Mahesh,

10.219.1.96/28 is being seen as an E2, which means that the metric is seen as 20 by the RR for the path coming from PE1, PE2 and PE3. Given that the IGP metric to these PEs is probably the same, the selection is probably based on lowest RID.

Try changing the default from E2 to E1. Note that either the path through PE2 -> CE1 or PE1 -> CE1 will be choosen if they have equal cost.

Regards

Hello,

Ok, this is a hard scenario to comprehend, but I think I am beginning to understand the problem.

When you trace, the packets are forwarded usually in the opposite direction of the route propagation. If you have a look at the diagram you posted (which is very helpful to visualize things and I have to congratulate you on drawing it), you will see what's happening with the route when it first comes from CE1.

CE1 advertises route to both PE1 and PE2. PE2 sends network to CE3, which sends it to PE3. So, PE3 has an OSPF path to the network and so does PE1. 2 routes reach your RR and RR chooses the route via PE3 and passes it to PE4.

If you have followed all this path, you will see that the basic problem is the following: the metric information is lost when you redistribute at various points. When you redistribute from OSPF into MP-BGP, PE uses OSPF metric to set the BGP MED. When PE redistributes routes back from BGP into OSPF, the PE uses MED to set the OSPF metric. When you use default-metric or metric option in redistribute command, this behavior is overridden, because you directly reset the metric.

Local preference might be better than manipulating metrics and MEDs, but you may also consider the following: when you redistribute, you could try not re-setting the metrics. This way you will be able to propagate a metric that reflects the fact that the alternative backup path is less preferred. I think you need to also have a metric-type 1 (external type 1 routes) in OSPF to achieve this metric propagation end-to-end, but honestly this case was so hard for me, I think my head is going to explode right now and I can't decide if it is actually needed (have a look at the "forward metric"'s of the OSPF routes at PE1 and PE3, those are different but the forward metric is not considered in external type 2 routes).

I also think that the MED is not the reason the route is chosen by RR (I think both routes have a metric of 20 propagated as a BGP MED of 20). I think the algorithm goes further down (I don't know your network in more detail, but it could be something like: closest backbone path to BGP next-hop), and you manage to overcome that by forcing the algorithm to stop at an earlier point and decide based on MEDs.

If I made a mistake, I am sorry, but this is not easy, especially without access to routers to see what's going on.

Kind Regards,

M.

Harold & Maria,

I might be misunderstanding some thing here, The original poster claimed that from PE4 thr route to Network 10.x.x.x is not taking the direct path , rather its taking the path toward the RR.

He said when he redistributed the Network from OSPF into BGP with a cost of 15, the path was choosen.

We are talking about PE4 here, PE4 doesnt know about OSPF and its only has 2 IBGP session with both RR and PE1. The route from PE4 was prefered cause it prefered the lowest MED value because the OSPF cost is copied into the BGP MED attribute.

Also, Cisco doesnt recommend changing the OSPF cost cause its copied into the BGP MED, and could result in a routing loop.

The recommendation has been introduced when RIP and OSPF is involved an being redistributed into BGP. Its also clear when Implementing a sham link to correct customer OSPF backdoor link in MPLS-VPN Scenarios.

Hope I clarified my point,

Mohamed

Hi Harold,

Pls look at the bellow paragraphs:

--""he is advt. me one pool 10.219.1.96/28 from PE1 .

But when I do trace from PE4 it goes to PE3---CE3---PE2---CE1.

Infact it should go directly to PE1-CE1

He is not advt this pool from CE3. When we change redistribution metric to 15 at PE1 to MPBGP problem get resolved ""--

My previous post was based on the above and the output from (sh ip route 10.x Network),He also mentioned a direct path between PE1-CE1 from PE4.

HTH

Mohamed

Hi Harold,

I know the route is being redistributed and advertised on PE3. what I mean the Route at PE4 is learned via BGP not OSPF.. (My main concern was PE4).

Mahesh,

The (Down-bit) in the LSA header is supported to prevent routing loop, what it does, it will certainely makes sure the packet are never back to the original router.

You can simply disable the (Down-bit) check, the only way to do this is by configuring (Capability vrf-lite) under the OSPF process at your PE.

Since you have a Multi VRf CE router, you need to disable the Down-bit check at your PE.

In order to have E1 route reflected at the ASBR, you will have to set it as External-type 1 when redistributed on your ASBR PE router.

HTH

Mohamed