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OSPF update flooding

Hi Everyone,

My question has to do with OSPF and the flooding of LSA's. With redundant links in an area how does OSPF control the flooding of LSA?s to make sure that there are no unnecessary loops of updates? I have heard it sets the TTL field however I am not sure if this is true and if it does how does it know the right hop count?

Thanks in advance

Graham

2 REPLIES

Re: OSPF update flooding

Graham,

Have a look at this PDF attachment that breaks down the whole process. As you can see it's not a Cisco link but it's quite comprehensive.

http://www.cs.princeton.edu/~jrex/teaching/spring2005/reading/shaikh01.pdf

HTH

Sundar

Hall of Fame Super Silver

Re: OSPF update flooding

Graham

In addition to the link from Sundar I would offer these comments. OSPF routers flood LSAs to their neighbors. Neighbors are by definition on connected interfaces. Therefore neighbors are always 1 hop away. I believe that it is true that the OSPF packets have TTL set to 1 since they are intended to stay on the local link and not be forwarded. It is also true that the multicast addresses used by OSPF are in a range of addresses intended for link local usage and are not intended to be forwarded to any other link.

If there are redundant links there will be LSAs on each link. There is no such thing as a loop for LSAs (this is particularly true since LSAs are not forwarded and if they are sent out an interface and do not go beyond that link there can not be a loop). The question of possible loops in OSPF is solved in the SPF algorithm which takes all the LSAs and builds a topological map of the network. The building of the map detects any possible loops and resolves them.

HTH

Rick

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