Describe the efficiency gains of MPLS in terms of forwarding

Describe the efficiency gains of MPLS in terms of forwarding equivalence classes (FECs). What is a practical analogy that describes an FEC?

An important concept to MPLS is the Forwarding Equivalence Class (FEC),

and it’s something new to the technology struggle to understand.

FECs and their role in MPLS:

An FEC is a set of packets that a single router:

(1) Forwards to the same next hop;

(2) Out the same interface; and

(3) With the same treatment (such as queuing).

Every router performing generic IP forwarding determines

the next hop to which the packet is to be forwarded, the interface out which

the packet is sent to get to that next hop, and how to queue the packet for

that interface. But we don’t often hear those very basic procedures presented

as “determining what FEC a packet belongs to.”

FECs are almost always discussed when introducing the fundamental

concepts of MPLS. The reason for this is that understanding how a packet’s FEC is

determined at an MPLS Label Switching Router (LSR) goes a long way toward understanding

MPLS itself.

In an MPLS network the FEC is determined only once, at the ingress to an LSP, rather

than at every router hop along the path.

MPLS is a technology that pushes the “intelligence”

to the edge of the network, leaving the core to do simple switching. In other words,

the network control plane is located at the edge and the forwarding plane is in the center.

MPLS, a packet switching protocol developed by the IETF. Initially developed to improve

switching speed, other benefits are now seen as being more important.

MPLS adds a 32- bit label to each packet to improve network efficiency and to enable routers

to direct packets along predefined routes in accordance with the required quality of service.

The label is added when the

packet enters the MPLS network, and is based on an analysis of the packet header. The label

contains information on the route along which the packet may travel, and the forwarding

equivalence class (FEC) of the packet. Packets with the same FEC are routed through the

network in the same way.

Routers make forwarding decisions based purely on the contents of the label. This simplifies

the work done by the router, leading to an increase in speed. At each router, the label is

replaced with a new label, which tells the next router how to forward the packet. The label

is removed when the packet leaves the MPLS network.