/w (GateD. Bay etc.) have the same limitation?
c) What happens if I try and feed routes with lots of community
   attributes to a Cisco, (perhaps I feed routes with less than
   16 communities on but have set community additive)? Does it
   drop some randomly? Given that some people use 666 style filtering
   to prevent advertisement of routes to peers, could this cause

(yes, I have an application for more than 16 communities per route).

Alex Bligh
GX Networks (formerly Xara Networks)

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