Tuesday, November 25, 2008

What is Forwarding Equivalence Class ?

FEC is forward equivalence class which means providing the same type of behaviour to reach the destination. What does the same behaviour means? Actually when the packet comes to router following steps need to be made for its forwarding
Step 1:- Forward the packet to the same next-hop. It is like the same as in fast switching. (Fast switching uses interrupt based switching).
Step 2:- Determine the outgoing interface & all packets should be forwarded out to the same interface. (Please check the Cef Load Balancing Problem)
Step 3:- The same type of treatment should be given to all packets. Treatment means the queuing behaviour like fifo etc.
The above steps will be performed at each and every hop of the router. No issue still the router is using the interrupt context switching (cache switching) but still it has to make the forwarding decision on the basics of the steps which requires lot of cpu processing.
Now you know about the basics of FEC but it is the forwarding is same in case of MPLS. The answer is definitely no. Let’s check what happens in case of MPLS.
In MPLS path is labelled switched. So when the packet enters the router the same above mentioned steps performed but at the next hop router only the packet is switched from one interface to another. It means the core is able to switch the packets and FEC will be performed at the ingress of the router. So the control plane will be at the PE router & forwarding plane at the core. Control plane of router should be kept in mind while desiging the edge routers.

shivlu jain

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