Help understanding EIGR and OSPF

[comptek]

Where does OSPF and EIGRP look when it has to reconfigure a downed network? Is it the Topology Table? If so, if I were to do a sh ip eigrp topology will that tell me what route the router will use to find a new route? Or is it the Neighbor table sh ip eigrp neighbor. I think I may be over thinking this too much only becaue I have failed the ccna 2 already. If this is too vague I apologize.

Thanks in advance.

Comptek
A+, Network+

 

[lerdalt]

OSPF does not keep track of additional paths if a route goes down. It will have to run the SPF algorithm again to find the best routes.

EIGRP does keep a list of successor routes in the topology table. The neighbor table just is a list of the EIGRP neighbors.

My EIGRP is a little rusty, so I can't explain it too in depth right now.

 

[comptek]

lerdalt,

Thanks for your response, I am going over some online review questions and I dont remember exactly the question but it was something along the lines of "If you were running eigrp/ospf (not sure which one the question was asking) where will the router go to get the next route. I remember seeing a table (route table, or eigrp topology table). But you know what again I think I am over thinking the subject.

Thanks

Comptek
A+, Network+

 

[Cluebird]

EIGRP will initially look in the topology table (sh ip eigrp topology) to see if it has an alternate route called a "feasible successor". If there is no feasible successor, EIGRP will go "active" on the route and send a query to all neighbors to find a usable route. Routers that are queried about a "lost" route either query their neighbors (following split horizon rules) or send a reply with information about the lost route. If there is another path, the original requestor will install that path in the topology table as a successor route (sh ip eigrp top) and then in the forwarding database (routing table (sh ip route)).

OSPF doesn't share routes so any change will require OSPF to update the link-state database through triggered updates and flooded LSAs. Once the LSDB is updated (sh ip ospf database), the router recalculates the SPF and installs routes in the forwarding database (sh ip route).

HTH