An engineer implemented LDP protocol on the ISP network. The engineer must ensure that there are no packet loss issues when IGP and LDP protocols are not synchronized. Which configuring must the engineer implement so that the IGP routing protocol will wait until LDP convergence is completed?

The core issue described is a race condition between the Interior Gateway Protocol (IGP) and the Label Distribution Protocol (LDP). When a network link recovers, the IGP (like OSPF or IS-IS) can converge and install a new route in the routing table faster than LDP can establish a session and exchange labels for that path. This results in traffic being forwarded to a next-hop for which no MPLS label exists, causing packets to be dropped (black-holed). The MPLS LDP IGP Synchronization feature is designed specifically to prevent this. It instructs the IGP to advertise a link with a maximum metric until the LDP session is established and synchronized over that link, effectively delaying its use for traffic forwarding until the MPLS data plane is ready.

A. Disabling IP CEF is incorrect because Cisco Express Forwarding (CEF) is a prerequisite for MPLS operation on Cisco routers and is essential for high-performance forwarding.
C. LDP Session Protection is a feature that maintains LDP sessions during link failures by using a different IP path, but it does not address the initial convergence synchronization issue between IGP and LDP.
D. Disabling MPLS LDP IGP synchronization is the opposite of the required action; it would ensure the problem of unsynchronized convergence and potential packet loss persists.

1. Cisco IOS XE MPLS Configuration Guide, Release Gibraltar 16.11.x, "MPLS LDP-IGP Synchronization" section: "The MPLS LDP-IGP Synchronization feature ensures that the Label Distribution Protocol (LDP) is fully established before the Interior Gateway Protocol (IGP) starts to forward traffic through a link... To prevent packet loss, the MPLS LDP-IGP Synchronization feature delays the IGP from advertising a link until LDP is fully synchronized."

2. Cisco IOS XR MPLS Configuration Guide for the Cisco CRS Router, "Implementing LDP-IGP Synchronization" chapter: "LDP-IGP synchronization is a mechanism that allows you to synchronize LDP and an IGP. This synchronization prevents the loss of MPLS packets that can occur because the IGP has converged on a link, but LDP has not."

3. Cisco IOS MPLS Configuration Guide, Release 12.4, "MPLS LDP-IGP Synchronization" section: "When a link comes up, the IGP can converge faster than the LDP. If the IGP converges faster, the router may start to forward IP packets before the MPLS forwarding table is updated with the labels for the new link. The packets are then dropped. To prevent packet loss, the MPLS LDP-IGP Synchronization feature delays the IGP from advertising a link until LDP is fully synchronized."