Examine a Type 4 ASBR Summary LSA

Action

To examine a Type 4 ASBR summary LSA, enter the following CLI operational mode command:

user@host> show ospf database asbrsummary extensive

Sample Output

user@R3> show ospf database asbrsummary extensive

    OSPF link state database, area 0.0.0.0

[...Output truncated...]

ASBRSum *10.0.0.6         10.0.0.3         0x80000042  1023  0x2  0xb943  28

  mask 0.0.0.0

  TOS 0x0, metric 1

  Gen timer 00:27:57

  Aging timer 00:42:57

  Installed 00:17:03 ago, expires in 00:42:57, sent 00:17:01 ago

  Ours, 

[...Output truncated...]

What It Means

The sample output shows that an LSA within the backbone area, *10.0.0.6, is generated by ASBR R3, as indicated by the asterisk (*) next to the link-state ID field and ours in the last line of the LSA.

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.

Because the router ID of all the ASBR summary LSAs is a full 32-bit value, the network mask is not needed and is set to a value of 0.0.0.0. The metric for the LSA within the backbone area is set to 1, which is the cost to the advertising router (R3) from the originating router (R6). The metric is calculated before the SPF algorithm is calculated.

In general, each ABR that must transmit information about an ASBR from one OSPF area into another generates a Type 4 LSA. This LSA is flooded to each router in the OSPF area. A Type 4 LSA is defined as having an area scope so that another ABR does not reflood it across the area boundary.