3.7.d Implement, optimize and troubleshoot routing policies

3.7.d [iv] Communities, extended communities

A community is a BGP attribute that may be added to each prefix. Communities are transitive optional attributes, meaning BGP implementations do not have to recognize the attribute and at the network operator’s discretion carry it through an AS or pass it on to another AS. The community attribute can be thought of as simply a flat , 32-bit value that can be applied to any set of prefixes. It can be read as a 32-bit value or split into two portions, the first 2 bytes representing an ASN and the last 2 bytes as a value with a predetermined meaning. The values 0x00000000 through 0x0000FFFF and 0xFFFF0000 through 0xFFFFFFFF are reserved. Most modern router software displays communities as ASN:VALUE. In this format the communities 1: 0 through 65534: 65535 are available for use. The convention is to use the ASN of your own network as the leading 16 bits for your internal communities and communities that you accept from and send to your customers. When OSPF is used as PE-CE routing protocol, BGP uses extended communities to convey various OSPF attributes however there are a few exceptions (e.g. network type) to what attributes are conveyed.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 3997-3998).  . Kindle Edition.