Henry,
I don't think it is IOS incompatibilities per se causing your problem for
simple BGP configurations such as the one you describe, although there are
some inconsistencies in how BGP handles your network statements according
to IOS version. It is difficult to tell what's going on without configs
and a little more explanation, but here are some things to check in order
to troubleshoot your BGP connectivity:
1. Is your next-hop address reachable? In the router you're trying to
ping from, do "show ip bgp" and locate the destination network entry for
the address you're trying to ping. Find the next-hop IP address listed for
this entry. Now, do "show ip route <next-hop>" using the next-hop for your
destination that you just found. This will tell you if this router knows
how to get to that address. If it doesn't, you won't find the entry in
your IP routing table even if your router is learning of it from BGP, and
you won't be able to ping it. Your router MUST know how to get to that next
hop FROM SOME OTHER MEANS THAN BGP.
2. Synchronization may be keeping the routes from being advertised to an
external BGP neighbor if the router has learned about them from an internal
BGP neighbor. If all the routers in your AS are running BGP, then you can
turn synchronization off on all your routers. That might make those routes
that are in your BGP table pop into your IP routing table.
3. If your routes are not even getting into the BGP table on the router
where your network statements are configured, you need to check those
statements. One thing that may cause you trouble with IOS versions older
than 12.0 is the BGP auto-summarization feature. By default,
auto-summarization is enabled for those versions.
The way auto-summarization will affect you is that if you enter a BGP
network statement with a classful network number and no mask (e.g.,
"network 10.0.0.0"), the BGP router will pick up and advertise a BGP prefix
entry for the classful network number (10.0.0.0/8) if it finds any subnets
of it in your IP routing table, but will not advertise the individual
subnets. This is the opposite of the way your 12.0 IOS versions will
behave--newer IOS versions (at least the 12.0 IOS images I've had
experience with over the last 2 years) will pick up the subnets of any
classful network statement put them in the BGP table, but will not create
the summary. This despite that the 12.0 documentation still asserts that
BGP auto-summary is on by default.
To make the 11.x routers advertise your subnets for you, you will need to
configure a separate network statement for each subnet you want advertised,
with the exact mask that corresponds to its prefix length as it appears in
the IP routing table. As an alternative, you might want to try, "no
auto-summary" in your configurations, but I have had inconsistent results
with it. Just to make it more complicated, I have seen routers with IOS
versions earlier than 11.2 refuse even to advertise the summary if one of
the subnets exists, with BGP auto-summarization enabled. Can't say whether
that was a bug or not.
With respect to pinging, remember that you not only have to see a route in
your routing table for the destination, but all the routers along the path
have to have a way to get back to the router you're pinging from in THEIR
routing tables as well.
Pamela
At 12:59 AM 12/31/00 -0700, henryguzman wrote:
>Hello All,
> I just started doing CCNP labs at home and am having some problems
>getting BGP to work correctly. My home lab consists of 2 Cisco 3000 series
>routers with v12.0 IOS, two 2501's (borrowed) with version 11.0, and one
>Cisco Access Pro router card with version 11.3. My question is could the mix
>and match nature of the different IOS's in my home lab cause problems with
>BGP syncing up correctly? For instance, in one lab I did with 3 routers, all
>the routers "found" each others routes via BGP but I could not ping end to
>end thru the middle router. In other words if the routes are seen in the
>routing table, why can't I ping? Yes I checked clocking and the ports are
>all up & up. Or what's going on if routes don't show up and I've correctly
>followed the instructions for entering the neighbor and network statements?
>I can ping from point to point. BTW, the labs I'm doing are from The Cisco
>Networking Academy Level 5 Chapter 8.
> Is it just me messing up somewhere, or should I get all the routers
>talking the same IOS? Problem is, I don't have a login at Cisco's web site
>that will enable me to download an IOS image. Suggestions anyone?
>
>Thanks for your time and input.
> Happy New Year!
>
>Henry Guzman
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