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May 152012
 

A distance Vector protocol only exchanges remote routes, if it learns the route from the same protocol. It means that it looks at the RIB for the remote routes learned and then it sends an update regarding this to other routers.

OSPF has the same characteristics when doing inter area routing. The main reason for an Autonomous System division into logical areas is to reduce the size of the LSDB . This implies that a router in the non-backbone will not have the complete view of the topology as the backbone area routers. They only have the information told to them from the ABR.

The ABR generates a Type 3 summary LSA, only if it has a corresponding Type 1 LSA or an Inter-area route in its RIB. If it does not have any of them, the Type 3 LSA will not be generated or regenerated.

In the above topology R4 will have an inter area route to the loopback of R3. This O IA route exists at R4 because R2, which is an ABR has a respective route via OSPF to the loopback of R3. This also means that R4 will lose the Type 3 LSA to the 3.3.3.3/32 network, if R2 does not have it in its RIB as an OSPF learned route.

Let us check this case. We can apply a static route to 3.3.3.3/32 on R2, which will then remove the OSPF route from its RIB. Thus it will be incapable to regenerate the Type 3 LSA which it learned from R1.

OSPF: Detect change in LSA type 3, LSID 3.3.3.3, from 22.22.22.22 area 24
OSPF: Schedule partial SPF - type 3 id 3.3.3.3 adv rtr 22.22.22.22
OSPF: Service partial SPF 1/0/0
OSPF: process partial spfQ entry
OSPF: process partial spfQ LSA id 3.3.3.3: mask 255.255.255.255, type 3 adv_rtr 22.22.22.22, age 3600, seq 0x80000005 (Area 24)
OSPF: process summary partial ABR 0x0 txit 0x0 LSA 3.3.3.3: mask 255.255.255.255, t3 adv 22.22.22.22, age 3600, seq 0x80000005 (Area 24)
OSPF: Start partial processing Summary LSA 3.3.3.3, mask 255.255.255.255, adv 22.22.22.22, age 3600, seq 0x80000005 (Area 24) type 3
OSPF: inter-route to 3.3.3.3/32 became unreachable, check externals

The ‘debug ip opsf spf’ output shows that R2 (RID 22.22.22.22) sends a max age LSA for the network 3.3.3.3/32. This causes R4 to remove the Type 3 LSA .

R2 did not regenerate the Type 3 LSA because it does not have an O IA route installed in its RIB. But the case would be different with R1. Let us again install a static route at R1 for the loopback of R3. R1 will still generate a Type 3 LSA because it has a Type 1 LSA for the 3.3.3.3/32 network.

R1#sh ip route static
3.0.0.0/32 is subnetted, 1 subnets
S       3.3.3.3 [1/0] via 10.0.13.3

R1#sh ip ospf database self-originate

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

                Router Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Link count
11.11.11.11     11.11.11.11     652         0x80000007 0x00B906 2

                Summary Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
3.3.3.3         11.11.11.11     652         0x80000006 0x003688
10.0.13.0       11.11.11.11     652         0x80000006 0x00A410

                Router Link States (Area 13)

Link ID         ADV Router      Age         Seq#       Checksum Link count
11.11.11.11     11.11.11.11     652         0x80000007 0x00533E 2

                Summary Net Link States (Area 13)

Link ID         ADV Router      Age         Seq#       Checksum
10.0.12.0       11.11.11.11     652         0x80000006 0x00AF06
10.0.24.0       11.11.11.11     652         0x80000006 0x00ADBB

To conclude, an ABR will only generate a Type 3 LSA when it has a corresponding Type 1 LSA. An ABR will regenerate a Type 3 LSA only if it has a corresponding Inter area route.