Examine a Type 5 AS External LSA

Action

To examine a Type 5 AS external LSA, enter the following CLI operational mode command:

user@host> show ospf database extern extensive

Sample Output

user@R2> show ospf database extern extensive

    OSPF AS SCOPE link state database

 Type       ID               Adv Rtr           Seq      Age  Opt  Cksum  Len 

Extern  *10.0.0.100       10.0.0.2         0x80000047  1377  0x2  0xfd09  36

  mask 255.255.255.255

  Type 2, TOS 0x0, metric 0, fwd addr 10.0.0.1, tag 0.0.0.0

  Gen timer 00:21:02

  Aging timer 00:37:02

  Installed 00:22:57 ago, expires in 00:37:03, sent 00:22:55 ago

  Ours, 

[...Output truncated...]

What It Means

The sample output shows one Type 5 external LSA, *10.0.0.100. The status of the router represented by this LSA is indicated by the fwd addr field, which shows that it does not belong to any particular OSPF area. The forwarding address provides the address toward which packets should be sent to reach the external router (10.0.0.1). R1 is the ASBR with the connection to external router A.

The mask field represents the subnet mask associated with the advertised router. It is used with the link-state ID field (10.0.0.100), which encapsulates the network address in a Type 5 LSA. This LSA has a metric value of 0, the default value, indicating that this is a Type 2 external metric. Thus, any local router should use the default metric (0) when performing an SPF algorithm.

Each time the LSA is updated, the sequence (seq) field increments, indicating that the router has the most recent version of the LSA. Values range from 0x80000001 to 0x7FFFFFFF. If the sequence field is not incrementing, there may be problems with the connection.

In general, each ASBR generates a Type 5 LSA to advertise any routers external to the OSPF AS. This LSA is flooded to each non-stub router in the entire AS.