OSPF routing issue

Unanswered Question
Jul 15th, 2010
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Hi networkers,

This is a scenario I'm sure some of you guys have encountered.

I have two routers. Each one has a link to the telco provider. One is 10M (line1) and the other is 2M (line2). R1 is connected to line1, and R2 is connected to line2. They are backbone area routers.

The issue is the following: most of the LANs of our branches are learned through R2. But some routes are learned through R1.

I first thought it's a matter of OSPF costs. Both my routers are connected to the telco, so I thought that R2, when it computes routes to branches, finds that it's preferable to reach them through ISP PE2 (connected on line2) than through R1 (which has a better WAN bandwidth).

Telco told me to configure "filters" as they said :-), I thought of distribute lists and route-maps.

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Overall Rating: 1.8 (4 ratings)
davidsudjiman Thu, 07/15/2010 - 00:59
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Hi wass.aouadi,

You can start checking the OSPF DB via "sh ip ospf database" on both R1 and R2. Especially R1, check whether it receives both from R2 and from ISP and compare it.

rgds, dsu

davidsudjiman Thu, 07/15/2010 - 02:34
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What about

sh ip ospf database summary

sh ip ospf database summary

sh ip ospf database external

sh ip ospf database external

on both R1 and R2

davidsudjiman Thu, 07/15/2010 - 03:55
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Issue: R1 will prefer from over due to its intra-area path.

To Do:

1. You can make not to advertise, or

2. you can filter LSA3 on R1 and R2 from appearing.

Wassim Aouadi Thu, 07/15/2010 - 23:59
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I did some shut/no shut on R1 and R2 interfaces. Here's what I noticed:

  • when both line1 and line2 are up, R1 builds and prefers inter-area routes
  • when line1 is down, the only way R1 builds routes is by learning LSAs from R2
  • when line1 is up and line2 is down, R1 builds routes based on External type 2 LSAs, and R2 builds its routes after learning LSAs from R1.

So I noticed that OSPF process prefers inter-area LSAs over External LSA.

the questions here are:

  1. how can I make OSPF prefer External LSAs for those routes? is it on the Service Provider side because they are doing redistribution of OSPF into/out of BGP?
  2. and if I do that, will it impact OSPF routes learned by R2 when line1 gets down?


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