From: Howard C. Berkowitz (hcb@gettcomm.com)
Date: Wed Jul 07 2004 - 15:38:17 GMT-3
At 7:23 PM +0100 7/7/04, Pierre-Alex Guanel wrote:
>How does OSPF add the cost of the links?
>
>>From what I have seen, it does not seem like the cost in the packet gets
>updated from hop to hope.
>
>The process of flooding just passes the original lsa along, unaltered.
>
>>From what I read, the OSPF algorithm knows how to find all the costs and add
>them up from the LSA using the Djakstra algorithm.
>
>Yes I know its all explained in RFC 2178 ...section 16, but I am looking at
>a simple explaination.
>
>thanks,
>
>Pierre
Let's keep this at the intra-area case at the moment, which is all
the Dijkstra algorithm cares about. Link state protocols do not
transmit routes, but router links (and a list of subnets connected to
them) and network/subnet links (with a list of router interfaces
connected to them).
The Dijkstra algorithm builds a tree, matching up the
networks-with-routers and the routers-with-networks. Each time it
steps closer to a destination, it picks the lowest interface cost at
that hop. When the final best paths are selected, OSPF adds up the
costs along the path, and then sends the route to the routing table
installation process. The intra-area routes are never passed to other
routers during flooding. Route metrics are computed independently by
each router, from its own position in the topology.
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