OSPF LSA4 Generation

[arakeshr]

Step1- Routes redistributed into Area0 by the edge router which is connected to BGP.
Step2- Now the redistributing router becomes ASBR and generates LSA 5 to the rest of the OSPF topology.
Step3- LSA 5 is received by all routers within area0.
How do these routers within area0 know this is a ASBR router?
coz in LSA5 E bit is set notifying that "im the ASBR, please solve SPT towards me for external routers"
Hence routers within area0 knows who is ASBR and how to reach solving SPT.

Ok now moving on..

Setp5- LSA 5 which was originated by ASBR reaches ABR. As E bit is set, ABR generates LSA4 to notify routers WITHIN AREA1 ONLY.

This is one way of the generation of LSA4, but there is another way.

Say if LSA4 which is generated by ABR is reached towards another ABR in the area, then the receiving ABR generates LSA4 within the area.

To summarize:

LSA 4 is generated if ABR receives LSA5 with E bit set.
LSA 4 is generated if the ABR receives LSA4.

There is lot more to it. I just simplified as much as i can.

Please find a attachment for reference

[Giuseppe Larosa]

Hello @arakeshr ,

it should be a little different

LSA type 5 are domain wide flooded to all standard areas and to area 0

the E flag bit is set in the Router LSA generated by the ASBR node meaning that it is currently injecting in the OSPF domain at least one prefix.

The ABR between area 0 and the area where the ASBR is located generates the first copy of LSA type 4 as a way to propagate the ASBR liveness, in all other areas each router will check if an LSA type 4 exists for the OSPF RID associated to the   the LSA type 5 originator ID,  as an additional check before using the info in the external LSA data structure.

The ABRs connected to area 0 will re-generate the LSA type 4 in each standard area

Hope to help

Giuseppe

[MHM Cisco World]

check below comment 

[MHM Cisco World]

there is NO LSA4 as I guess first time, 
LSA5+LSA4 is only appear in OSPF 
but if there is BGP in-between OSPF then there is only LSA5 there is no LSA4 at all. 
the router that redistribute BGP into OPSF is appear as adv-router in LSA5