Re: BGP Path Selection weirdness regarding next hops

Hi Marko, Brian

How does a BGP neighbor is formed by a default route. Two routers cannot
be neighbors if the only route to reach the neighbor is a default route.
the minimum prefix to reach teh neighbor should be /1 and max is /32 and i
have tested it several times.

How will the BGP decide that its neighbor went down, by just seeing whether
it has a route to reach the neighbor with atelast /1 route to it. But in
this case, the next hop to reach the neighbor is not reachable,
So in this case , it should check for any other path to reach the neighbor
and why it is searching for a another path for the next hop to reach the
neighbor ..

It should check for the another path for the neighbor address and not the
next hop which is used previously to reach the neighbor. In ALU box and
MX960, it works this way and why not it is not working this way in Cisco.

Correct me if i am wrong in this logic

On Sat, Dec 1, 2012 at 8:07 AM, Marko Milivojevic <markom_at_ipexpert.com>wrote:

> Without going any deeper (some topology information is missing and m
> pod is otherwise busy to try this, no matter how FUN it sounds), I'd
> venture a guess that yes, "igp" metric is compared.
>
> The "igp metric" in this sense is really "the metric to reach the
> protocol, no matter what that protocol might be". In your case, one of
> these protocols happens to be BGP. You may want to test this hypotesis
> by tweaking the BGP's MED value for the default route to make it
> numerically higher than OSPF cost to reach the next-hop of the other
> route.
>
> Funnily enough, this is one of the few places where numerical metric
> values of different protocols are directly compared, regardless of the
> AD and/or longest-match.
>
> --
> Marko Milivojevic - CCIE #18427 (SP R&S)
> Senior CCIE Instructor - IPexpert
>
> On Fri, Nov 30, 2012 at 6:21 PM, John Neiberger <jneiberger_at_gmail.com>
> wrote:
> > I posted this question to the Cisco NSP list and I've also talked to a
> > couple of guys from Cisco Advanced Services and I'm still stumped about
> > something. I'll try my best to phrase it in a way that makes sense.
> >
> > Router A is learning about a prefix from two route reflector clients. In
> > both cases, the next hop for the prefix is the loopback address of the
> > advertising routers. Their loopback addresses are being advertised into
> > OSPF.
> >
> > So, from the perspective of Router A, it's BGP table for this prefix has
> > two paths:
> >
> > 1: 4.4.4.4 (loopback address of Router B, learned via OSPF) * winner due
> > to lower IGP metric
> > 2. 5.5.5.5 (loopback address of Router C, learned via OSPF)
> >
> > Now for the weirdness to begin. A network event occurs that causes the
> > loopback address of Router C to go away. This shouldn't affect Router A
> > because it is already selecting the shortest path to the network via
> Router
> > B (4.4.4.4).
> >
> > However, Router A is also learning a default via BGP. That means that
> even
> > though 5.5.5.5 (loopback of Router C) disappeared and is unreachable, the
> > router is doing a recursive lookup and keeps the path in the BGP table;
> > 5.5.5.5 is still reachable, it thinks, by using the default route.
> >
> > The weird thing is that this causes Router A to start using the wrong
> path!
> > It seems to be preferring a path with a next hop learned via BGP to a
> path
> > with a next hop learned via OSPF. Why would it do this? I see no
> > documentation that would explain why a BGP-learned next hop is preferred
> > over an IGP-learned next hop.
> >
> > Is the router still comparing IGP metrics even though the "wrong" path
> now
> > has no IGP metric?
> >
> > It's not changing due to router ID, cluster length, or neighbor IP
> address.
> > I checked. So, why is it switching?
> >
> > As soon as the BGP session from Router A to Router C times out, the
> > extraneous path gets removed from the BGP table and the router goes back
> to
> > using the correct path it should have been using all along.
> >
> > So, is a BGP-learned next hop preferred over an IGP-learned next hop? If
> > so, why? If not, any idea why my router switches paths? I've turned on
> BGP
> > debugging and IP routing debugging and haven't found a suitable
> explanation
> > for the switch.
> >
> > John
> >
> >
> > Blogs and organic groups at http://www.ccie.net
> >
> > _______________________________________________________________________
> > Subscription information may be found at:
> > http://www.groupstudy.com/list/CCIELab.html
>
>
> Blogs and organic groups at http://www.ccie.net
>
> _______________________________________________________________________
> Subscription information may be found at:
> http://www.groupstudy.com/list/CCIELab.html

Blogs and organic groups at http://www.ccie.net
Received on Mon Dec 03 2012 - 21:54:22 ART