Routers in an LSP

Each router in an LSP performs one of the following functions:

• Ingress router—The router at the beginning of an LSP. This router encapsulates IP packets with an MPLS Layer 2 frame and forwards it to the next router in the path. Each LSP can have only one ingress router.
• Egress router—The router at the end of an LSP. This router removes the MPLS encapsulation, thus transforming it from an MPLS packet to an IP packet, and forwards the packet to its final destination using information in the IP forwarding table. Each LSP can have only one egress router. The ingress and egress routers in an LSP cannot be the same router.
• Transit router—Any intermediate router in the LSP between the ingress and egress routers. A transit router forwards received MPLS packets to the next router in the MPLS path. An LSP can contain zero or more transit routers, up to a maximum of 253 transit routers in a single LSP.

A single router can be part of multiple LSPs. It can be the ingress or egress router for one or more LSPs, and it also can be a transit router in one or more LSPs. The functions that each router supports depend on your network design.

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