From: Adam Quiggle (aquiggle@xxxxxxxxx)
Date: Tue Jan 01 2002 - 23:31:11 GMT-3
Yup, you are right.....digging too deep. :-)
Thanks,
AQ
At 08:46 PM 1/1/02, Chua, Parry wrote:
>Hi,
>
>I think that the topic is focusing on BUILDING PEERING SESSIONS, it main
>purpose is to show different way of EBGP session as well as IBGP session
>is build using loopback interface on one router.
>
>
>Parry Chua
>
>-----Original Message-----
>From: Adam Quiggle [mailto:aquiggle@nc.rr.com]
>Sent: Wednesday, January 02, 2002 9:11 AM
>To: EA Louie; ccielab@groupstudy.com
>Subject: Re: eBGP multihop & Halabi page 300 (anyone have any better
>ideas?)
>
>
>Louie,
>
>I don't think there is anything misstated in Halabi's book, just a lack
>of clarification. As a theoretical example I understand how running
>OSPF between RTD, RTE and RTF resolves a local reachability problem.
>However, I should have stated that I was looking at this from a global
>reachability perspective.
>
>For example, what if the network 172.31.10.0/24 was hanging off of RTC
>and we advertised it into BGP. Without injecting the BGP routes into
>OSPF
>RTE will never know how to get to 172.31.10.0/24 and subsequently AS2
>will not be able to reach that network even though RTD has it in his
>BGP table.
>
>Maybe this is an obscure point, but given the configs, I think you could
>be mislead to think that you can run eBGP between RTF and RTD without
>having to address RTE and the global reachability problem.
>
>Looks like I was just over-analyzing what was supposed to be a very
>simple configuration.
>
>Thanks,
>AQ
>
>At 04:04 AM 12/31/01, EA Louie wrote:
> >dude, you missed the obvious in this scenario - ospf is running on the
> >192.68 network. read those configs again, and look at the sh ip bgp
>nei on
> >p 307 (2nd edition). obviously RTF found the nexthop to RTD.
> >
> >of course, i'm reading from 2ed of Halabi - check the errata for 1ed of
> >Halabi if that's what you're using.
> >
> >-e-
> >
> >----- Original Message -----
> >From: "Adam Quiggle" <aquiggle@nc.rr.com>
> >To: <ccielab@groupstudy.com>
> >Sent: Sunday, December 30, 2001 11:49 PM
> >Subject: eBGP multihop & Halabi page 300 (anyone have any better
>ideas?)
> >
> >
> > > Hey gang,
> > >
> > > Just fooling around with eBGP multihop on page 300 of Halabi's book.
> >While
> > > looking at Figure 10-1 on page 300 I thought about an interesting
> >situation
> > > that doesn't seem to be discussed in this section (maybe it does and
>I
> >just
> > > haven't gotten to it).
> > >
> > > If you look at the configurations provided on pages 301-305 you will
> >notice
> > > that the configuration of RTE is conspicuously missing. I assume
>that
> >this
> > > was intentional to let the reader stumble across and think through
>this
> > > problem. The most obvious problem is that using the configurations
> > > provided RTD and RTF will not be able to form an eBGP multihop
>session
> > > since neither router has a path to the ip address used to form the
>BGP
> > > session. This can be easily solved by one of two methods:
> > >
> > > (a) static routes on RTF and RTD
> > > (b) configure an IGP between RTF, RTE and RTD, thus providing the
> > > connectivity needed to form the BGP session.
> > >
> > > This all seems simple enough and I'm sure everyone can see that
>solution
> > > (a) would not be allowed on the CCIE lab...anyway.
> > >
> > > Everything seems merry until you start thinking about routing
>packets
> > > between AS1, AS2 and AS3. It seems that RTE would cause a lot of
>problems
> > > very quickly since it does not have a path to any of the networks in
>AS3
> >or
> > > AS1 (assuming that AS2 would have its own IGP that would inform it
>of
> > > routes within its own AS). In addition we can not redistribute BGP
>routes
> > > from AS1 and AS3 into the IGP of AS2 from RTD since that will just
>cause a
> > > routing loop between RTE and RTD.
> > >
> > > It seems to me that in order to solve this problem the routes for
>AS3 and
> > > AS1 must come from RTF. To solve this problem I see one of two
>solutions:
> > >
> > > (a) configure a default route on RTE pointed at RTF and rely on the
>IGP to
> > > inform router RTE about routes within AS2 (this means AS2 could not
>be a
> > > transit AS)
> > > (b) Redistribute BGP on RTF into the IGP running between RTF, RTE
>and RTD.
> > >
> > > Does anyone see another solution to this problem?
> > >
> > > Thanks,
> > > AQ
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