[c-nsp] OSPF design

[Rin]

Dear all, 

Thank you for your replies. 

We use OSPF basically to advertise each router's loopback so that we can
deploy L2 , L3 VPN between routers. There'll be no other external route
advertised into OSPF. Thus, we will not configure summarization on any ABR
router as well as stubby areas. 

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. 

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. 

"OSPF uses Dijkstra's SPF algorithm to compute the shortest path tree (SPT).
During the computation of the SPT, the shortest path to each node is
discovered. The topology tree is used to populate the routing table with
routes to IP networks. When changes to a Type-1 or Type-2 link-state
advertisement (LSA) occur in an area, the entire SPT is recomputed. In many
cases, the entire SPT need not be recomputed because most of the tree
remains unchanged. Incremental SPF allows the system to recompute only the
affected part of the tree. Recomputing only a portion of the tree rather
than the entire tree results in faster OSPF convergence and saves CPU
resources. Note that if the change to a Type-1 or Type-2 LSA occurs in the
calculating router itself, then the full SPT is performed"  (source:
http://www.cisco.com/en/US/docs/ios/12_0s/feature/guide/ospfispf.html) 

>From your advice, I'm more likely to configure those 100 routers inside an
OSPF area now.

The reason why we design OSPF up to UPE devices because we also have FTTH
switches configure as Layer 2, also we can deploy different Layer 3
redundancy techniques such Layer 3 loop prevention, MPLS TE..up to UPE
layer.

Thanks,

 Rin

-----Original Message-----
From: cisco-nsp-bounces at puck.nether.net
[mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of Heath Jones
Sent: Saturday, October 23, 2010 6:05 AM
To: Robert Crowe (rocrowe)
Cc: cisco-nsp at puck.nether.net
Subject: Re: [c-nsp] OSPF design

> Just remember
> that you cannot summarize (today) your main Loopback used for your
> LDP/BGP ID as there needs to be a full LSP from ingress-to-egress PE
> across areas, if you providing L2/L3VPN services.

Is this because the lsp is label in label (outer being pe, inner being
customer route)?
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