OSPF LSA Types Explained

Lessons Discussion
101

Hi @bluephoenix71,

The type 4 LSA is advertised by the ASBR. Here’s a quick example, look at this picture:
ospf-r1-r2-r3-loopback-33-33-33-33
R1 and R2 are in area 0, R2 and R3 in area 1. The loopback interface is redistributed in OSPF, making R3 an ASBR:

R3(config)#interface Loopback 1
R3(config-if)#ip address 33.33.33.33 255.255.255.255

R3(config)#route-map R3_L1 permit 10
R3(config-route-map)#match interface loopback 1

R3(config)#router ospf 1
R3(config-router)#redistribute connected route-map R3_L1 subnets

R1 sees the route:

R1#show ip route | include 33.33.33.33
O E2     33.33.33.33 [110/20] via 192.168.12.2, 00:00:32, GigabitEthernet0/1

Here’s how to see it in the LSDB:

R1#show ip ospf database asbr-summary 

            OSPF Router with ID (11.11.11.11) (Process ID 1)

                Summary ASB Link States (Area 0)

  LS age: 67
  Options: (No TOS-capability, DC, Upward)
  LS Type: Summary Links(AS Boundary Router)
  Link State ID: 3.3.3.3 (AS Boundary Router address)
  Advertising Router: 2.2.2.2
  LS Seq Number: 80000001
  Checksum: 0xBE68
  Length: 28
  Network Mask: /0
        MTID: 0         Metric: 1

You can see above that R2 advertises this LSA.