Routing

Hi all,

In my topology (please see attached pic) as IGP is used OSPF with areas. When all links up, traffic from Host1 to Host 2 goes through R3->R1->R4 and return path is the same.

When link between R3-R1 is down, traffic goes through R3->R2->R4. Despite of cost between R2-R1 is 1 and cost between R2-R4 is 10,  10.10.10.0/24 is inter-area route and R2 prefer route to Host 2 through R4 and not through R1.

However in that case the return path doesn't the same, it's through R4->R1->R2->R3. R1 and R2 are SRX3600, so R1 is blocking unknown traffic flow and return traffic is dropped on R1.

To avoid asymmetric routing I created static route on R2, as next hop is R1. In that case traffic from Host 1 to Host 2 goes R3->R2->R1->R4 and return traffic is the same. However if link between R1-R4 go down, traffic will be dropped on R1.

So how with OPSF to do, that when link R3-R1 is down, traffic go R3->R2->R1->R4 ?

Thanks

You have 2 options:

1. R2->R3 metric 10, R3->R2 metric 10. All remaining metrcis set to 1.
2. Move R2-R1 link in area 2 and keep your current metrcics. In that case you might epperience suboptimal routing between R1 and R2 (in case the lo0 interfaces of R1 and R2 are in 0 area, again because intra-area is preferred over inter-area )

Regards,

Krasi

Dear Krasi, thanks for reply.

In my full topology I have two links between R1 and R4, R2 and R4 (please see new attached file). Again, I want to avoid asymmetric routing. Also I can't change link between R1-R2 area 0 to area 2, but I can add new link in area2.

What can you suggest for this new topology ?

Then follow the option 1. Your new topology( with R2<=>R4 metrics 10, and 20) will suffer asymetric routing in case R3-R1 link fail. I would suggest  R2<=>R4 metrics mirror R4<=>R1 metrics.  Why not use link aggregation for redundant links - this will prevent assigning them in different security zones(by design or mistake) which is a potentintial problem in case asymetric routing (involving these links) arise and at the same time reduce the topology complexity to the first one. 

Krasi