BGP Help

Unanswered Question
Apr 22nd, 2010


Can someone explain to me what's going on in this config in regards to Send-Community, route maps, and BGP in general?


router bgp xxxx
no synchronization
bgp router-id xxxx
bgp log-neighbor-changes
network mask
timers bgp 15 45
neighbor remote-as xxxx
neighbor next-hop-self
ip bgp-community new-format
ip http server
ip http authentication local
ip http secure-server

I have this problem too.
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Giuseppe Larosa Thu, 04/22/2010 - 13:11

Hello Isalem,

the neigh send-community allows to send the community BGP attribute to the peer

the route-map that is applied to the neighbor allows for route filtering and for setting specific BGP attributes like BGP community at the same time.

IP prefixes matching first prefix-list are sent out with BGP community attribute 13979:110, IP prefixes matching second prefix-list are sent out with BGP community 13979:90

if there are other IP prefixes in BGP table they are advertised with bo BGP community settings for the presence of  the line:

route-map AVPN-OUTBOUND permit 30

the command

ip bgp-community new-format

is very useful to see BGP communities in the format ASN:value otherwise you would see a big number in show commands

BGP advertises IP networks if they are known in IP routing table:

the various network commands can create a BGP advertisement if an exact match is found (including the mask) in the IP routing table.

Hope to help


isalem Thu, 04/22/2010 - 13:21

Thank you, I am somewhat used to the use of AS Path, but kind of new to the send community.

So what would be the difference when you send the community with a value of 90 vs 110.  What would that do for the routing?


milan.kulik Thu, 04/22/2010 - 13:34


you can use a community to tell your neighbor (provider) which Local Preference you would like him to assign to the prefix you sent him, e.g.

See RFC 1998.



Giuseppe Larosa Thu, 04/22/2010 - 13:38

Hello Isalem,

the BGP community is a sort of tag of colour that you can give to a group of routes.

The advantage of the BGP community is that the provider that receives the BGP routes can use the BGP community as a criteria to decide some actions without having to deal with IP prefixes.

One community can mean national users or base users another community can be premium users or international users and so on.

it provides a level of abstraction to providers: if a route should be moved from prefix-list 1 to prefix-list2 or a new route has to be sent, only the customer changes its configuration the provider using BGP communities is able to dynamically adapt to the change

Hope to help



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