Unanswered Question
Jul 4th, 2007

Could you explain me how different method between FIB and normal routing table, thanks.

I have this problem too.
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pccthailand Wed, 07/04/2007 - 02:02


I have read this doc but still confuse. FIB contain the similar information like routing table such as prefix, subnet-mask, next-hop and outgoing interface but i'm sure what the different between them about information.

Anyone can explain me ?


mohammedmahmoud Wed, 07/04/2007 - 02:51


In brief:

CEF overcomes the main shortcoming of fast switching. CEF optimizes the route lookup process by using a construct called the Forwarding Information Base (FIB). The FIB contains information about all the known routes in the routing table. Rather than use a table that is updated when new flows appear, as did Cisco's earlier fast-switching technology (using only the routing table and flow cache), CEF loads FIB entries as routes are added and removed from the routing table (prebuild - won't wait for a flow). CEF does not require the first packet to a destination to be process switched, and allows much more effective load balancing over equal-cost routes.

When a new packet arrives, CEF routers first search the FIB. Cisco designed the CEF FIB structure as a special kind of tree, called an mtrie, that significantly reduces the time taken to match the packet destination address to the right CEF FIB entry as compared to the routing table. The matched FIB entry points to an entry in the CEF adjacency table. The adjacency table lists the outgoing interface, along with all the information needed to build the Data Link header and trailer before sending the packet. When a router forwards a packet using CEF, it easily and quickly finds the corresponding CEF FIB entry, after which it has a pointer to the adjacency table entry, which tells the router how to forward the packet.


Mohammed Mahmoud.


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