Reason of OSPF multi area

[acbenny]

Hi all,

What is the advantage to perform multi-area in ospf?

Suppose there are 100 router in Area 0 and 100 router in Area 1

which these areas are connceted by 2 ABR via Fast Ethernet

I just know if performing route summarization in both ABR router. If it can reduce the routing table of each side.

As a result to reduce CPU and memory. Any more reason ?

[Richard Burts]

Jack

There are multiple aspects that are benefits of running multi-area OSPF. In addition to what you have listed is that when any link within an area changes state, it generates a type 1 and/or type 2 LSA and ALL routers within that area must calculate a full SPF. When a link in another area changes state the router will receive a type 3 LSA and must process it but it does not require a full calculation of the SPF. So the impact of a link state change is reduced when you run multi-area OSPF.

HTH

Rick

[acbenny]

Do you mean that let's say if any network changes in Area 0, then all routers in Area 0 will calculate FULL SFP, but routers in Area 1 no need calculate FULL SFP of Area 0 on this event. If so, why it will be that ?

[Richard Burts]

Jack

Yes I do mean that if a link changes state in area 0 that all routers in area 0 must calculate the full SPF and that routers in area 1 will not need to calculate the full SPF.

The reason for this is that a link state change in area 0 changes the topology map for area 0 and all area 0 routers must calculate the full SPF to get the correct topology map. But since the link state change in area 0 does not change the topology map of area 1 that routers in area 1 do not caluclate the full SPF. The routers in area 1 must process the type 3 LSA about the change in area 0 but that does not require a full SPF calculation.

HTH

Rick