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BGP Communities – Lets have a look

BGP communities are the nothing but a method of tagging. Tagging can help in path manipulations and filtering. Tagging can be done on all prefixes in a BGP table. BGP communities are OPTIONAL TRANSITIVE path attribute type.

  • BGP community is 32 bit no
  • Its format used in as AS NO:NUMBER (example 100:10. 200:2)
  • 64 bit long community is known as extended community.
  • for IPv6 we have 160 bit long community no.
  1. NO EXPORT – When this community is applied, BGP updates are sent to only iBGP peers. Will not send to EBGP peers.
  2. NO ADVERTIZE – In this updates are not sent to EBGP and iBGP peers.
  3. INTERNET – This community is for sending updates to all EBGP and iBGP peers.
  4. LOCAL AS – This community is used in BGP confederation. Due to this updates are sent to iBGP peers but not EBGP peer within a confederation.

Own communities are defined by user. There are two methods of tagging :-
1. Old method – In this method communities values were single block. Example – 100, 200, 201 were community tags
2. New method – This method is used now a days. In this method communities are defined by mixing AS NO and TAG. Example – 100:10, 200:20 etc.

So we have talked about the community and how to define the communities. But how do we utilize them :-

We use COMMUNITY LIST (in same context as we use prefix list) for communities. It works same as prefix list or access list but identifies on the basis of communities(or TAG).
We then call these communities in route maps for further manipulations.
There are two types of Community list :-
1. Standard – 1-99
2. Extended – 100-500

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