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BGP Communities


  • BGP communities are optional transitive attributes
  • They are used mainly to associate an administrative tag to a BGP route
  • In spite of the communities being transitive, Cisco IOS routers do not pass them across the BGP sessions by default. The feature needs to be activated using the command “neighbor send-community”
  • To display the community in the structured notation, you need to enter the global configuration command “ip bgp-community new-format”
  • In order to set a community value, use the route-map command “ set community etc” or to add the community value on the existing community values use “set community additive ”
  • The routes with communities assigned to them can be matched using “community-list” and which can then be referenced in a route-map later. There are two types of community lists : standard and extended. For eg.
  • ip community-list 1 permit 101:1 101:2
  • ip community list 1 deny no-export Std community list
  • NO-ADVERTISE community attribute: The well-known NO_ADVERTISE BGP community signals BGP router not to advertise the particular prefix to any BGP peer.
  • NO_EXPORT community attribute: The well-known NO_EXPORT community instructs the BGP router to advertise the prefix only across iBGP peering links. It ensures that the prefix is not advertised anywhere outside its AS

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