OSPF LOADBALANCING PROBLEM

Unanswered Question
Aug 28th, 2008

Please find the attached file.

I have 5 routers named R0 to R4. R2 is doing static routing with R0 & R3. R0 is ASBR for area 0, R3 is ASBR for area 1 & R1 is ABR for area 0 & 1. R4 is the simple router which is participating in area 1. A route 20.20.20.20 which is the loopback of router R2 and redistributed from the ASBRs. OSPF cost fo link R3-R4 has been changed to 1000. Now the 20.20.20.20 route is preferred via R1, From R1 it should perform the load balancing but it is not performing. On R1 I am getting 20.20.20.20 route from the both ASBRs but it always follow the R3 path. But If I trace from r1 to 20.20.20.20 load balancing is actually happening but not when the packet comes from R4.

After that I disable the CEF from the R1 and no ip route-cache command is added to serial 0/0 & serial 0/1. After that load balancing starts from R1 when traffic comes form R4. I am not able to understand the reason for that.

regards

shivlu

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Giuseppe Larosa Thu, 08/28/2008 - 02:19

Hello Shivlu,

when having multiple ASBRs redistributing the same prefixes is better to use external type 1.

E2 metric is not added to the IGP metric to the ASBR.

E1 instead will compute the metric as the sum of the seed metric and the metric to ASBR router including intra and inter-area costs.

O E1 are preferred over O E2 routes even with a greater seed metric becuse they are more detailed.

So these strange results can derive from this.

I would suggest you to repeat your tests using O E1 routes for net 20.20.20.20/32.

Hope to help

Giuseppe

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