Forwarding Equivalence Class

A Forwarding Equivalence Class (FEC) is a group or flow of packets that are forwarded along the same path and are treated the same with regard to the forwarding treatment. All packets belonging

Forwarding Equivalence Class 31

to the same FEC have the same label. However, not all packets that have the same label belong to the same FEC, because their EXP values might differ; the forwarding treatment could be different, and they could belong to a different FEC. The router that decides which packets belong to which FEC is the ingress LSR. This is logical because the ingress LSR classifies and labels the packets. Following are some examples of FECs:

■ Packets with Layer 3 destination IP addresses matching a certain prefix

■ Multicast packets belonging to a certain group

■ Packets with the same forwarding treatment, based on the precedence or IP DiffServ Code Point (DSCP) field

■ Layer 2 frames carried across an MPLS network received on one VC or (sub)interface on the ingress LSR and transmitted on one VC or (sub)interface on the egress LSR

■ Packets with Layer 3 destination IP addresses that belong to a set of Border Gateway Protocol (BGP) prefixes, all with the same BGP next hop

This last example of a FEC is a particularly interesting one. All packets on the ingress LSR for which the destination IP address points to a set of BGP routes in the routing table—all with the same BGP next-hop address—belong to one FEC. It means that all packets that enter the MPLS network get a label depending on what the BGP next hop is. Figure 2-7 shows an MPLS network in which all the edge LSRs run internal BGP (iBGP).

Figure 2-7 An MPLS Network Running iBGP

Internal BGP

Figure 2-7 An MPLS Network Running iBGP

Internal BGP

Edge LSR

Edge LSR

The destination IP address of all IP packets entering the ingress LSR will be looked up in the IP forwarding table. All these addresses belong to a set of prefixes that are known in the routing table as BGP prefixes. Many BGP prefixes in the routing table have the same BGP next-hop address, namely one egress LSR. All packets with a destination IP address for which the IP lookup in the routing table recurses to the same BGP next-hop address will be mapped to the same FEC. As already mentioned, all packets that belong to the same FEC get the same label imposed by the ingress LSR.

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