path selection

[gongya001]

R2#show ip ospf data external

            OSPF Router with ID (2.2.2.2) (Process ID 1)

                Type-5 AS External Link States

  Routing Bit Set on this LSA in topology Base with MTID 0
  LS age: 1256
  Options: (No TOS-capability, DC, Upward)
  LS Type: AS External Link
  Link State ID: 6.6.6.6 (External Network Number )
  Advertising Router: 3.3.3.3
  LS Seq Number: 80000002
  Checksum: 0x3423
  Length: 36
  Network Mask: /32
        Metric Type: 2 (Larger than any link state path)
        MTID: 0
        Metric: 20
        Forward Address: 10.10.10.6
        External Route Tag: 0

R2#show ip route 6.6.6.6
Routing entry for 6.6.6.6/32
  Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 20
  Last update from 172.16.4.4 on Ethernet0/2, 00:21:50 ago
  Routing Descriptor Blocks:
  * 172.16.4.4, from 3.3.3.3, 00:21:50 ago, via Ethernet0/2
      Route metric is 20, traffic share count is 1

R2#traceroute 6.6.6.6 num
Type escape sequence to abort.
Tracing the route to 6.6.6.6
VRF info: (vrf in name/id, vrf out name/id)
  1 172.16.4.4 1 msec 0 msec 0 msec
  2 10.10.10.6 1 msec 0 msec 1 msec

Anyone can help me to understand why R2 selects R4 as the path to 6.6.6.6 instead of R3 ?

thanks !!

[LaserNinja]

I think it's because through the R4, you have an External Type 2 Route, as it says here

R2#show ip route 6.6.6.6
Routing entry for 6.6.6.6/32
Known via "ospf 1", distance 110, metric 20, type extern 2, forward metric 20

so that means it doesn't count the internal metric to the R4, thus lowering the path cost, compared to going through the R3.

[Cristian Matei]

Hi,

   It has to do with the forwarding address from the LSA Type5. As the FA is set to non-zero, it will follow the path to reach the FA/nexthop, which is 10.10.10.6 (this is internal OSPF path selection and SPF calculation). Can you confirm that "show ip route 10.10.10.6" follows the path towards R4?

Regards,

Cristian Matei.