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BGP Communities Part 1 – Well known Communities

As mentioned by Jeff Doyle in his iconic “TCPIP volume 2” book – communities allows you to apply policies to a group of routes. This route group can be from a peer, from a AS etc. Communities travel with route and passed from peer to peer. Community is a 32-bit number and ranges from 1-4294967200 or AA:NN where AA is 16 bit number and NN is arbitrary value. Remember both formats are okay and cisco accepts both forms ( there is third format also that is in hexadecimal but it is used rarely). Cisco by default shows community value in decimal format but you can change it to AA:NN format by using “ip bgp-community new-format”. 

Now communities can be user defined or standard (Also known as well known communities). Well known communities are as follows :- 

No-Advertise ( 65535:65281)
The No_Advertise community (0xFFFFFF02 or 4,294,967,042) specifies that routes with this community should not be advertised to any BGP peer. The No-Advertise BGP community can be advertised from an upstream BGP peer or locally with an inbound BGP policy. In either method, the No-Advertise community is set in the BGP Loc-RIB table that affects outbound route advertisement.

No-Export ( 65535:65282)
The No_Export community (0xFFFFFF01 or 4,294,967,041) specifies that when a route is received with this community, the route is not advertised to any EBGP peer. If the router receiving the No-Export route is a confederation member, the route can be advertised to other sub-ASs in the confederation.
Notice that the R3 and R4 displays “not advertised to EBGP peer.” R3 is able to advertise the network to R4 (via Update Group #3) because they are members of the same confederation, even though their ASNs are different.

Local-AS (No Export SubConfed)( 65535:65283)
The No_Export_SubConfed community (0xFFFFFF03 or 4,294,967,043) known as the Local-AS community specifies that a route with this community is not advertised outside of the local AS. If the router receiving a route with the Local-AS community is a confederation member, the route can be advertised only within the sub-AS (Member-AS) and is not advertised between Member-ASs.

The fourth well known community is INTERNET (0x0 or 0:0) – This enables router to advertise the route to everyone. It is permit all of BGP communities.

Well that was all about the well known communities. We will study about the extended communities as well as communities list (standard list and expanded list) in BGP Communities Part 2 article…

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