CEF Basics

When the packet is being received by the router what it does. Actually it looks for the destination network in the routing table and corresponding to that which next hop is used and which interface is used for outgoing. It means if the next hop is reachable then it will look for the arp entry for the directly connected router and header rewrite process will occur & packet will be forwarded towards the destination. All the packets are fast switched, I will let you know how to check the fast and cef switched packets later on my discussion. So it means on every packet the same process has to be initiated & uses most of the CPU processes and all. To overcome this problem cisco introduces a new switching mechanism that is CEF (Cisco Express Forwarding). CEF maintains two tables

a)      FIB (Forwarding Information Base)

FIB is forwarding information base which is as such the copy of the routing table. Whatever route comes in the routing table a same copy is created in the CEF table and that is known as FIB. So we can say FIB is nothing but a copy of the routing table. With the help of show ip cef you can check the cef table

 

b)      Adjacency Table (Which is used to store arp information)

This is the table which actually stores your outgoing interface with the arp of that interface.

You can check with the help of given command

Show adjacency internal

 

Structure Of CEF

FIB	Adjacency Table

 

                                                                                        

 

So we can say FIB & Adjacency tables are the data structures which are using for handing the information.

 

FIB		Adjacency Table
10.10.10.0	Pointer  à	Next Hop 1.1.1.1,Arp & Outgoing Interface
		Next Hop 2.2.2.2 & Arp & Outgoing Interface

 

Routing table is having entry of 10.10.10.0 with next hop 20.20.20.20 and which is reachable by 1.1.1.1 if this interface is down then the pointer will move towards the 2.2.2.2 so there is no change in the routing table no change in the FIB table only change occurs at the pointer end which actually saves lots of processes and of course calculations. 

regards

shivlu