Using the BGP community attribute

Whilst studying for my CCNP I came across the BGP community attribute. This isn’t covered in much detail in the Building Scalable Cisco Internetworks (BSCI) self-study guide, so off I went to google.

I managed to find a couple of sites that had some information on this subject.

The first site, Cisco IOS Hints and Tricks, has some good background information, but not much detail on how to configure this.

The second site,, has a example of using communities to influence an ISP’s routing decisions. This is the one I found most useful. The example focuses on a customer that has a multi-homed connection to a single ISP. This ISP publishes a list of communties, that can be used to affect the connection used to connect to routes advertised from the customers network.

BGP communites in-effect tag a prefix (or set of prefixes). Once these prefixes are tagged, operations can be performed on that group of prefixes, rather than having to identifiy each prefix. Route Maps are heavily used in BGP communites and are used to both classify the prefixes and to set what actions occur.

One use for a BGP Community could be to determine which prefixes are not to be advertised out of your AS – Autonomous System. All internal networks could be tagged with a certain community at the point they are distributed into the BGP AS if they are to remain private. On your exit routers you could then have a route filter that will stop any prefixes that are tagged with the specified community from being exported out of the AS.

Hopefully this makes sense, but if not then please leave your comments and I will try and answer them.

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