Need Help with Changing Route Path

Unanswered Question
Jan 7th, 2008

Here is my topology and my problem

Router (R1)

Serial 1/1 connected to s1/1 of R2 12.0.0.1/30

Loopback 0 100.0.0.1/24

Router (R2)

Serial1/1 connected to s1/1 of R1 12.0.0.2/30

Serial1/3 connected to s1/3 of R3 23.0.0.1/30

Serial1/2 connected to s1/1 of R4 24.0.0.1/30

Loopbak0 1.1.1.1/24

Looback1 3.3.3.3/24

Router (R3)

Serial1/3 connected to R2 s1/3 23.0.0.2/30

Serial1/1 connected to R5 s1/1 35.0.0.1/30

Looback0 2.2.2.2/24

Looback1 4.4.4.4/24

Router (R4)

Serial1/1 connected to R2 s1/2 24.0.0.2/30

Serial1/2 connected to R5 s1/2 45.0.0.1/30

Loopback0 5.5.5.5/24

Router (R5)

Serial 1/1 connected to R3 s1/1 35.0.0.1/30

Serial 1/2 connected to R4 s/2 45.0.0.2/30

Loopback0 6.6.6.6/24

Loopback1 9.9.9.9/24

Note:-Ok here i'm running OSPF area 10 just keeping it simple i'm able to set the network up and ping everyone now what i want on R4 is this in the routing table the path to network 6.6.6.6 is out S1/2 on R4 now i want to change it so that the path is out S1/1 without shutting the interface on s1/2. What i want is redundancy so that if the path is not available through S1/1 on router R4 then it should switch to s1/2 on R4 but s1/1 should be my primary path.

Also i have tried incresing the administrative distance of the route comming on s1/2 but has not helped. It just does not change

Also tried using distribute list the route from s1/2 gets out from the routing table but the other path from s1/1 does not appear.

Have also tried using route-maps incresing the metric compared to the path from s1/1 but still the route path does not switch just stays as it is through S1/2 .

I must be making a silly mistake and i hope u guys out there can help me out.

Waiting for your reply.

Thanks & Regards,

Ryel D'souza

I have this problem too.
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marikakis Mon, 01/07/2008 - 02:38

Hello,

You report that IP address 35.0.0.1/30 is used both in R3 and R5. Is that so on the routers as well or is this just a typo here?

Kind Regards,

M.

ryldsz123 Mon, 01/07/2008 - 02:44

oh sorry just a typo sorry for the confusion

R5 s1/1 is 35.0.0.2/30

shrikar.dange Mon, 01/07/2008 - 02:46

hi,

can you try changing the bandwidth on R4? for serial 1/2 keep it very low and for s1/1 keep it high.

hope this helps,

regards,

shri

s.arunkumar Mon, 01/07/2008 - 03:09

Interseting..This is just coming out of my small experience ;).Lets hear more 4rm experts too..

Now let me tell u that ospf an link state protocol.In ur area 10 all the lsa received will be type 1 or 2(for broadcast network).

it then run spf and then based on that it put in routing table.As i understand from ur senario the best path after spf calculation might be towards R5 via ser 1/2.When this int goes down again an spf might run to take the path via ser 1/1.

If u want the way as u mentioned i feel first bring the route to routing table by manipulating cost via ser 1/1 less than via 1/2 for the route.I know u tried this but make sure the cost towards route via ser 1/2 is more than ser 1/1.For the adjust the cost

for this route on all routers for the path via ser 1/1.ie on router R2,R3 as well.

But this may change the routing path of other routes also..

marikakis Mon, 01/07/2008 - 03:57

Hello,

The suggestion to adjust bandwidth or ospf cost from previous replies I believe is fine. Just want to add 2 things.

1. You do not have to adjust bandwidths/costs all around the network. Just do it on the "shortest configuration path", that is the interface between R4 and R5. Make it look like it is very low bandwidth, so the other way round is preferred.

2. You should adjust the bandwidth in both directions of the link, that's is you should configure both R4 and R5 serial interfaces, so that both routers have the same understanding on what the preferred path is. (Each router runs its own Dijkstra and has its own view of the network graph.)

Kind Regards,

M.

Kevin Dorrell Mon, 01/07/2008 - 04:08

Ryel,

The Administrative Distance is not really relevant here. AD is used only arbitrate between different routing protocol domains, and not between routes in the same domain.

What you need to do is increase the cost of the path R4:S1/2 <--> R5:S1/2 until it is greater than the total round the path R4-R2-R3-R5. Have a look at the path cost on R4s current route to 6.6.6.6 via S1/2. Then use ip ospf cost n on R5-S1/2 (or should that be on R4-S1/2 ?)

Now, I am guessing that does not really answer your question, and actually what you are trying to do is make the route for 6.6.6.6 go round R4-R2-R3-R5, while the route for 9.9.9.9 goes direct R4-R5. Here is the crunch that you will not like - you cannot do it with your current topology!

The reason is that OSPF within a single area is a link state protocol, and so it knows about the cost of links, but knows nothing about costs of prefixes. It knows what router(s) the prefix is found, and it knows the cumulative cost of the links it can use to get to that router, but it cannot attach a cost to a prefix. Each router works out the cost to the destination purely on the basis of the costs of the intermediate links.

Kevin Dorrell

Luxembourg

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