BGP Attribute - COMMUNITY

COMMUNITY is a optional transitive attribute. Even if community is not recognized by router it is propagated to other neighbors.

1) Format :  There are three format of community (in Cisco IOS 12.0 and later) :
 

• Decimal, range of0 to 4,294,967,200. Range of 0 to 65535 and 4294901760 to 4294967295 are reserved
• Hexadecimal
• New AA:NN format (AS number, 2-byte number). 

By default IOS uses decimal format. Nowadays most popular is AA:NN format, to use it you need issue the ip bgp-community new-format global configuration command.

2) Well known communites : 

• INTERNET : all routes belongs to this community by default, doesn't have a value
• NO_EXPORT (4294967041 or 0xFFFFFF01) : routes carrying this value cannot be advertised to EBGP peers or outside of the confederation
• NO_ADVERTISE (4294967042 or 0xFFFFFF02) : routes carrying this values cannot be advertised at all to either EBGP or IBGP peers
• LOCAL_AS (4294967043 or 0FFFFFF03) : routes received carrying this value cannot be advertised to EBGP oeers including peers in other AS within a confederation

3) Example of configuration : 

To set the community attributes you can use route maps :

• set community community-number [additive] [well-known-community] 

• route-map test

  match ip address 1

  set community no-export

  route-map test2

  match as-path 1

  set community 100 additive