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  • 1.  BGP Extended Community

    Posted 07-23-2011 00:39

    Hi All

     

    Can any one explain:

     

    1- What is the difference between normal BGP community and BGP extended community

    2- In which scenario we need to define these communities

    3- When we define BGP comminity there is option invert-match. What is this exactly?

     

    Thanks



  • 2.  RE: BGP Extended Community
    Best Answer

    Posted 07-23-2011 01:30

    Hello,

     

    1- What is the difference between normal BGP community and BGP extended community 

    The standard community has 32 bits and is represented in the following format ASN:VALUE.The values 0x00000000 through 0x0000FFFF and 0xFFFF0000 through 0xFFFFFFFF are reserved. RFC 1997 defines 3 standard communities: NO-EXPORT (0xFFFFFF01 ), NO-ADVERTISE (0xFFFFFF02 ), NO-ADVERTISE-SUBCONFED (0xFFFFFF03 )

    Extended community are 64 bits long. Example of Extended communities are: "Route Target Community", "Link Bandwidth Community".

    2- In which scenario we need to define these communities

    Communities may be added to BGP prefixes and based on received community you can apply desired policy, for example to filter them or to change some BGP attributes etc.

    "Route Target" extended community is used in MP-BGP for L3VPNs as you know.

    "Link Bandwidth" extended community is used for unequal load sharing of traffic between BGP peers.

    3- When we define BGP community there is option invert-match. What is this exactly?

    "community" command actually defines a regular expression and if it matches the community attribute of received prefix it returns the "True" result, if not then "False". "invert-match" makes contrary, if there's a match it returns "False" otherwise "True".

     



  • 3.  RE: BGP Extended Community

    Posted 07-23-2011 07:34

    Thanks Dear. My second question was why extended community is needed? In which scenario normal community would be used for route tagging and in which scenario extended BGP community would be used?

     

    thanks



  • 4.  RE: BGP Extended Community

    Posted 07-24-2011 00:08

    Hi,

     

    Usually normal communities are used for the sake of BGP policy application, for instance take a look at the following link:

    http://onesc.net/communities/as701/

    Here, based on received community Verizon's BGP routers can alter some BGP attributes and those change the traffic flow...

    Extended communities are a special case, they are mostly used in MPLS VPN environment, for example: RT, SOO