Communities BCP [was: RE: BGP Path Filtering]

Stephen J. Wilcox steve at
Fri May 16 23:28:43 UTC 2003

On Fri, 16 May 2003, Guy Tal wrote:
> On Fri, 16 May 2003, Stephen J. Wilcox wrote:
> > On Fri, 16 May 2003, Danny McPherson wrote:
> > > On 5/16/03 1:26 PM, "Jay Ford" <jay-ford at> wrote:
> > >
> > > > You can & definitely should strip those community values on announcements you
> > > > receive from EBGP peers.  Interesting things happen if you let others turn
> > > > your routing policy knobs when you think they can't reach them.\
> > >
> > > Agreed.
> > >
> > > Likewise, when you receive communities (and MEDs, ugh!) [if possible] you
> > > should reset them (v. employing 'additive').  Unnecessary propagation and/or
> >
> > Hmm.. assuming you're interconnecting at multiple points with the same ASN they
> > will probably want to indicate to you where to send traffic to them using MEDs,
> > if you go stripping them out you lose that info, check the peering policy.. this
> > may put you in breach.
> >
> > Even without a breach of policy the MED will help find the best path to a route
> > thats identical at two points and if you take it out you lose that.
> All a matter of perspective. I would assume most people want to use
> closest exit on their networks. Imagine how nice it would be if you could
> hand off traffic to a peer that is your closest exit, then force them to
> carry the traffic back to you at that same point from across the country
> or across the globe!

Okay perhaps I didnt add enough detail, I'm not suggesting that you should 
accept them without sanity but within a city or region it helps to know where 
the best exit point is for that route.

In my own situation I peer with some networks several times within the same 
building and its nice for them to hint to me where they want my traffic and vice 
versa, if I want to distribute load I send my hints to them.

> > Altho I'd agree if you'd say to clear the communities and MEDs on egress, thats
> > an okay thing to do.
> Spank me if I'm wrong here, but you don't pass MEDs on transitively
> anyway. And if you strip communities off, you limit your
> peers/customers/providers from being able to use the same control that you
> are asking for.

Hmm is the first bit optional? ;) Okay again I perhaps wasnt specific enough, 
I'm suggesting the MEDs and communities are useful between two peers who may be 
peering at multiple points, so this is engineering choices when the two paths 
are identical and all you have is the entry/exit point to choose.


> Guy Tal

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