OSPF LSA Type 4

[Yair Sofee]

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Hey guys,

I've been diving deep into OSPF recently and found myself lost when it comes the LSA Type 4. Why do routers in other areas need to know the ASBR's RID? Why isn't the RID of the ABR advertised in the LSA Type 5 instead of the ASBR'S? In the end all traffic going to the ASBR would first travel through the ABR of the same area so it should be sufficient for the routers to know that if they want to get to an external route they can travel to the ABR of the area where the ASBR resides and then the ABR would take care of the traffic. Also I would love to know what is the content of the LSA Types 3, 4 and 5 I can't seem to find it anywhere online. Thanks!

[balaji.bandi · Accepted solution]

there is a good explanation with example: very useful.

https://community.cisco.com/t5/networking-documents/ospf-lsa-5-and-lsa-4/ta-p/3292502

[Giuseppe Larosa · Accepted solution]

Hello @Yair Sofee ,

in OSPF there is the concept of owner / originator of each LSA data structure. The owner or originator of an LSA type 5 is the ASBR that injected it into the OSPF domain.

The LSA type 5 is propagated unchanged to area 0 and to to all non stub areas.

So what is the meaning and usage of LSA type 4 ?

It provides a validation that the ASBR RID is actually acting as ASBR.

An OSPF router in the same area of the ASBR can examine the ASBR Router LSA and can find a flag bit  E set meaning the device is acting as an ASBR.

This check cannot be performed by devices in other areas as the ASBR Router LSA is limited to the original area.

So the LSA type 4 provides a way for all OSPF routers in the domain to control if an LSA type 5 can be used as the corresponding LSA type 4 is present if at least one prefix is injected in OSPF domain.

Hope to help

Giuseppe

[Yair Sofee]

But why is the LSA type 5 propagated unchanged? I knew it happens but logically it makes more sense to change the RID of the source router to the ABR and that way LSA Type 4 is rendered useless and we still get the same result.

[Giuseppe Larosa]

Hello @Yair Sofee ,

the LSA type 5 is propagated unchanged among areas for a protocol design choice of the protocol designers.

See RFC 2328 OSPFv2

I can agree that other choices could be possible, but the current implementations follow the standard as thought by OSPF protocol designers .

Hope to help

Giuseppe

[Martin L]

Book Routing TCP/IP part 1 by Jeff Doyle has detailed info about insides of OSPF LSA packets.