OSPF External route selection

[Shahzad Qadir]

Hello,

I have got this lab setup where ospf is learning two routes via two different processes. But it is prefering the route with higher metric. can someone please explain why ospf is taking higher cost route.

My ospf database and routing table is as follows:

R2(config-if)#do sr

Gateway of last resort is not set

     3.0.0.0/24 is subnetted, 1 subnets

O E1    3.3.3.0 [110/378] via 10.10.10.1, 00:00:29, Serial0/0

     4.0.0.0/24 is subnetted, 1 subnets

O E1    4.4.4.0 [110/428] via 10.10.10.1, 00:00:29, Serial0/0

     10.0.0.0/30 is subnetted, 2 subnets

C       10.10.10.0 is directly connected, Serial0/0

C       10.10.10.4 is directly connected, Serial0/1

R2(config-if)#do sh ip ospf data ext 4.4.4.0

            OSPF Router with ID (10.10.10.2) (Process ID 2)

                Type-5 AS External Link States

  Routing Bit Set on this LSA

  LS age: 326

  Options: (No TOS-capability, DC)

  LS Type: AS External Link

  Link State ID: 4.4.4.0 (External Network Number )

  Advertising Router: 10.10.10.5

  LS Seq Number: 80000001

  Checksum: 0xE75A

  Length: 36

  Network Mask: /24

        Metric Type: 1 (Comparable directly to link state metric)

        TOS: 0

        Metric: 250

        Forward Address: 0.0.0.0

        External Route Tag: 200

            OSPF Router with ID (10.10.10.6) (Process ID 1)

                Type-5 AS External Link States

  Routing Bit Set on this LSA

  LS age: 483

  Options: (No TOS-capability, DC)

  LS Type: AS External Link

  Link State ID: 4.4.4.0 (External Network Number )

  Advertising Router: 10.10.10.9

  LS Seq Number: 80000001

  Checksum: 0xB5BA

  Length: 36

  Network Mask: /24

        Metric Type: 1 (Comparable directly to link state metric)

        TOS: 0

        Metric: 300

        Forward Address: 0.0.0.0

        External Route Tag: 100

There are 2 routes in database but router is selecting route with higher metric.

Thanks

Shahzad

[lgijssel · Accepted solution]

The OSPF route selection rule is that intra-area routes are preferred over inter-area routes, which are preferred over external routes. However, this rule should apply to routes learned via the same process. In other words, there is no preference between external routes from one process compared to internal routes of other process.

The preference rule between a given OSPF process and any other process (be it OSPF or another routing protocol) should follow the administrative distance rule. However, because different OSPF processes will have the same administrative distance by default, OSPF distance should be configured explicitly for different OSPF processes, to achieve the desired behavior.

http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a0080531fd2.shtml#routepref

regards,

Leo

[Shahzad Qadir]

Thanks. Good explaination, it solve the mystry for me.