01-10-2012 01:43 PM - edited 03-07-2019 04:16 AM
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 !
01-10-2012 01:56 PM
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.
01-10-2012 02:32 PM
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
01-11-2012 07:23 AM
I stand corrected, thanks Peter
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