North American Network Operators Group

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Re: Communities BCP [was: RE: BGP Path Filtering]

  • From: Joe Abley
  • Date: Fri May 16 15:21:55 2003

On Friday, May 16, 2003, at 14:50 Canada/Eastern, Deepak Jain wrote:

The question is that changing your communities down the road might be a big
headache depending on whether you'll come to need more advanced ways of
filtering your announcements than coarse communities allow. Especially when
going (through growth or acquisition) from a single metro network to a
multi-metro or national/international one.

What criteria does the community think should be used when specifying
communities. IMO, overspecificity [sp?] doesn't hurt, but others may (and
have) disagreed. What's the BCP?
I don't know that this is "best", but I like it:

+ decide on a set of characteristics that will help make routing policy decisions today
+ allocate one unique 16-bit number to each of those characteristics (N)
+ apply corresponding community string attributes (THIS_AS:N) to routes in sensible places
+ configure your route export policy based on community string attributes present on routes
+ worry about other characteristics as and when they become useful/necessary, and not before

Examples of characteristics I have seen in the wild are:

+ I was learnt from a peer
+ I was learnt from a transit provider
+ I was learnt from a customer
+ I was learnt over an exchange point
+ I was learnt over a private peering connection

These are all markers to be used in setting internal policy, so the choice of numbers really doesn't matter. You can add or remove characteristics from your list when necessary without having to renumber anything. You can (and probably should) strip all these attributes before sending routes to EBGP peers, so nobody else has to see them.

Specifying a list of characteristics that seem like they might one day be useful but which today do nothing to influence routing policy seems like a waste of effort. People might better spend their time checking that all their customer sessions have accurate import prefix filters.