Penultimate hop popping is performed in MPLS-based networks where the router upstream to the Edge LSR removes the top label in the label stack and forwards only the resulting packet (either labeled IP or IP packet) for a particular FEC. This process is signaled by the downstream Edge LSR during label distribution with LDP. The downstream Edge LSR distributes an implicit-null (POP) label to the upstream router, which signals it to pop the top label in the label stack and forward the resulting labeled or IP packet. When the packet is received by the Edge LSR, no lookup is performed in the LIB if the incoming packet is an IP packet. Therefore, penultimate hop popping saves a single lookup on edge routers. The operation of penultimate hop popping is depicted in Figure 1-13.
Figure 1-13. Penultimate Hop Popping
As illustrated in Figure 1-13, the downstream Edge LSR1 distributes an implicit-null label mapping for network 172.16.10.0/24 to its upstream LSR1. Upon receiving a labeled packet, LSR1 pops the top label and sends the resulting IP packet to the Edge-LSR1.
Basic MPLS Configuration
Basic MPLS VPN Overview and Configuration
PE-CE Routing Protocol-Static and RIP
PE-CE Routing Protocol-OSPF and EIGRP
Implementing BGP in MPLS VPNs
Carrier Supporting Carriers
MPLS Traffic Engineering
Implementing VPNs with Layer 2 Tunneling Protocol Version 3
Any Transport over MPLS (AToM)
Virtual Private LAN Service (VPLS)
Implementing Quality of Service in MPLS Networks
MPLS Features and Case Studies