ospf area network types

Unanswered Question
Oct 2nd, 2008

Hi, I have been reading a little on ospf, what area types are there, ie NSSA, SA, etc, what does this mean to ospf, is there any config difference on the router and what benefit does it have ?

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satish_zanjurne Thu, 10/02/2008 - 21:15

Hi,

1.Area concept of OSPF , is integrated as part of scalability & faster convergence feature of routing protocol.

2.As your network grows, the single change in network causes all routers to rerun the SPF algorithm & causing delay in convergence.

3.If you add more areas in the Network either geographically or logically , the SPF calculations are restricted to the area's & ABR area border routers advertise summary across area's resulting in faster convergence & high scalability.

4.Types of area's are

Backbone area Area 0

Stub Area

Totally Stubby

Not So Stubby

5.All these areas's are either some types of Link State Advertisements ( LSA ) & don't take other LSA, you need to decide which type of area you want as part of design

5.There is config differernce of few commands for every type of area.

6.Overall benefit is faster convergence & scalability

refer below links for more clarification

http://cisco.com/en/US/tech/tk365/technologies_q_and_a_item09186a0080094704.shtml

http://cisco.com/application/pdf/en/us/guest/tech/tk480/c1550/ccmigration_09186a0080187c6d.pdf

HTH..rate if helpful...

carl_townshend Fri, 10/03/2008 - 01:25

Hi there

Thanks for that

another question about ospf,

If my router has a link failure, does my router send an update to the DR, does thnis then send info out to all routers ?

with this information does every router have to re run the SPF alogorithm ?

carl_townshend Fri, 10/03/2008 - 03:04

what are they doing when running this algorithm? what are they looking to do? are they trying to find another route to the network? if so, shouldnt ospf already know about this other route?

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.

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