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It's a best practice to strip the BGP communities from received prefixes which might have a significance in local administrative domain. What are the reasons to keep the rest of the BGP communities received from the uplinks?

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Communities can be used for all sorts of things. One of them is to label specific prefixes with information, like the location it was learned or the relationship with the network they learned the prefix from.

It can be useful to retain these communities for debugging purposes, and you could even adjust your own routing policies based on them to do route optimization. For example: depref routes learned by my upstream from yet another upstream if they're learned in another continent.

Also, if you have downstream customers, they may be interested in this information to improve their routing policies.

Some public looking glasses (like NLNOG and BGP.tools) collect information on these communities, so they can show them in their output. this LG output is a nice example on how a prefix is tagged by various networks.

Of course, any community you use internally should be stripped on all prefixes learned from external sources.

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