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OSPF multi area config with 2 ABRs

Posted on 2009-02-23
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Last Modified: 2012-05-06
Hi, in a OSPF design with 2 areas and 2 ABRs... I have three routers in Area 0. It is hub-and-spoke FR non-broadcast topology. Hub router (R5) is DR, and two spokes (r1 and R4) are ABR between Area0 and Area1. Area1 is one router R6, plus those two ABRs and it is one ethernet segment.
When I ping network X, which is loopback on R6, from R5 hub ping is OK. When I trace same IP I get this:

Tracing the route to 150.1.6.6 (network X)

  1 155.1.0.1 24 msec
    155.1.0.4 28 msec
    155.1.0.1 28 msec
  2 155.1.146.6 28 msec *  28 msec

155.1.0.1 and 155.1.0.4 are ABRs. Since the cost to network X are same via R1 nad R4, I have this in routing table:

R5#sh ip ro ospf
     155.1.0.0/24 is subnetted, 2 subnets
O IA    155.1.146.0 [110/65] via 155.1.0.4, 00:40:26, Serial0/0/0
                    [110/65] via 155.1.0.1, 00:40:26, Serial0/0/0
     150.1.0.0/16 is variably subnetted, 4 subnets, 2 masks
O IA    150.1.6.6/32 [110/66] via 155.1.0.4, 00:35:27, Serial0/0/0
                     [110/66] via 155.1.0.1, 00:35:27, Serial0/0/0
O IA    150.1.4.4/32 [110/65] via 155.1.0.4, 00:35:56, Serial0/0/0
O IA    150.1.1.1/32 [110/65] via 155.1.0.1, 00:36:32, Serial0/0/0

In the lab solution guide it shouldn't be two paths to reach network X. Why do I get this ping-pong between two ABRs?
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Question by:barkaHR
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Expert Comment

by:Don Johnston
ID: 23710339
>In the lab solution guide it shouldn't be two paths to reach network X.

Without seeing the lab guide and understanding your topology, it's impossible to say why you're seeing two path instead of one.

>Why do I get this ping-pong between two ABRs?

Because you've got two paths to the same network.
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Author Comment

by:barkaHR
ID: 23710590
I understand. Here, I'm sending you some materials. Hope it would help.
topology2.jpg
R1.txt
R4.txt
R5.txt
R6.txt
R5-sh-ip-route-lab-solution.jpg
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Don Johnston earned 750 total points
ID: 23710632
Based on your information, you should be seeing two equal cost paths to the destination.

Unless you change the bandwidth parameter of the R1 or R4 connection to the frame network, the paths will be the same cost.
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Author Comment

by:barkaHR
ID: 23710667
BW on FR links is default. Nothing is changed. So, you are saying that lab solution is wrong? They have only one path to dest.
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Expert Comment

by:Don Johnston
ID: 23711036
Like I said before, without seeing the lab document, I can't say what's right and what's wrong. But based on the diagram you posted, it's behaving correctly.
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Author Comment

by:barkaHR
ID: 23751467
ok. thanks.
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