OSPF Type-3 Network-Summary-LSAs Generations and Deletions

The following scenarios were setup to observe the generations and deletions of OSPF Type-3 Network-Summary-LSAs in a looped network topology:
1) OSPF is configured on RT1, RT2, and RT3. RT4 not participating in OSPF routing yet. Area 1 is separated by Area 0.
2) RT4 participates in OSPF routing after includes Serial0/0 in the router ospf configuration.
3) RT4 includes Serial0/1 in the router ospf configuration.

Below shows the routing table and OSPF LSDB on RT1 upon Scenario 1. RT1 calculates the route to 40.40.40.0/24 based on the (Area 1  Area 0) Type-3 LSA generated by RT3. Note that RT1 also generates a (Area 0  Area 1) Type-3 LSA into Area 1 on its Serial0/0. This is due to RT1 assumes that it is the only router that resides in Area 1.

RT1#sh ip route

Gateway of last resort is not set

--- output omitted ---
     40.0.0.0/24 is subnetted, 1 subnets
O IA    40.40.40.0 [110/192] via 10.10.10.2, 00:00:29, Serial0/1
RT1#
RT1#sh ip ospf database

            OSPF Router with ID (3.3.3.3) (Process ID 100)

--- output omitted ---
                Summary Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
30.30.30.0      1.1.1.1         39          0x80000001 0x00B1EC
40.40.40.0      3.3.3.3         40          0x80000001 0x000C6C

--- output omitted ---
                Summary Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
10.10.10.0      1.1.1.1         39          0x80000001 0x008456
20.20.20.0      1.1.1.1         29          0x80000001 0x009DDE
40.40.40.0      1.1.1.1         29          0x80000001 0x004DB2
RT1#

The similar behaviors can be observed on RT3 as well – RT3 advertises 30.30.30.0/24 into Area 1 which is known via RT1 when RT1 advertises the network from Area 1 to Area 0.

RT3#sh ip route

Gateway of last resort is not set

--- output omitted ---
     30.0.0.0/24 is subnetted, 1 subnets
O IA    30.30.30.0 [110/192] via 20.20.20.1, 00:00:31, Serial0/0
RT3#
RT3#sh ip ospf database

            OSPF Router with ID (3.3.3.3) (Process ID 100)

--- output omitted ---
                Summary Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
30.30.30.0      1.1.1.1         44          0x80000001 0x00B1EC
40.40.40.0      3.3.3.3         41          0x80000001 0x000C6C

--- output omitted ---
                Summary Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
10.10.10.0      3.3.3.3         41          0x80000001 0x00CAC7
20.20.20.0      3.3.3.3         41          0x80000001 0x00DED5
30.30.30.0      3.3.3.3         31          0x80000001 0x007A9B
RT3#

Conclusion: RT1 and RT3 advertise the Type-3 LSAs learnt from each other via Area 0 as Type-3 LSAs into Area 1.

Below shows the routing table and OSPF LSDB on RT4 upon Scenario 2. It receives a Type-3 LSA for 40.40.40.0/24 from RT1. Note that the routing tables and OSPF LSDBs on RT1, RT2, and RT3 are identical as upon Scenario 1.

RT4#sh ip route

Gateway of last resort is not set

     20.0.0.0/24 is subnetted, 1 subnets
O IA    20.20.20.0 [110/192] via 30.30.30.1, 00:00:58, Serial0/0
     40.0.0.0/24 is subnetted, 1 subnets
C       40.40.40.0 is directly connected, Serial0/1
     10.0.0.0/24 is subnetted, 1 subnets
O IA    10.10.10.0 [110/128] via 30.30.30.1, 00:00:58, Serial0/0
     30.0.0.0/24 is subnetted, 1 subnets
C       30.30.30.0 is directly connected, Serial0/0
RT4#
RT4#sh ip ospf database

            OSPF Router with ID (4.4.4.4) (Process ID 100)

--- output omitted ---
                Summary Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
10.10.10.0      1.1.1.1         277         0x80000001 0x008456
20.20.20.0      1.1.1.1         267         0x80000001 0x009DDE
40.40.40.0      1.1.1.1         267         0x80000001 0x004DB2
RT4#

Below shows the routing table and OSPF LSDB on RT4 upon Scenario 3. Take note upon the Type-3 LSAs advertised by RT1 and RT3 from Area 0 into Area 1. RT1 and RT3 no longer advertising 30.30.30.0/24 and 40.40.40.0/24 into Area 1, as RT1, RT3, and RT4 recognize each other in Area 1 via Type-1 Router-LSAs in Area 1.

RT4#sh ip route

Gateway of last resort is not set

     20.0.0.0/24 is subnetted, 1 subnets
O IA    20.20.20.0 [110/128] via 40.40.40.1, 00:01:02, Serial0/1
     40.0.0.0/24 is subnetted, 1 subnets
C       40.40.40.0 is directly connected, Serial0/1
     10.0.0.0/24 is subnetted, 1 subnets
O IA    10.10.10.0 [110/128] via 30.30.30.1, 00:01:02, Serial0/0
     30.0.0.0/24 is subnetted, 1 subnets
C       30.30.30.0 is directly connected, Serial0/0
RT4#
RT4#sh ip ospf database

            OSPF Router with ID (4.4.4.4) (Process ID 100)

                Router Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum Link count
1.1.1.1         1.1.1.1         311         0x80000002 0x00DCF7 2
3.3.3.3         3.3.3.3         77          0x80000002 0x000880 2
4.4.4.4         4.4.4.4         77          0x80000002 0x00C466 4

                Summary Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
10.10.10.0      1.1.1.1         515         0x80000001 0x008456
10.10.10.0      3.3.3.3         514         0x80000001 0x00CAC7
20.20.20.0      1.1.1.1         505         0x80000001 0x009DDE
20.20.20.0      3.3.3.3         514         0x80000001 0x00DED5
RT4#