BGP Community Attribute

Community attribute is one of those non-standard BGP attributes that you really need to know well if you wish to use it. The big advantage is that from time to time you will just swoop in and solve a big architecture problem your colleague Network Engineers are having. The down side is that it's a bit tacky. For example, these are the communities you can set within the route-map configuration>

Cisqueros_R1(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) <--- ADVERTISE these networks
  local-AS        Do not send outside local AS (well-known community) <--- ONLY advertise within the AS
  no-advertise    Do not advertise to any peer (well-known community) <--- The neighbor will not advertise what was matched to neither eBGP nor iBGP neighbors
  no-export       Do not export to next AS (well-known community) <--- Like previous, but ONLY for eBGP
  none            No community attribute
  <cr>


*IMPORTANT: Do not forget to actually SEND the community to the neighbor, otherwise your configuration will not work!!!

 (config-router)#neighbor x.x.x.x send-community

You can of course apply the BGP community attributes on the INBOUND and OUTBOUND direction, where you automatically override the existing value.

Besides these well-known community values, you can also assign a random community number and use them later as BGP TAGS.