Hi Prabu,
Standard communities in fact do not influence the BGP best path algorithm at all. They constitute only numbers attached to prefixes, and these numbers allow you, the administrator, to perform filtering or more customized modification of other BGP attributes on paths with selected community attribute values. There are certain selected standard community values that have impact on the depth of route propagation, such as no-export (prevent the route from being advertised over eBGP), no-advertise (prevent the route from being advertised to anyone), local-as (prevent the route from being advertised to another sub-AS in a confederation), but these do not really affect the best path selection.
http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/26634-bgp-toc.html#communityattribute
Extended communities are almost similar; however, the situation is slightly more complicated. There is a set of extened communities whose values can influence the best path algorithm by inserting themselves at certain selected steps, or points, of the best path algorithm. This modification to BGP is not yet official, however, Cisco does implement it, for example, in MPLS VPNs. The Internet Draft describing this extension is called BGP Custom Decision Process and is accessible here:
http://tools.ietf.org/html/draft-ietf-idr-custom-decision-04
In most situations, however, the communities are used to perform filtering or subsequent attribute modification but they do not themselves modify the best path selection process.
Best regards,
Peter
