From: George Spahl (g.spahl@xxxxxxxxxxxxx)
Date: Mon May 20 2002 - 21:32:57 GMT-3
Jeongwoo,
I guess it's kind of a minor point, but I was really trying to find out
what the standard practice was on the Internet for forwarding
communities within an AS; specifically a transit AS.
For example, what if you in AS1 want all of AS2 to have the route(s)you
send, but not other ASs that are connected to AS2? You would send it
from AS1 to AS2 with "no export" and the receiving router in AS2 would
share the route(s) with its IBGP peers, but it won't forward the
"no-export" attribute on to them unless it has been explicitly
configured to do so. Since they no longer have the "no-export"
attribute there's nothing to keep them from forwarding the route(s) on
to another AS.
It just seems kind of odd that whether or not the "no-export" works
would depend on how the AS2 peers have or haven't been configured. I
had always just assumed that the community attributes would be
automatically forwarded to the IBGP peers in an AS. Apparently they
aren't, which led me to wonder if it was standard practice on the
Internet to configure the transit ASs so that the communities would be
forwarded along to the other peers within an AS. In other words, is
that something you could pretty much count on as you send out a route
with "no export" to a transit AS?
Thanks,
George
-----Original Message-----
From: Jeongwoo Park [mailto:jpark@wams.com]
Sent: Monday, May 20, 2002 7:27 PM
To: 'George Spahl'; ccielab@groupstudy.com
Subject: RE: BGP Communities - somewhat long, confusing
George,
When sending router A in AS 1 sends a route with community attribute
(no-export) attached, the receiving router in AS 2 will share that route
with its peer routers within the AS 2.
If you don't want that route to be shared with its peer routers, the
sending
router should send the router with community attribute (no-advertise)
attached.
JP
-----Original Message-----
From: George Spahl [mailto:g.spahl@insightbb.com]
Sent: Monday, May 20, 2002 3:14 PM
To: ccielab@groupstudy.com
Subject: BGP Communities - somewhat long, confusing
Thought I would send this out one more time now that everyone's back
from the weekend. What I'm trying to figure out is whether or not the
"no export" community relies on a transit AS sending the community to
its IBGP peers in order to prevent the route(s) from being advertised
outside that AS. Maybe I'm just missing something fundamental about how
it all works...
****************************************************
Greetings,
It seems that if a route enters an AS with the community attribute
attached (set to "no-export" for example), then the EBGP peer that
receives the route will not advertise this route to any other EBGP peer.
However, he is not obliged to send the community attribute to his IBGP
peers along with the route. Instead the "neighbor send community" must
be configured for any IBGP peer of his that needs to see the community
attribute. Is that correct so far?
If so, then whenever I send a route to an AS with "no-export" set,
expecting that my route won't be advertised beyond that AS, I'm very
much relying on the assumption that the peers within the receiving AS
have been configured to send the community attributes on to their
internal peers, especially those which have EBGP connections to other
ASs. This way those peers would know that my route should not be
advertised to any other AS. Otherwise, I suppose, the first peer upon
receiving the route would send the route along to his IBGP peers without
the "no-export" attached and they would think it's OK to advertise to
other ASs. Is this also correct?
If this is actually how it works then is it common practice on the
Internet to configure the BGP peers within a transit AS so that the
community attributes are forwarded on to each other? Corrections,
clarifications, comments anyone?
Thanks,
George
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