How Cisco switches use CEF (Cisco Express Forwarding)
CEF is the predominately used method for layer 3 switches nowadays. CEF contains a forwarding information base (FIB) for processing L3 information, and an adjacency table that is used for L2 information. Both of them sit in a high-speed cache that is located in the ASIC hardware of the switch (NOT the IOS Software, the slow layer 3/routing layer)
The FIB contains the entire routing table of the network and the adjacency table contains the entire mac-address-table of the network, thus when a packet enters the layer 3 switch, it first goes to the FIB, the FIB then strips open the packet, matches the destination ip with the ip it has in it's routing table matching it with the mac-address from the adjacency table and because it holds all that info it can manage to directly transfer the packet at wire speed.
This is the key concept and one of the big differences between a switch and a switch using CEF that is able to do all this forwarding between VLAN's to routed ports to other routers, all moving at wire speed. It's one of the main advantages of buying a layer 3 switch.
Exceptions to CEF (Packets that don't qualify):
-Packet with header options
-Packet with TTL expired
-Packets destined to a tunnel interface
-Packets with unsupported encapsulations
-Packets requiring fragmentation (MTU exceeded)
Configuring CEF:
Most layer 3 switches that shift from Cisco support CEF by default (as in you have to turn it off if you don't want to use it because it is 'on' by default).
S2#conf t
S2(config)#ip cef (Turns on CEF on the switch)
CEF: Show commands
To verify if CEF is running:
S2#show ip cef
CEF also lets you monitor traffic/statistics -> Use the ? feature: S2(config)#ip cef ?
example (1) S2#show ip cef vlan 20 (Will show all the cached info in the CEF table)
example (2) S2#show ip cef 172.30.2.0 (to see if CEF has learnt about a particular entry, in this
case 172.30.2.0)
'show ip cef summary' is another handy command to use.
nice and clean explaination!
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