Loop Preventative in Eigrp and Ospf

[nivetha01]

Hi All,

Can anyone let me know the loop prevention mechanism in eigrp and ospf?

[Richard Burts]

Each of the protocols has its own mechanism to prevent loops. With OSPF it is pretty simple - as a link state protocol OSPF uses LSA to build a map of all of the links within the area and based on this map OSPF can detect and prevent any loop within the area. EIGRP is somewhat more complex in its loop prevention mechanism. EIGRP uses the DUAL mechanism and the feasibility test to prevent loops.

HTH

Rick

[pascalfr0]

I don't know EIGRP so I can't say anything about it.

For OSPF, each node builds the same representation of the network based on the LSA it receives. This representation is a graph (see http://en.wikipedia.org/wiki/Graph_(mathematics)), where LSA are used to create nodes/vertices (routers and networks) and edges (costs to go from one node to another).

Each router performs dijkstra algorithm  to find out  the shortest path from itself  to every other destination in the network.

As Dijsktra algorithm is in fact the determination of an optimal tree where the router is the root and every destination is a leaf, with optimal root to leaf cost, there is by essence no possibility for any loop.

So, by essence, OSPF won't ever allow for a loop.

Beware the redistribution policies you configure at the boundaries of the OSPF cloud though. Loops could occur through hasardous redistribution at the boundaries of the OSPF domain I guess, but never inside, OSPF design being what it is.

[Joseph W. Doherty]

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Posting

I just wanted to add, both EIGRP and OSPF will compute a loop free topology, as the other posters have noted.  However, it's possible to have transient loops when there's a change in the network topology and all the routers have not yet converged on an accurate representation of it.