OSPF Type 4 LSA origin

mimullin · ‎09-04-2007

I'm reading through the CCNP BSCI official Prep Guide (Latest Version) and it states that the type 4 LSA is produced by the ASBR. The CCNP BSCI Exam Cert Guide (Self-Study 3rd edition) states that the type 4 LSA is sent from the ABR to the ASBR. So where is the type 4 LSA generated and TO where is it sent? I have two official exam prep books written by the same author (Clare Gough) that contradict each other. According to the standard both type 3 and 4 LSAs are originated at the ABR (RFC 2328 - 12.1.4).

Thanks

arun kumar · ‎09-04-2007

In OSPF, any router that injects external routing information into OSPF is considered to be an ASBR and the routes are called external routes (E1 or E2). i.e. in OSPF apart from the routes originated by putting network command, others by "redistribute static or connected".

router ospf 1

router-id 11.11.11.11

network 10.0.0.0 0.0.0.255 area 0.0.0.0

redistribute static subnets

ip route 124.0.0.0 255.0.0.0 PO1/0.

In the above example, 10.0.0.0/8 is considered to be originated by OSPF itself (because of the network command) and 124.0.0.0/8 is considered external since its injected into OSPF by some means. So the router 11.11.11.11 is considered to be an ASBR since it injects external routing information to OSPF and 10.0.0.0/8 is considered to be external. Thus Type 4 LSA is generated by an ASBR indicating that it has an information about external routes and propagating those information to the rest of OSPF. Type 5 LSA in this case is 124.0.0.0/8.

Type 4 LSA provides the information to OSPF routers how to reach an external route.

So Type 4 LSA is generated by ASBR and always a host route (/32).

Type 3 LSA is used to summarize between ABR boundaries of two areas and generated by an ABR.

HTH

mimullin · ‎09-04-2007

okay - not trying to be arguementative, only looking to clarify. RFC 2328 says the type 4 LSA is originated at the ABR (12.1.4) and the latest exam cert guide says that type 4 LSAs show up in the routing table as O IA and are used to advertise the location of the ASBR, not E1 or E2 routes. And wouldn't the LSA for the redistributed route above be a type 5 LSA, which would show up in the routing table as an E1 or E2(by default)?

Example:

router ospf 1

network 10.0.0.0 0.0.255.255 area 0

network 11.0.0.0 0.0.255.255 area 1

redistribute rip metric-type 1 subnets

router rip

version 2

network 172.16.0.0 255.255.0.0

show ip route ospf would show the redistibuted RIP routes in the routing table as type 1 external routes (E1) and this would be displayed in the LSDB under the type 5 LSA.

I've just seen documentation that states the type 4 LSA is generated from the ASBR and the ABR. The standard and one of the study guides says it comes from the ABR. Another study guide says it is produced by the ASBR.

Thanks for the quick reply and sorry to keep pressing the issue.

mohammedmahmoud · ‎09-04-2007

Hi Mike,

I've been through this same road long ago :) and the final result to reach is a type 4 LSA describes the route to an ASBR in another area and is created by the ABR of the area containing the ASBR, i am not near a lab now, but you can test it.

HTH,

Mohammed Mahmoud.

mheusing · ‎09-05-2007

Hi,

As already said in a post, an ABR will create a LSA type 4. To clarify the situation further: why is a LSA4 needed and what is the information content?

To understand the need for LSA4 one should look first at LSA5. LSA5 is created by an ASBR (redistribution router) to announce external networks. The router ID found in LSA5 is the router ID of the ASBR. LSA5 are flooded throughout all normal OSPF areas unchanged. Assume an ASBR 1.1.1.1 in area 1; all routers in area 1 will know from the router LSAs (LSA1) where the router 1.1.1.1 is found in the topology and hence can insert the external network announced into the routing table with a proper next hop.

If the LSA5 is flooded into area 0 through an ABR 2.2.2.2 the router ID in LSA5 is still that of the original ASBR (1.1.1.1).

In area 0 the router ID 1.1.1.1 is not known and thus every router in area 0 needs an additional information: how can I reach router 1.1.1.1? This is exactly answered by LSA4. The ABR 2.2.2.2 will create LSA4 with the information: "Any traffic towards router 1.1.1.1 can be sent through me (2.2.2.2)".

It is only then that the area 0 routers can insert the external network into their routing tables with a proper next hop (towards the ABR).

So in brief: an ASBR creates LSA5 with their router ID to announce external networks; an ABR creates LSA4 when forwarding LSA5 (unchanged) into another area to to tell the other area routers how to reach the ASBR.

Hope this helps! Please rate all posts.

Regards, Martin