A 7705 SAR performs different functions based on its position in an LSP—ingress, egress, or transit—as described in the following list:
ingress Label Edge Router (iLER) — The router at the beginning of an LSP is the iLER. The ingress router encapsulates packets with an MPLS header and forwards the packets to the next router along the path. An LSP can only have one ingress router.
Label Switching Router (LSR) — An LSR can be any intermediate router in the LSP between the ingress and egress routers, swapping the incoming label with the outgoing MPLS label and forwarding the MPLS packets it receives to the next router in the LSP. An LSP can have 0 to 253 transit routers.
egress Label Edge Router (eLER) — The router at the end of an LSP is the eLER. The egress router strips the MPLS encapsulation, which changes it from an MPLS packet to a data packet, and then forwards the packet to its final destination using information in the forwarding table. An LSP can have only one egress router. The ingress and egress routers in an LSP cannot be the same router.
A router in a network can act as an ingress, egress, or transit router for one or more LSPs, depending on the network design.
Constrained-path LSPs are signaled and are confined to one Interior Gateway Protocol (IGP) area. These LSPs cannot cross an autonomous system (AS) boundary.
Static LSPs can cross AS boundaries. The intermediate hops are manually configured so that the LSP has no dependence on the IGP topology or a local forwarding table.