Communities

E.B. Dreger eddy+public+spam at noc.everquick.net
Mon Oct 15 14:49:31 UTC 2001


> Date: Mon, 15 Oct 2001 11:55:27 +0200 (CEST)
> From: Iljitsch van Beijnum <iljitsch at muada.com>



> I'm looking for information about the way networks use
> communities in BGP.
> 
> It seems that many of the larger networks only use communities
> to supply their customers with a mechanism to adjust the local
> preference to indicate which connection is preferred when a
> customer connects over more than one link (something that can
> also be done with the MED).

Remember that local-pref has higher priority than as-path
length; MED is the lowest priority before router ID.

For instance, I match "_asnthatIdontlike_" and penalize
local-pref to [try to] avoid routing traffic over an ASN that
I think has poor performance.  If I penalize AS65000, then

	me 3549 65500 65432 65432 65432 65123

will be preferred over

	me 6347 65000 65123

This is one reason that redistributing one's upstream routes via
BGP can be bad despite as-path length:  If someone uses
local-pref, it's quite conceivable that one will take the
erroneous path that some edge idiot[1] leaked into the table.

[1] I'm an edge-dweller.  I can insult them.  Note, however, that
upstreams _should_ filter their downstreams to prevent improper
adverts... but the root of the problem is the one at the edge.



[ snip ]

> And how about things like congestion?

How do you mean?



> Is there any need for more "well known" communities?

I wish that providers would set a community indicating route
ingress.  I know, for instance, that GBLX does this... but their
system with hundreds of communites leaves some to be desired,
IMHO.

I'd like to see providers tag "route learned in this region" at
various granularity levels.

As for providers listening to communities, I like selective
as-path padding... I'd have to dig up the thread, but this has
been discussed in the past few months.



Eddy

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