From: Waterman, Roy (Roy.Waterman@colt.net)
Date: Sat Dec 27 2008 - 19:22:39 ARST
Hi Gaurav
This sounds like the operation of partial SPF in OSPFv2 to me.
Taken from OSPF & IS-IS: Choosing an IGP for Large-Scale Networks (by
Jeff Doyle) under 8.1 SPF Enhancements
"...This is the basis of Partial Route Calculations (PRC). When a node
advertises the addition, deletion, or metric change of an IP prefix,
rather than unnecessarily rerunning the SPF calculation the other nodes
on the tree simply record the change."
"OSPFv2, on the other hand, incorporates IP address semantics into type
1 and type 2 LSAs. That is, routers or pseudonodes advertise both
topological information and their attached IP addresses in the same
LSAs. So for example, if an IP address attached to an OSPFv2 router
changes, the router must originate a type 1 LSA to advertise the change.
However, because node information is also carried in type 1 LSAs,
flooding the LSA triggers a full SPF calculation in all routers in an
area-even though the address change is irrelevant to the node topology.
As a result, only type 3, 4, 5, and 7 LSAs-whose only purpose is to
advertise prefix information-trigger a PRC in OSPFv2"
So, if by "change in lsa 1 and lsa 2 types only" you take it to mean
change in metric of an IP prefix, then this would result in a full spf
recalculation for OSPFv2.
Note that this applies to OSPFv2 and not OSPFv3 which removes IP
addressing info from Type 1 & Type 2 LSAs.
Now as far as I can tell, there doesn't seem to be a command related to
partial route calculation (prc) in ospfv2, so I imagine that this is
just standard behaviour but someone feel free to correct me here if this
is incorrect.
Hope this helps.
Regards
Roy Waterman
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
GAURAV MADAN
Sent: 27 December 2008 17:23
To: Cisco certification
Subject: ISPF
Hi
I wan just browsing the old archieves on groupstudy when found the
following
: http://www.groupstudy.com/archives/ccielab/200809/msg01414.html
One whos drafted the mail has written that there was a ques in his
workbook which asked :
to make ospf recalculate its area databases only when there is a change
in lsa 1 and lsa 2 types only.
Do you think ispf is correct option for making this work .?
If there is a change in lsa 1 ; i guess full tree need to b recalculated
. Plzz advice
Gaurav Madan
Blogs and organic groups at http://www.ccie.net
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