MPLS Fundamentals

Label Distribution

The first label is imposed on the ingress LSR and the label belongs to one LSP. The path of the packet through the MPLS network is bound to that one LSP. All that changes is that the top label in the label stack is swapped at each hop. The ingress LSR imposes one or more labels on the packet. The intermediate LSRs swap the top label (the incoming label) of the received labeled packet with another label (the outgoing label) and transmit the packet on the outgoing link. The egress LSR of the LSP strips off the labels of this LSP and forwards the packet.

Consider the example of plain IPv4-over-MPLS, which is the simplest example of an MPLS network. Plain IPv4-over-MPLS is a network that consists of LSRs that run an IPv4 Interior ...