Re: BGP Path Selection weirdness regarding next hops

i see tat. Is tat a fair point wat i have raised ?
wat are ur thoughts on this

On Tue, Dec 4, 2012 at 9:24 PM, Marko Milivojevic <markom_at_ipexpert.com>wrote:

> As I said - only that.
>
> --
> Marko Milivojevic - CCIE #18427 (SP R&S)
> Senior CCIE Instructor - IPexpert
>
> On Tue, Dec 4, 2012 at 6:54 AM, Routing Freak <routingfreak_at_gmail.com>
> wrote:
> > Marko
> >
> > What is wrong in my statement except that when default route is there it
> > will accept the peering from neighbor .
> > Could you explain me ?
> >
> >
> > On Tue, Dec 4, 2012 at 6:55 AM, Marko Milivojevic <markom_at_ipexpert.com>
> > wrote:
> >>
> >> You are not entirely wrong. BGP won't initiate session, but it will
> >> respond if a session is initiated from another router.
> >>
> >> You can read more about it here:
> >> http://blog.ipexpert.com/2010/11/08/bgp-peering-and-default-routes/
> >>
> >> --
> >> Marko Milivojevic - CCIE #18427 (SP R&S)
> >> Senior CCIE Instructor - IPexpert
> >>
> >> On Mon, Dec 3, 2012 at 8:24 AM, Routing Freak <routingfreak_at_gmail.com>
> >> wrote:
> >> > 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
> >> >> >
> >> >> >
> >> >> >
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Received on Tue Dec 04 2012 - 21:28:19 ART