BGP Communities
BGP Communities has to be one of my favorite features added to the BGP protocol. As you should know by now, BGP passes several attributes between peers that help influence the BGP best path selection algorithm. One of these is the BGP Community attribute. Think of this as another tag placed on the route advertisement that can give us additional information.
This tag is formatted as 32bit value that is typically displayed in two 16 bit parts. The most typical, and most widely accepted best practices treat these as your Autonomous System Number (ASN), followed by a 16 bit attribute. For example, if your ASN was 65248 and you wanted to tag this route with the number 666, you would set something like 65248:666.
As for the second half of our tag, this number is arbitrary. BGP Communities are a transitive attribute that is completely optional. This means that BGP treats these tags as purely information, and that it is up to the network engineer to decide what these values mean, and what to do with them.
RFC 1997 defines some well known communities that have global significance. These values and their operations should be respected by any community Continue reading