What Label Triggers Penultimate Hop Popping?

E.

In MPLS (Multiprotocol Label Switching) networks, the Penultimate Hop Popping (PHP) operation is used to reduce the processing load on the last LSR (Label Switching Router) by having the penultimate LSR remove the topmost label and forward the packet to the final destination. This reduces the load on the last LSR by eliminating the need to process the label stack.

To inform neighboring LSRs to perform the PHP operation, an LSR advertises a special label called the Implicit Null (IMP-null) label or the "implicit discard" label. When an LSR receives a packet with the IMP-null label, it removes the label and forwards the packet to the next hop without forwarding the label to the next hop.

The answer to the question is E. IMP-null, as it is the label advertised by an LSR to inform neighboring LSRs to perform the PHP operation. The other labels listed in the answers are used for different operations in MPLS networks.

A. 0x00 is not a valid MPLS label. B. PHP is not a label, but rather an operation performed by an LSR. C. Swap is a label operation used to replace an incoming label with a new label. D. Push is a label operation used to add a new label to the label stack. E. IMP-null is a label used to inform neighboring LSRs to perform the PHP operation.