350-501 Question #254: Real Exam Question with Answer & Explanation

Question

A network engineer is configuring a newly installed PE router at the regional gateway location. The new PE router must use MPLS core routing protocols with the existing P router, and LDP sessions between the two routers must be protected to provide faster MPLS convergence. Which configuration must the engineer perform on the network so that LDP sessions are established?

Options

• AEnable communication over TCP port 646 for T-LDP hello messages.
• BEnable RSVP-TE FRR on the LDP interface to protect the LDP session between routers.
• CEnable LDP session protection on either one of the routers, which allows them to autonegotiate.
• DSet the LDP session protection timer on each router to the same value.

Explanation

To establish LDP sessions with session protection, an engineer must enable LDP session protection on at least one of the routers, as this feature allows for autonegotiation between peers. LDP session protection maintains the LDP session even if the underlying IGP adjacency temporarily fails.

Common mistakes.

• A. LDP uses TCP port 646 for session establishment, but enabling communication over this port is a prerequisite for LDP itself, not specifically for LDP session protection which uses T-LDP.
• B. RSVP-TE FRR (Fast Reroute) is a mechanism for protecting MPLS TE tunnels, not specifically for protecting LDP sessions.
• D. While LDP timers are important for session keepalives, explicitly setting session protection timers to the same value on both routers is not the primary action for enabling session protection; the feature itself initiates the autonegotiation.

Concept tested. MPLS LDP session protection configuration

Reference. https://www.cisco.com/c/en/us/td/docs/iosxr/cisco_iosxr_software/mpls/config_guide/b-mpls-cg-iosxr-65x/b-mpls-cg-iosxr-65x_chapter_01.html#concept_E77651086C14457385055BB93E6F40B3

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