MPLS forwarding

As shown in Figure 5, a packet is forwarded over the MPLS network in the following steps:

1. Router B (the ingress LSR) receives a packet with no label. It identifies the FIB entry that matches the destination address of the packet, pushes the outgoing label (40 in this example) to the packet, and forwards the labeled packet out of the interface VLAN-interface 20 to the next hop LSR Router C.

2. When receiving the labeled packet, Router C identifies the LFIB entry that has an incoming label of 40, uses the outgoing label 50 of the entry to replace label 40 in the packet, and forwards the labeled packet out of the outgoing interface VLAN-interface 30 to the next hop LSR Router D.

3. When receiving the labeled packet, Router D (the egress) identifies the LFIB entry that has an incoming label of 50, removes the label from the packet, and forwards the packet out of the outgoing interface VLAN-interface 40 to the next hop LSR Router E. If the LFIB entry records no outgoing interface or next hop information, Router D identifies the FIB entry by the IP header and then forwards the packet according to the FIB entry.