OSPF Secondary Route

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Sep 23rd, 2007
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Hi Experts,

i want to know does ospf precalculates the secondary path in advance. like EIGRP when primary path fails in Eigrp it takes the fessible successor as a next best path for routing.

so does this is supported in ospf????????

Thanks in Advance


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paul.matthews Sun, 09/23/2007 - 23:39
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No, it does not. In that respect OSPF is totally reactive, but bear in mind we are comparing two totally different protcols.

EIGRP has two features that come into play here - the feasible sucessor and the query mechanism,, but they sit on top of a distance vector mechanism where the effect of an event is for the attached routers to update their routing tables and propagate that to their neighbours.

OSPF is link state so on the change of a link, the LSA is updated and that is propagated while routers are updatng their routing tables, with each router running the SPF calculation separately.

Depending upon your point of view, EIGRP plans ahead, but hopes that it is prepared for the changes, where OSPF waits till it knows what has actually hapenned.


Kevin Dorrell Mon, 09/24/2007 - 01:46
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You are right that EIGRP pre-calculates a secondary path in advance. However, it is not always guaranteed to find one that would be acceptable in all conditions. (See discussions about the feasability condition.) In that case, when something happens in the network, EIGRP could spend some time asking its neighbors what happened, ("going active") before it can work out a suitable alternate route.

OSPF, on the other hand, does not pre-calculate an alternate route. Instead, it keeps a database holding all the information it can possibly get about the whole network area. In this way, when something does happen, it already has all the information it needs to calculate the alternate route.

Kevin Dorrell



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