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Question about FIB table

Which statement is true about the FIB table?

a) The FIB is derived from the IP routing table and is optimized for maximum lookup

throughput.

b) When the FIB table is full, a wildcard entry redirects traffic to the Layer 3 engine.

The Bosson test engine says that it's only Answer A is ok, but i say A and B are correct.

what do you think ?

Thanks !

3 REPLIES
Hall of Fame Super Bronze

Re: Question about FIB table

It has to be A because B makes no sense.

When the FIB table is full, assuming we are talking about hardware assisted platforms, traffic will be forwarded in software. There isn't any wildcard entry for redirection, traffic is simply punted to the CPU.

Cisco Employee

Re: Question about FIB table

Edison,

I respectfully disagree. Several sources on CEF insist that if the TCAM is full, a wildcard entry is installed in the place of all FIB entries that did not make it into the TCAM, and this wildcard entry causes the packets to be processed in software. For example, this is how CCNP Networking Academy curricula put it.

This fact seems to be corroborated also by the following document:

http://www.cisco.com/en/US/tech/tk827/tk831/technologies_tech_note09186a0080094b27.shtml

Note: In the rare case in which the FIB table is full, the wildcard entry           is still present but, instead of dropping packets that match it, they are           forwarded to the MSFC2. This only occurs if you have more than a 256K prefix in           the FIB and if you cannot store the complete routing table and ARP adjacency in           the FIB. You then need to have the default mechanism sent to MSFC2 since the           MSFC2 can have a routing entry that is not present in the FIB.

Best regards,

Peter

Hall of Fame Super Bronze

Re: Question about FIB table

I stand corrected, thanks Peter

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