Communities BCP [was: RE: BGP Path Filtering]

Joe Abley jabley at
Fri May 16 19:19:22 UTC 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 
  + 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.


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