From: Adam Quiggle (aquiggle@xxxxxxxxx)
Date: Tue Jan 01 2002 - 22:11:23 GMT-3
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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