Apologies for how terrible the Topology is, I took an existing picture and put some purple arrows leading off the ABR’s, indicating that this is actually where Type 4 LSA’s are created.

Now ASBR’s create Type 5 LSA’s which of course are External / Redistributed routes which are shared throughout the entire routing domain across different Area boundaries by ABR’s, however the ASBR does NOT create a Type 4 LSA to go with it, which is the LSA Type that describes how to get back to the ASBR.

Clear as mud? Good. The reason why the ASBR does not send out Type 4 LSA’s is the same concept surround the SPF Tree, that every router worries about its own Area, and let the routers in other Areas make their own dag nab SPF Tree’s.

That being said, when an ASBR is configured to be an ASBR, it flips on a bit in it’s Hello / Update that it is as ASBR with some Type 5 LSA’s, and every router in the Area gets these Type of LSA that is not an ABR will not produce Type 4 LSA’s.

To demonstrate this, I have done “no router ospf 1” on R1 / R2 / R3, and added it back with only the /24 Area 0 network, with R1 doing default-information originate making it an ASBR.

The LSDB from R1 says it all:

R1#sh ip ospf data

            OSPF Router with ID ( (Process ID 1)

                Router Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Link count         294         0x80000002 0x009BDB 1         207         0x80000002 0x005D11 1     861         0x80000003 0x003BB8 1

                Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum     861         0x80000001 0x00E575

                Type-5 AS External Link States

Link ID         ADV Router      Age         Seq#       Checksum Tag     955         0x80000001 0x005594 1

Every router in the Area must match this exact table, and I went to R2 and R3 to verify they do have this exact same Database, I found that there is no Type 4 LSA’s because there are no ABR’s.

There is only a single Type 5 LSA and the Advertising router is R1, along with Type 1 and Type 2 LSA’s that also exist within a single Area. There is no LSA describing how to get back to the ASBR, because when R1 became an ASBR it flipped on a bit in it’s Hello / Update that indicated it’s now an ASBR, and all routers in the single Area installed the Type 5 LSA it is advertising with itself as the Advertising router.

If another Area is added to any router in this Area, it will create a Type 4 LSA to flood into that other Area, describing itself as the path back to the ASBR for that Area.

So say we had 5 routers total, this first is the ASBR, and the other 4 go hop by hop downstream to a destination. R1 being the ASBR and R2 being the first ABR in line, R2 will create a Type 4 LSA to flood into the new Area, saying “I am gateway to the ASBR holding X route your looking for.”

So R3 which is the next hop in line creates a Type 4 LSA, which tells the next that he (R3) is the gateway to the ASBR, when all he knows that R2 is saying he is the Gateway to the ASBR.

This goes on and one through different OSPF Areas, and is why ABR’s create the Type 4 LSA, but the ASBR will still have it installed in its Link State Database, because the SPF Tree Algorithm makes sure all OSPF Routers in an Area have matching LSDB’s.

And that, is the mystery unveiled about LSA Type 4’s. Sorry about the crappy Topology example, I figured it was more in the explanation and LSDB once we cleared out all other Areas and made a single Area 0.

I think that might be it for OSPF review, the rest of the material, I think I’ll risk taking a hit on exam day points, as I this stone has got to keep rolling on 🙂