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350-501 · Question #254

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…

The correct answer is C. Enable LDP session protection on either one of the routers, which allows them to autonegotiate. 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…

MPLS and Segment Routing

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.

How the community answered

(47 responses)
  • A
    2% (1)
  • B
    9% (4)
  • C
    83% (39)
  • D
    6% (3)

Why each option

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.

AEnable communication over TCP port 646 for T-LDP hello messages.

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.

BEnable RSVP-TE FRR on the LDP interface to protect the LDP session between routers.

RSVP-TE FRR (Fast Reroute) is a mechanism for protecting MPLS TE tunnels, not specifically for protecting LDP sessions.

CEnable LDP session protection on either one of the routers, which allows them to autonegotiate.Correct

LDP session protection, also known as Targeted LDP (T-LDP) session protection, is enabled to protect LDP sessions from temporary IGP adjacency failures, leading to faster MPLS convergence. When enabled on one or both routers, they can autonegotiate the protection, establishing a targeted LDP session over the loopback interface, which is more stable than an interface-based session.

DSet the LDP session protection timer on each router to the same value.

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

Source: 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

Topics

#MPLS#LDP Session Protection#MPLS Convergence#LDP Configuration

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