[30673] in North American Network Operators' Group
Re: Community NO-EXPORT
daemon@ATHENA.MIT.EDU (Brett Frankenberger)
Tue Aug 22 20:24:44 2000
Message-Id: <200008222336.SAA06706@rbfux.rbfnet.com>
To: dcahill@salesforce.com (Dave Cahill)
Date: Tue, 22 Aug 2000 18:36:37 -0500 (CDT)
From: "Brett Frankenberger" <rbf@rbfnet.com>
Cc: nanog@merit.edu
In-Reply-To: <1AAE1D2D1003D311882400902771AA470112E9F1@mail.salesforce.com> from "Dave Cahill" at Aug 22, 2000 08:12:34 AM
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Errors-To: owner-nanog-outgoing@merit.edu
>
> |-------------------| |--------|
> | |---------| |-----OTHERS
> | AS1 | | AS2 |
> | 192.168.0.0/16 | | |
> | |---------| |-----OTHERS
> |-------------------| |--------|
>
> I advertise my class B range from AS1 for example 192.168.0.0/16 on
> both routers but to achieve some load-sharing I addvertise
> 192.168.0.0/17 on one router and 192.168.128.0/17 on the other router
> as well.
>
> I setup a community NO-EXPORT for the /17 so AS2 won't advertise them
> out to other AS's.
>
> The question In AS2 BGP selects the best route and /17 should be
> selected because of the longest match rule. BGP places the /17 in
> its routing table. BGP should only UPDATE the other IBGP Peers with
> its routing table entry (IE Each router won't send it's entire BGP
> database). So if the /17 will be sent to other IBGP with the
> community NO-EXPORT how do other AS's learn about the /16 ?
AS2 will have both /17's and the /16 in it's routing table. Those are
three separate destinations. The /16 will never actually be used,
because there's a more specific (one of the /17's) for every possible
address within that /16, but that's irrelevant. There are three
distinct destinations (196.168/16, 192.168/17, 192.168.128/17) and BGP
will (absent some sort of filtering) install the best route to each
such destination in it's table.
Absent filtering, AS2 would send all three routes to it's BGP peers,
but your NO EXPORT community will cause the two /17s to be filtered, so
only the /16 will make it beyond AS2.
-- Brett
