In this post we will learn the importance of LSA 4 and see which router actually propagates the LSA 4 and what it actually contains. To achieve this we will make use of the topology below –
- In the above topology R1 and R2 form neighborship in Area 10.
- R1 and R3 form OSPF neighborship in Area 0.
- R3 –R4 –R5 form neighborship in Area 20.
- R5 form EIGRP neighborship with EIGRP. Hence R5 router acts as the ASBR in our topology since it is connected to a different routing domain as well.
- R3 is the ABR for Area 20 and Area 0.
- R2 is the ABR for Area 0 and Area 10.
The LSA for the external network 220.127.116.11 is originated by the ASBR router R5 as LSA 5 in the area 20. ASBR, router 5 also generates a LSA 1 in Area 20 for its own router ID. This LSA 1 when reaches the ABR for the Area 20 is converted to LSA type 4 and propagated in Area 0 and Area 10 both.
R5#sh ip ospf database
Notice for the external network the Link ID is external network and the Adv. Router is the ASBR router ID which in our case is 18.104.22.168.Now R3 propagates this LSA 5 from R5 into area 0 and there onwards router R1 propagates this LSA into area 10.The Adv. router ID in this LSA 5 is the router ID of the ASBR which is 22.214.171.124.In order to reach the external network each router must also know how to reach the Adv. Router 126.96.36.199. This is where the LSA 4 comes into picture. R5 generated as LSA 1 for its own router ID 188.8.131.52 in area 20 but R3 propagates this LSA 1 as LSA type 4 into the other areas i.e. area 0 and area 10. Hence the router in areas 0 and 10 get to know how they can reach the ASBR and hence finally get to the external network 184.108.40.206.
R1#sh ip ospf database
LSA 4 is generated by the ABR. Link ID in that is the router ID of ASBR and Adv. Router is the ASBR itself. LSA 4 is required to tell the routers in other areas how to reach the ASBR to get to external network eventually.