OSPF Path Selection question

Answered Question
Mar 20th, 2010

Hi Friends;

  Any one has idea what's going on in the belwo

           R1

            | (BGP)

|------------------------|

R3---------------------R2

|   OSPF Area1   |

R5-------------------- R4

     OSPF Area 0

-R1 is running BGP with R2 and R3

-R2 is primary router So it's redistribute BGP into OSPF metric 100 type E1

-R3 is Secondry router So it's redistribute BGP into OSPF metric 200 type E1

The problem is R5 always using R3 to reach R1 however the metric os this path is higher than the one coming from R4, Any Idea, Thanks

Best Regards;

M.Kamar

I have this problem too.
0 votes
Correct Answer by Lei Tian about 6 years 8 months ago

No, you cann't disable it; you can create a tunnel and put the tunnel in area 1, and change the tunnel cost to make it the prefer path.

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Lei Tian Sat, 03/20/2010 - 19:08

Hi M.Kamar,

Because intro-area route always prefer over inter-area route.

HTH,

Lei Tian

mkamar Sun, 03/21/2010 - 00:11

Thanks Lei;

  Is there a way to disable this feature and make and let the router consider the cost 1st?

Best regards;

  Mohamed Kamar

Correct Answer
Lei Tian Sun, 03/21/2010 - 05:17

No, you cann't disable it; you can create a tunnel and put the tunnel in area 1, and change the tunnel cost to make it the prefer path.

mkamar Sun, 03/21/2010 - 17:19

Hi Letian;

Ops, This is a wonderful idea, I think i'll go it

Best Regards;

  Mohamed Kamar

ex-engineer Sun, 03/21/2010 - 18:03

Mr. Kamar:

Can you please show the complete route table entry on R5 for the network you are trying to reach?

Im not sure I understand which is the intra-area and which is the inter-area route...

The BGP routes that R5 receives will be OSPF external type 1 routes (O E1).

R5 and R4 are both ABRs, arent they?

[EDIT] And R1, R2 and R3 have all OSPF interfaces in Area 1, while R4 and R5 have one interface in Area 1 and the other in Area 0. [EDIT]

Please correct me if I am misunderstanding something.

Thanks

mkamar Tue, 03/23/2010 - 01:56

Hi ex-engineer ;

I took 3 routes from R5 for you, fa 1/0 is the link to R3, note how much the cost is, and the routes are preffered, note also when i shut down this link the routes came from R4 fa 0/0, you will note also that all these routes are External but in the 1st case before shut down it came from Intra-area and in the 2nd case after shut down it came from Inter-Area

That's way we conclude that there is no way to change this OSPF behavior (Intra better than inter) and the work around is either put area 1 link between R4 and R5 or create a tunnel between them to force the traffic to take this path, Thanks

O E1    10.0.0.0/24 [110/1110] via 10.201.0.68, 00:02:13, FastEthernet1/0
O E1    10.0.0.0/23 [110/1200] via 10.201.0.68, 00:02:08, FastEthernet1/0
O E1    10.0.1.0/24 [110/1110] via 10.201.0.68, 00:02:13, FastEthernet1/0

R5(config)#int fas 1/0
R5(config-if)#shutdown

O E1    10.0.0.0/24 [110/111] via 10.201.0.25, 00:00:55, FastEthernet0/0
O E1    10.0.0.0/23 [110/221] via 10.201.0.25, 00:00:55, FastEthernet0/0
O E1    10.0.1.0/24 [110/111] via 10.201.0.25, 00:00:55, FastEthernet0/0

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