From: Todd, Douglas M. (DTODD@PARTNERS.ORG)
Date: Mon May 05 2008 - 17:46:00 ART
All depends on what you are trying to accomplish:
Transitive/non-transitive - well-known/not-well-known - Internet Architectures
(cisco press) goes through all this very well. The ultimate bgp book.
Thoughts:
Typically you will see senerios like, dont all this route to export from your
domain. Do not use filter-lists, prefix-lists, to accomplish this task. Or don't
be a transit for your ISP. Here you might put the no-export on your routes
(inbound on ISP routes, outbound for other routes). There for your other ibgp
routers will note export those routes outside your domain.
no-advertise - means do not advertise this to any other peer. If used inbound,
routes are not advertised to any peer. Outbound, peer/s do not advertise those
routes to anyone else.. no-export is somewhat the same, but the radius of the
route is much less. Ie., no-export can be multiple hops within YOUR AS.
no-advertise is next hop only (depending on in vs out appliacation). (more can
be added here,but you get the idea).
Local-as: kind of combines no-advertise an no-export to some degree. Routes do
not leave your as (works in confederations issues ). Routes float around in
your AS only. Do not leave you AS.
Additive: keeps the original community, but "ORS" the additional. Basically, if
no additive the inbound community is over written with what you set.
AA:NN(New format) - great for tagging routes for policies. Look at this as a
route tag for filtering etc.
Dont forget to use the send communty on your neighbors.
Internet - well... not much other use....
None - null out any inbound community seen...
Here is the basics for setting the community, replace with a match if you need
to match what you set.
(config-route-map)#set community ?
<1-4294967295> community number
aa:nn community number in aa:nn format
additive Add to the existing community
internet Internet (well-known community)
local-AS Do not send outside local AS (well-known community)
no-advertise Do not advertise to any peer (well-known community)
no-export Do not export to next AS (well-known community)
none No community attribute
please correct/add where necessary...
Thanks
DMT
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of Ashwani
Sent: Monday, May 05, 2008 4:07 PM
To: Cisco certification
Subject: Help understanding BGP community
Hello Experts,
Can any one please explain how to use BGP community? I know the basic
understanding of community to influence inbound traffic , why cant we use MED or
AS_ PATH prepend to do that? What's advantages or disadvantages of one over
another?
Any help with example will be highly appreciate.
Thanks,
Ashwani
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