OSPF LSAs / flooding

[carl_townshend]

Hi Gents

Its been a while since I looked into the LSA process, can you remind me of the following?

- On an ABR, if no summarization is done, does it flood all the type 1 LSA's from one area into another? and the way to supress the number of LSA's getting flooded will be to summarize?

- Secondly, on a normal broadcast network where you have a DR/BDR, do the routers only flood the type 1 LSA's to the DR, then the DR floods them to everywhere else?

Cheers

[balaji.bandi]

- On an ABR, if no summarization is done, does it flood all the type 1 LSA's from one area into another? and the way to supress the number of LSA's getting flooded will be to summarize?

BB -  ABR Flood the Type 3 LSA ( this describes as inter-area routes) -

- Secondly, on a normal broadcast network where you have a DR/BDR, do the routers only flood the type 1 LSA's to the DR, then the DR floods them to everywhere else?

BB - Type 1 LSA will learn by DR from other Routers

[Vishnu Vardhan S]

- On an ABR, if no summarization is done, does it flood all the type 1 LSA's from one area into another? and the way to supress the number of LSA's getting flooded will be to summarize?

Ans: The ABR router floods Type 3 LSA, basically it receives Type 1 and Type 2 from one area and converts that to Type 3 and shares to another area. 

- Secondly, on a normal broadcast network where you have a DR/BDR, do the routers only flood the type 1 LSA's to the DR, then the DR floods them to everywhere else?

A normal router send Type 1 LSA to DR and he floods to everyone and DR floods Type 2 to everyone. 

[carl_townshend]

Hi

So was I correct in saying that all LSA's from one area are flooded to another unless summarization is done?

[Giuseppe Larosa]

Hello @carl_townshend ,

LSA type 1 and LSA type 2 have single area scope and are not flooded out the original area.

However, the IPv4 prefixes described in LSA type 1 and LSA type 2 are sent within LSA type 3 in other areas by the ABR(s) between original area and area 0. Remote ABRs then re-generate those LSA type 3 to those areas that are standard or not totally stubby.

Area summarization gives a level of control on what IP subnets are propagated to other areas a summary route can replace multiple component routes in the original area and provides stability to the OSPF domain hiding single IP subnet that is flapping for example.

Hope to help

Giuseppe

[balaji.bandi]

High level kind of - Type 3 LSA also kind of Summary LSA.