Routing: Configuration OSPF To Filter Type-5 LSAs

 Today I am going to talk about OSPF protocol where we will discuss on the filtering of the type-5 LSAs from the network. These type 5 LSAs are external LSAs which are originated by ASBR router and can be learned from external OSPF or another routing protocol like EIGRP, RIP or BGP.


Here today I am going to talk about the LSA 5 learning from the external network and we are using EIGRP for this case.


An OSPF type 5 LSA is originated by an Autonomous System Boundary Router (ASBR) and flooded within the OSPF area. These routes are generated by means of redistribution into OSPF from other protocols or by the redistribution of connected or static routes.

Fig 1.1- OSPF EIGRP Topology


Here in our discussion, we are going to talk about the method to filter LSA type 5. Let's have a sample network topology.

    Configuration at San Jose Router 

   Configuration at Toronto router

    Configuration at Dubai router


   Configuration at Shri Amritsar Sahib router


Above is the basic configuration on all the routers, now we will going to filter the LSA type 5 with the help of the distribute list

    Configuration on ASBR router ( Dubai Router)-Distribute List

The summary-address command along with the not-advertise keyword can be used on the ASBR under the Router OSPF process to stop the propagation of a Type 5 LSA. On Dubai, the summary-address command has been used to stop the advertisement of the 172.16.1.0 Type 5 LSA.

    Configuration on ASBR router (Dubai Router)-Summary Address


While redistributing an IGP into OSPF, a route-map can also be used to stop the TYPE 5 LSA from being generated on the ASBR. On Dubai, while redistributing from EIGRP, a route-map is used to deny the 192.168.1.0 subnet from being advertised into OSPF

    Configuration on ASBR router (Dubai Router)-Route Map


You can check now by using the command “show ip ospf database external” to confirm whether the LSAs that were filtered have indeed been blocked.