[c-nsp] OSPF design

[Mark Tinka]

On Monday, October 25, 2010 04:50:08 pm Rin wrote:

> I agree with Geoff's post that separating network into
> different OSPF areas cannot reduce LSDB size. If we
> separate into different areas, LSA1,2,3 are generated
> and all routers must trigger SPF for a topology change
> inside an area. If we do not separate into different
> areas, only LSA1,2 are generated and all routers must
> also trigger SPF for a topology change inside an area.

This is one of the problems we found with OSPFv2, and one of 
the reasons we transitioned to IS-IS.

OSPFv2 doesn't separate topology and prefix information 
within an area. A change in prefix (not topology) 
information means Type 1 LSA's are generated. But since Type 
1 LSA's also carry topology information, a full SPF must be 
run within the local area. Not very useful if topology 
information remained static and only prefix information 
changed, e.g., metrics.

Of course, OSPFv3 handles this issue in the same way IS-IS 
does, but unless you're planning on running IPv4 in OSPFv3 
(which works in Juniper), that won't help you much.

> According to below statement, iSPF helps each router to
> run SPF only on the changed portion of the topology.
> This means neither separating network into areas nor
> configuring inside an area will benefit from iSPF.
> Correct me if I'm wrong at this.

Well, actual PRC would work for Type 3, 4, 5 and 7 LSA's in 
OSPFv2 as their only purpose is to signal prefix information 
(external areas).

Cheers,

Mark.
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