MPLS Penultimate-Hop Popping (PHP)

BC

B: In penultimate-hop popping, the final MPLS label is popped from the IP packet at the last provider router in the network before being forwarded to the PE router.

The PE router receives the packet and checks the IP address, and then the packet is forwarded to its destination.

C: Ultimate-hop popping is disabled by default on LSPs. Penultimate-hop popping is the default behavior.

MPLS (Multiprotocol Label Switching) is a technology used in computer networks to forward traffic between network nodes using labels instead of network addresses. Penultimate-hop popping (PHP) is an MPLS optimization technique where the label of the penultimate (second to last) hop is popped or removed, leaving only the label of the egress (last) router.

The two statements that correctly describe MPLS PHP are:

B. With PHP enabled, the router upstream from the egress router pops the MPLS label.

D. With PHP enabled, the egress router pops the MPLS label.

Explanation:

A. In the Junos OS, PHP is disabled by default.

This statement is not entirely correct. In the Junos OS, PHP is enabled by default for RSVP-TE (Resource Reservation Protocol-Traffic Engineering) LSPs (Label Switched Paths) and BGP (Border Gateway Protocol) labeled unicast routes. However, PHP is disabled by default for other types of LSPs, such as LDP (Label Distribution Protocol) LSPs.

B. With PHP enabled, the router upstream from the egress router pops the MPLS label.

This statement is correct. With PHP enabled, the router upstream from the egress router pops the MPLS label. This means that the penultimate hop router removes the topmost MPLS label before forwarding the packet to the egress router. The egress router can then use the remaining label to forward the packet to the correct destination.

C. In the Junos OS, PHP is enabled by default.

This statement is partly correct. As mentioned earlier, PHP is enabled by default for some types of LSPs in the Junos OS, but not all.

D. With PHP enabled, the egress router pops the MPLS label.

This statement is also correct. With PHP enabled, the egress router pops the MPLS label, which means that the egress router removes the topmost label before forwarding the packet to the final destination. This can help reduce the processing load on the egress router and improve network performance.

In summary, MPLS PHP is an optimization technique that can improve network performance by reducing the processing load on routers. When PHP is enabled, the penultimate hop router or the egress router can pop the MPLS label, depending on the network configuration. In the Junos OS, PHP is enabled by default for some types of LSPs, but not all.