From: Curtis Call (curtiscall@xxxxxxx)
Date: Wed Nov 14 2001 - 19:44:06 GMT-3
Good point, I wasn't thinking of that since it's use hadn't been mentioned.
Here's what I think is happening, Router C is a Route-Reflector for Router A
and Router B. Router A is receiving a path with a med of 50 and Router B is
receiving a path with a med of 60. These two paths are sent to Router C which
reflects them to the two clients. Router B, upon receiving indication of a
better path (the one with a med of 50) withdraws it's previous advertisement
of a path with a med of 60 and instead will use the route with a med of 50.
This leaves just one path possible to the destination, through Router A.
To test this hypothesis disable route-reflection and the two paths should
appear.
"Charles Manafa" <charles.manafa@blueyonder.co.uk> wrote:
> I think you will find that "maximum-paths" BGP router config, will allow
BGP
> to use up to 6 equal paths to the destination.
>
> ----- Original Message -----
> From: "Curtis Call" <curtiscall@usa.net>
> To: <ccielab@groupstudy.com>; "Jobson Eddie" <EJobson@thrupoint.net>
> Sent: Wednesday, November 14, 2001 3:44 PM
> Subject: Re: [MED problem]
>
>
> > BGP just selects one route, you won't see two BGP routes to the same
> > destination in your routing table. (At least you shouldn't)
> >
> > "Jobson, Eddie" <EJobson@thrupoint.net> wrote:
> > > I am having problems getting both routes to an external BGP neighbor
> when
> > > setting different metrics. Can anyone tell me what I'm missing. The
only
> > > thing I haven't tried yet is the deterministic commands.
> > >
> > > Diagram
> > >
> > > ------------
> > > | Ext. |
> > > | BGP peer |
> > > -------------
> > > | |
> > > | |
> > > | |
> > > --------- ---------
> > > | Rtr A | | Rtr B |
> > > TKR1---------| | | |-------TKR2
> > > ---------- ----------
> > > | |
> > > WAN 0 | | WAN 1
> > > | |
> > > --------
> > > | Rtr C |
> > > | RR |
> > > ---------
> > >
> > > The EBGPpeer is peering with both Rtr A & Rtr B but want it to prefer
> TKR1
> > > on RtrA and TKR2 on RtrB, unless either of the links fail. If so and
the
> > Rtr
> > > B to EBGP peer fails use the Router reflector RtrC to get to TKR2 and
> vice
> > > versa.
> > >
> > >
> > > I've tried various enabling of next hop self, redistribute connected,
> > > network staements all in different combinations but no luck. I get TKR1
> > > showing up as a metric of 50 when all links are up and TKR2 as a metric
> of
> > > 50, but I would have expected a less preferred route with a metric of
60
> > > also shown without the > sign in the show ip bgp table. If I pull one
of
> > the
> > > links I loose the route, but I have full connectivity between
> RtrA-RR-RtrB
> > > over IBGP/IGP.
> > >
> > > Configs
> > > RtrA
> > > router bgp 1
> > > neighbor EBGPpeer remote-as 100
> > > neighbor EBGPpeer upd lo0
> > > neighbor EBGPpeer route-map med out
> > > neighbor EBGPpeer ebgp-multihop 5 (there is another router in the way
> not
> > > shown on the diag)
> > > network TKR1 mask 255.255.255.0
> > > network TKR2 mask 255.255.255.252
> > > neighbor RtrC remote-as 1
> > > neighbor RtrC upd lo0
> > > neighbor RtrC next-hop-self
> > > no sync
> > > route-map med permit 10
> > > match ip add 1
> > > set metric 50
> > > route-map med permit 20
> > > match ip add 2
> > > set metric 60
> > > route-map med permit 30
> > > access-list 1 permit TKR1
> > > access-list 2 permit TKR2
> > >
> > > RtrB
> > > router bgp 1
> > > neighbor EBGPpeer remote-as 100
> > > neighbor EBGPpeer upd lo0
> > > neighbor EBGPpeer route-map med out
> > > network TKR1 mask 255.255.255.0
> > > network TKR2 mask 255.255.255.252
> > > neighbor RtrC remote-as 1
> > > neighbor RtrC upd lo0
> > > neighbor RtrC next-hop-self
> > > no sync
> > > route-map med permit 10
> > > match ip add 1
> > > set metric 50
> > > route-map med permit 20
> > > match ip add 2
> > > set metric 60
> > > route-map med permit 30
> > > access-list 1 permit TKR2
> > > access-list 2 permit TKR1
> > >
> > > RtrC
> > >
> > > router bgp 1
> > > neighbor RtrA remote-as 1
> > > neighbor RtrA upd lo0
> > > neighbor RtrA route-reflector-client
> > > neighbor RtrB remote-as 1
> > > neighbor RtrB upd lo0
> > > neighbor RtrB route-reflector-client
> > > no sync
> > >
> > > EBGPpeer
> > > router bgp 100
> > > neighbor RtrA remote-as 1
> > > neighbor RtrA ebgp-multihop 5 (hidden router)
> > > neighbor RtrB remote-as 1
> > >
> > > That's it! As I say full IP connectivity and routes being received when
> all
> > > links are up, but less preffered metric 60 routes do not appear?
> > >
> > > Any help appreciated,
> > >
> > > Eddie.
> >
> >
> >
> > ----------
> > curtiscall@usa.net
> > OSPF/BGP Practice Exams
> > www.boson.com\tests\Advanced.htm
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