Ospf installing a high metric path in routing table

Unanswered Question
Sep 8th, 2007

Hi,

We have follwoing set for routers.

1. R1 and R2 (installed at same location ) having channelised E1 controllers , and each is connected to different locations on 64 kbps serial link time slots on Ch E1. ( each timeslot connected to diffreen location.)

2. R1 and R2 are connected to each other on ethernet.

3. R1 and R2 ethernet is in the OSPF area 0.

4. All locations on channelised E1 controller that is all 64 kbps serial links are defined in area 4.1.0.0

5. All well so far so good !

6. When i am giving command on R1

# sh ip route SUB_NET_Connected_R2.

I am getting proper output as know via ospf and next hop is R2 ethernet. So that packets will be forwarded to R2 ethernet address and the R2 will forward those to corresponding serial link on channelised E1 controller. so far so good.

7. We identified one location ( R3 router ) as very critical and went for one more serial link on that router. One serial link was terminated on R1 channelised E1 controller and other link

was terminated on R2. On R2 it is not terminated on any timeslot of E1 controller but on a Channelised E1 controller all time slots clubbed together forming a point to point link. Bandwidth of both the link is configured same as 64 kbps. So ospf cost is same as 1562 for both links.

8. Now R3 is connected to R1 and also to R2 by 64 kbps link. This is causing problem. Both links are in area 4.1.0.0

9. when we check ip route ( on R1 ) for subnet connected to R2 via ch E1. , to our surpise OSPF poined all those to R3 serial link with higher metric that what was earlier with only one link on R3

Thne we admin down link R3 serial link on R2. And in 10 seconds all routes were back to normal as pointing to ethernet of R2.

10. In short traffic from R1 destined for some subnet on R2, would take route as following ( with both links on R3 are operational ) First packets will be forwared to R3 serial connected to R1 then R3 ( 64 kbps link ) will forward those to it's next serial port connected to R2 ( another 64 link ). Packets will then reach R2 and then R2 will forward those to corresponding E1 time slot.

All this will add additinal 64 link for all subnets. ( this jams both 64 kbps links connected to R3) althouhg this path is of higher metric as compared to path via ethernet connected between routes. ( so currently we have kept one link as admin down on R3 to keep the network up )

I hope I have made my point clear ! As it is too much of text.

Any similar problem if seen or came across please share, definitely appreciated.

Thanks in advance.

Subodh

I have this problem too.
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Edison Ortiz Sat, 09/08/2007 - 20:24

The subnets from R2 going via R3 to R1, are those in what OSPF area ? If they are in OSPF area 4.1.0.0, then it is behaving as designed.

Intra-area routes will be preferred over Inter-area routes regardless the cost of the link.

Can you post the 'show ip os int bri' from R1, R2 and R3 ?

bapatsubodh Sat, 09/08/2007 - 20:47

Hi,

Thankx for so fast reply in just 2 minutes after i posted my problem.

1. all Ch E1 links are in 4.1.0.0 area.

2. Packet source ip and packet Detination IP are in the same area as 4.1.0.0.

3. so it is taking 64 kbps path as it is in the same area, as compared to path via area 0 and then again to area 4.1.0.0.

4. R2--> serail link to R3 ( area 4.1.0.0 )---> Serial link on R3 to R1 ( area 4.1.0.0 )

--> to some subnet on Serial link on R1 ( area 4.1.0.0 ) This is what the path is taken as all these subnets are intra area.

5. What we expected is like this

R2--> to R1 ethernet ( area 0 ) ---> R1 to corresponding time slot on that serial link on ChE1 controller. ( area 4.1.0.0)

So if this is going to be sure and it is as OSPF would prefer intra area route over inter area routes ( where it need to forward traffic to area 0 and then again to area 4.1.0.0)

In order to have redundancy what is possible solution ?

Thanks for ur help !

Subodh

Edison Ortiz Sun, 09/09/2007 - 04:52

I recommend creating a different OSPF area between R2 and R3 serial link. Do the same for R3 to R1 serial link.

However, with this design, R3 must have an area 0 or you need to create virtual links from the routers with area 0 (in this case R2 and R1) pointing towards R3.

OSPF can become a bit tricky when designing preferred paths. Intra-area routes will always be chosen over any other routes (Inter-area or External).

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