LSA type 4 not present

[Jonn cos]

Hi all experts.

I labbed the following scenario

R1----------------------------------R2-------------------------------------R3->lo0 (3.3.3.3/8)

All 3 routers are running ospf area 1 (single area topology). Now from R3 when i redistribute lo0, i can surely see the route O E2 on R1, but what i dont get is the following

R1#sh ip os database

            OSPF Router with ID (10.0.0.1) (Process ID 100)

                Router Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum Link count
10.0.0.1        10.0.0.1        260         0x80000002 0x006C90 1
11.0.0.2        11.0.0.2        198         0x80000003 0x00E9D8 2
11.0.0.3        11.0.0.3        198         0x80000003 0x007C71 1

                Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
10.0.0.1        10.0.0.1        260         0x80000001 0x009873
11.0.0.3        11.0.0.3        199         0x80000001 0x007D85

                Type-5 AS External Link States

Link ID         ADV Router      Age         Seq#       Checksum Tag
3.0.0.0         11.0.0.3        213         0x80000001 0x004B44 0

Where is type 4 LSA ?

Kindly guide me

[Jonn cos]

Sorry, my bad. I just read the RFC and here is what i found

Summary-LSAs are the Type 3 and 4 LSAs.  These LSAs are originated
    by area border routers. Summary-LSAs describe inter-area
    destinations.

So in my case, since i didnt had any ABRs, thats why no type 4 lsa was present.

Kindly confirm me

[Peter Paluch · Accepted solution]

Hello Jonn,

Yes, you are correct. LSA-4 are created only by ABRs when external routes (contained in LSA-5) originated in a particular area are being injected to a different area. Within the same area where the LSA-5 are originated, no LSA-4 are needed.

The reason for existence of LSA-4 is somewhat convoluted. The LSA-5 describes an external network and it represents it as being directly connected (or at least directly reachable) through the ASBR which originated the LSA-5. This is done by the LSA-5 referring to the OSPF Router ID of the ASBR that originated it. Now, the area where the ASBR is located knows where to find that ASBR because it knows its LSA-1, so every router in the area can find a route to the ASBR, and therefore, through it, to the external networks it advertises. So here, no LSA-4 is necessary.

However, the LSA-1 from an area are not flooded to another areas while LSA-5 are. In a different area, the ASBR that the LSA-5 refers to is unknown because its LSA-1 has not been flooded there. Thus, the LSA-5 is "hanging in the void" because it refers to an non-existent object in the link-state database. This problem is rectified by having the ABR originate the LSA-4 that informs the area about the the existence of the particular ASBR, and the distance from the originating ABR to the ASBR in question. This way, the LSA-5 is valid again as it references an existing object.

I am not sure if I managed to put this in a comprehensive way... please feel welcome to ask further.

Best regards,

Peter

[Jonn cos]

Thanks alot sir. Your replies are always generous and helpful.

Thanks alot again