OSPF Forward Metric in External Type-2 (E2) Route Selection

RT4 and RT5 redistribute their directly connected network 192.168.1.0/24 into the OSPF routing domain as External Type-2 (E2) route. RT1 selects the path via RT3 instead of RT2 as the forward metric to the E2 route is 2 (the metric between RT1-RT3 and RT3-RT5 links is 1). Forward metric is the cost to reach an ASBR. Note that RT2 also selects the path via RT1-RT3 as it has a lower forward metric to reach RT5, the nearest ASBR in the OSPF routing domain.

Below shows the routing table and OSPF LSDB on RT1:

RT1#sh ip route

Gateway of last resort is not set

     35.0.0.0/24 is subnetted, 1 subnets
O       35.35.35.0 [110/2] via 13.13.13.3, 00:00:36, FastEthernet1/0
     24.0.0.0/24 is subnetted, 1 subnets
O       24.24.24.0 [110/65] via 12.12.12.2, 00:00:36, FastEthernet0/0
     12.0.0.0/24 is subnetted, 1 subnets
C       12.12.12.0 is directly connected, FastEthernet0/0
O E2 192.168.1.0/24 [110/20] via 13.13.13.3, 00:00:36, FastEthernet1/0
     13.0.0.0/24 is subnetted, 1 subnets
C       13.13.13.0 is directly connected, FastEthernet1/0
RT1#
RT1#sh ip ospf database

--- output omitted ---

                Type-5 AS External Link States

Link ID         ADV Router      Age         Seq#       Checksum Tag
192.168.1.0     4.4.4.4         91          0x80000001 0x00B273 0
192.168.1.0     5.5.5.5         91          0x80000001 0x00948D 0
RT1#
RT1#sh ip route 192.168.1.1
Routing entry for 192.168.1.0/24
  Known via "ospf 100", distance 110, metric 20, type extern 2, forward metric 2
  Last update from 13.13.13.3 on FastEthernet1/0, 00:00:36 ago
  Routing Descriptor Blocks:
  * 13.13.13.3, from 5.5.5.5, 00:00:36 ago, via FastEthernet1/0
      Route metric is 20, traffic share count is 1

RT1#