Which two tasks must you perform when you implement LDP NSF on your network? (Choose two.)

LDP Non-Stop Forwarding (NSF), also known as LDP Graceful Restart, is a high-availability feature that preserves the MPLS forwarding table (LFIB) during a control plane failure or restart. For LDP NSF to function correctly, it relies on the underlying Interior Gateway Protocol (IGP) also being NSF-capable. This ensures the routing information (FIB), upon which LDP builds its label bindings, remains stable. Therefore, enabling NSF for the link-state IGP (OSPF or IS-IS) is a mandatory prerequisite. Additionally, the LDP Graceful Restart mechanism is designed to operate between directly connected LDP peers, which is a stated requirement in the Cisco implementation.

A. This is too specific. The requirement is for the active IGP to be NSF-aware, which is more generally stated in option B, as LDP NSF is primarily documented with link-state protocols.
C. Cisco Express Forwarding (CEF) is a fundamental prerequisite for both MPLS and NSF. Disabling it would prevent the separation of control and data planes that NSF relies upon.
E. BGP NSF is a complementary but separate high-availability feature. It is not a direct dependency for implementing LDP NSF, which handles transport labels within the core.

1. Cisco Systems

"MPLS Label Distribution Protocol Configuration Guide

Cisco IOS XE Gibraltar 16.12.x". In the "LDP Graceful Restart" section

under "Prerequisites for LDP Graceful Restart

" the guide explicitly states:

"You must enable graceful restart for the link-state IGP (OSPF or IS-IS) that is in use on the network." (Supports option B)

"You must implement direct connections for LDP peers." (Supports option D)

"You must enable Cisco Express Forwarding (CEF)." (Refutes option C)

2. Cisco Systems

"MPLS High Availability Configuration Guide

Cisco IOS XE Release 3S". The section "How to Configure MPLS High Availability Features" details the configuration for "MPLS LDP Nonstop Forwarding." It lists the same prerequisites

confirming that the IGP must be NSF-capable and LDP peers must be directly connected. It makes no mention of BGP NSF being a prerequisite for LDP NSF.