Forwarding Equivalence Class in MPLS

Forwarding Equivalence Class (FEC) : - As noted in RFC 3031(MPLS architecture), this group of packets are forwarded in the same manner (over the same path with the same forwarding treatment).

In MPLS Domain FEC will not perform on every HOP, It will perform only on Ingress & Egress router in MPLS domain.

But in traditional IP network FEC will perform on every HOP that comes between source & destination.

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.