The BGP community attribute is a very powerful tool for assisting and scaling BGP at any level.
Most of the ISPs make extensive use of BGP communities while defining internal policies, Inter-provider relationships and Customer traffic engineering. There is no such standard defined for these communities but still RFC 1998 has community values which defined to have particular meaning:-
• ASx:100 :- This community says set local preference to 100 and make this path as preferred path.
• ASx:90:- This community says set local preference to 90 and make this path as backup if dual homed to ASx.
• ASx:80:- This community says set local preference to 80. This link is to another ISP with same AS path length.
• ASx:70:- This community says set local preference to 70. This link is to another ISP.
These communities are defined by the upstream ISP. All the customers which are homing to ISP, they can attach these communities while advertising the routes.
If upstream ISP is AS 200; To declare a particular path as a backup path, their customer would announce the prefix with community 100:70 to AS200. ISP with AS200 would receive the prefix with the community 100:70 tag, and then set local preference to 70.