Riverstone Networks WICT1-12 17.1 MPLS ARCHITECTURE OVERVIEW

17-2 Riverstone Networks RS Switch Router User Guide Release 8.0

MPLS Architecture Overview MPLS Configuration

17.1 MPLS ARCHITECTURE OVERVIEW

A forwarding equivalence class (FEC) is a group of IP packets that are forwarded over the same path with the same

forwarding treatment. Examples of FECs include:

•unicast packets whose destination address matches a specified IP address prefix

•unicast packets whose destination address matches a specified IP address prefix and whose type of

service (ToS) bit matches a specified value

•multicast packets with the same source and destination addresses

In “traditional,” non-MPLS networks, each router maps a packet to an FEC based on the destination IP address in the

packet’s network layer header. Each FEC is then mapped to a next hop. At each hop, the packet is examined and

assigned an FEC and a next hop.

In an MPLS network, packets are forwarded in one direction across a label switched path (LSP), as shown in

Figure17-1. Labels are the mapping of network layer routing to data link layer switched paths. With MPLS, the

assignment of a specific packet to an FEC is only done once—at the first router in the path, the ingress label switching

router (LSR). Only the ingress LSR1 in the path needs to analyze the layer 3 header information. If the destination

address in the incoming packet matches an entry in the routing table, the appropriate label is applied to the packet and

the packet is forwarded on to the next hop. At subsequent hops, intermediate or transit LSRs in the path need to only

look at the packet label to forward the packet through the MPLS network; no further examination of the packet’s

network layer header is required.

Figure 17-1 MPLS label switched path

At the egress LSR at the other edge of the path, the label is stripped off. The packet is then forwarded to its next

destination using information in the IP forwarding table. There is one ingress router and one egress router for each LSP.

Note that a router cannot be both an ingress and egress LSR for the same LSP.

An MPLS label is a 20-bit integer between 0 and 1048575 that identifies a particular FEC. The label is encapsulated

in the packet’s layer-2 header, as shown in Figure17-2.

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Ingress Egress to packet

incoming Label

label assigned label switched label removed

destination

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MPLS Network

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Label

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Transit Transit