cef and routing table

Answered Question
Dec 6th, 2008
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hi every body!

I read that routing table is copied into fib in CEF.

let say routing table has following entry


s 10.10.10.0 via 11.11.11.0

D 11.11.11.0 via 199.199.199.0 s0.

My question is if this entry is also present in fib, would switch cef- switch a packet destined to 10.10.10.1 or packet would be forwarded to layer 3 engine(essentiaaly router)?

thanks alot!

Correct Answer by Edison Ortiz about 8 years 4 months ago

It will be CEF-Switched as well but it's a recursive route.


Rack1R4#sh run | i ip route

ip route 10.10.10.1 255.255.255.255 31.3.0.1


Rack1R4#sh ip route | i 31.3.0.0

R 31.3.0.0 [120/5] via 148.1.0.3, 00:00:00, Serial0/0.401


Rack1R4#sh ip cef 10.10.10.1

10.10.10.1/32, version 97, epoch 0, cached adjacency to Serial0/0.401

0 packets, 0 bytes

via 31.3.0.1, 0 dependencies, recursive

next hop 148.1.0.3, Serial0/0.401 via 31.3.0.0/16


Rack1R4#sh ip cef 31.3.0.1

31.3.0.0/16, version 31, epoch 0, cached adjacency to Serial0/0.401

0 packets, 0 bytes

via 148.1.0.3, Serial0/0.401, 0 dependencies

next hop 148.1.0.3, Serial0/0.401

valid cached adjacency


Excellent question, Sarah !


__


Edison.

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royalblues Sat, 12/06/2008 - 21:52
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The CEF FIB table is built with the help of routing table. It holds the next-hop information.

In addition CEF also builts a adjacency table which holds the necessary layer 2 information (encap, mac-address etc) which would be required for sending the packet using that route.


Hence when a packet arrives, the FIB is used to check the next-hop information and then using the adjacency table the L2 packet rewrite is performed and the packet is forwarded.


HTH

Narayan


Narayan

Correct Answer
Edison Ortiz Sat, 12/06/2008 - 21:57
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It will be CEF-Switched as well but it's a recursive route.


Rack1R4#sh run | i ip route

ip route 10.10.10.1 255.255.255.255 31.3.0.1


Rack1R4#sh ip route | i 31.3.0.0

R 31.3.0.0 [120/5] via 148.1.0.3, 00:00:00, Serial0/0.401


Rack1R4#sh ip cef 10.10.10.1

10.10.10.1/32, version 97, epoch 0, cached adjacency to Serial0/0.401

0 packets, 0 bytes

via 31.3.0.1, 0 dependencies, recursive

next hop 148.1.0.3, Serial0/0.401 via 31.3.0.0/16


Rack1R4#sh ip cef 31.3.0.1

31.3.0.0/16, version 31, epoch 0, cached adjacency to Serial0/0.401

0 packets, 0 bytes

via 148.1.0.3, Serial0/0.401, 0 dependencies

next hop 148.1.0.3, Serial0/0.401

valid cached adjacency


Excellent question, Sarah !


__


Edison.

sarahr202 Sun, 12/07/2008 - 05:35
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Thanks alot Edison for your reply and kind remarks!

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