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1.3.- Forwarding Equivalence Class (FEC)  

For any routing protocol having some opportunity to survive, the scalability problem should be resolved from the beginning. To ensure scalability, flow states should be managed on aggregation and never on individual flows. MPLS ensures scalability supporting the aggregation with what is called Forwarding Equivalence Class (FEC).
The LER is the place where aggregation is completed. LER is responsible for classifying incoming packets and relating them to FECs. Each FEC is associated with an appropriate label and forwarding path. LER uses several modes to classify traffic. For example, using the packet destination adress and port as is indicated in the following table:

When packets leave the LER to go into the MPLS domain they will be forwarded using LSRs. To do this, the LSR looks just for labels on the MPLS packet and matches it with labels within its forwarding table. This forwarding table is called the Label Information Base (LIB). The LSR will push, pop or swap labels and forward packets according with LIB instructions. One representation of such a table is as follows:
 

 
Finally when the packet reaches again another LER to leave the MPLS domain, the LER removes the MPLS header and forward the packet to an IP network.
 
LER performs what is called the initial multi-field (MF) classification. It can map layer-2 to MPLS, MPLS to layer-3 and makes MF classification using a very fine granularity. This classification will decide which IP packets will be converted to MPLS packets and which will traverse the router being untouched.
 
By pushing several labels instead of one, we can create a stack of them, representing each one a network hierarchy. For example, let's suppose a packet enters an MPLS domain. When it enters the first network into the domain, the label 45 is pushed. The packet then travels through the domain using this first label. Routers will forward the packet following instructions given by label 45. Then, somewhere, when the packet enters a second network within the same domain, a new label is pushed, i.e. label 56. Now the packet will be forwarded using a different set of instructions which correspond to the label 56. When the packet reaches the second network frontier, the last LER router pops the label 56 (really penultimate hop is used to do this) and forwards the packet again to the first network using its original label number 45. Here you have a 2-level hierarchy. As you see, the network hierarchy possibilities are endless.
 

   

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