Dell PowerConnect J-8216 Software Guide for JUNOS version 10.3 - Page 3141

Chapter 120: MPLS-Overview The pop operation removes a label from the beginning of the packet. Once the label is removed, the TTL is copied from the label into the IP packet header, and the underlying IP packet is forwarded as a native IP packet The swap operation removes an existing MPLS label from an IP packet and replaces it with a new MPLS label, based on the following: • Incoming interface • Label • Label forwarding table Figure 82 on page 3069 shows an IP packet without a label arriving on the customer-edge interface (ge-0/0/1) of the ingress PE switch. The ingress PE switch examines the packet and identifies that packet's destination is the egress PE switch. The ingress PE switch applies label 100 to the packet and sends the MPLS packet to its outgoing MPLS core interface (ge-0/0/5). The MPLS packet is transmitted on the MPLS tunnel through the provider switch, where it arrives at interface ge-0/0/5 with label 100. The provider switch swaps label 100 to label 200 and forwards the MPLS packet through its core interface (ge-0/0/7) to the next hop on the tunnel, which is the egress PE switch. The egress PE switch receives the MPLS packet through its core interface (ge-0/0/7), removes the MPLS label and sends the IP packet out of its customer-edge interface (ge-0/0/1) to a destination that is beyond the scope of the tunnel. Figure 82: MPLS Label Swapping Figure 82 on page 3069 shows the path of a packet as it passes in one direction from the ingress PE switch to the egress PE switch. However, the MPLS configuration also allows traffic to travel in the reverse direction. Thus, each provider edge switch operates as both an ingress switch and an egress switch. Ultimate and Penultimate Hop Popping J-EX Series switches support ultimate and penultimate hop popping (that is, popping off the MPLS label) as follows: • With circuit cross-connect (CCC), ultimate hop popping (UHP) is enabled by default and label 0 (IPv4 Explicit Null Label) is advertised. With UHP, the egress PE switch is responsible for popping the MPLS label at the termination of the CCC. • With IP and MPLS, penultimate hop popping (PHP) is enabled by default. With PHP, the penultimate provider switch is responsible for popping the MPLS label and forwarding the traffic to the egress PE switch. The egress PE switch then performs an IP route lookup and forwards the traffic. This reduces the processing load on the egress PE switch, because it is not responsible for popping off the MPLS label. 3069