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352-001 · Question #632

A large ISP is analyzing the following IGP requirements: The network must be resilient against unstable MTU in one side for newly deployed transmission equipment. The network must support MPLS traffic

The correct answer is D. ISIS: adjacency remains up even if MTU changes. IS-IS is the correct IGP choice when MTU instability exists because, unlike OSPF, IS-IS does not perform MTU checks during adjacency formation and therefore remains stable when MTU values change.

Layer 3 Control Plane

Question

A large ISP is analyzing the following IGP requirements:

The network must be resilient against unstable MTU in one side for newly deployed transmission equipment. The network must support MPLS traffic engineering solution for future use. Which IGP must be selected and why?

Options

  • AISIS : in case MTU changes your TE tunnels keep the LSP stable
  • BOSPF: adjacency remains up even if MTU changes
  • COSPF: in case MTU changes your TE tunnels keep the LSP stable
  • DISIS: adjacency remains up even if MTU changes

How the community answered

(31 responses)
  • A
    19% (6)
  • B
    10% (3)
  • C
    6% (2)
  • D
    65% (20)

Why each option

IS-IS is the correct IGP choice when MTU instability exists because, unlike OSPF, IS-IS does not perform MTU checks during adjacency formation and therefore remains stable when MTU values change.

AISIS : in case MTU changes your TE tunnels keep the LSP stable

This incorrectly attributes IS-IS's advantage to TE tunnel LSP stability rather than the actual reason - that IS-IS adjacency formation is inherently unaffected by MTU changes.

BOSPF: adjacency remains up even if MTU changes

OSPF adjacency does NOT remain up when MTU values differ; OSPF performs an MTU check during the DBD exchange phase and will stall in ExStart state when a mismatch exists between neighbors.

COSPF: in case MTU changes your TE tunnels keep the LSP stable

This is wrong on both claims - OSPF adjacency breaks on MTU mismatch, and TE tunnel LSP stability is not the basis for protocol selection in this scenario.

DISIS: adjacency remains up even if MTU changesCorrect

IS-IS operates at Layer 2 and does not include MTU information in its Hello PDUs, so adjacencies are not disrupted by MTU mismatches on newly deployed transmission equipment. Both IS-IS and OSPF support MPLS TE extensions, but IS-IS is the correct selection here specifically because its adjacency mechanism is MTU-independent, satisfying the resilience requirement.

Concept tested: IS-IS vs OSPF MTU handling for adjacency stability

Source: https://www.cisco.com/c/en/us/support/docs/ip/integrated-intermediate-system-to-intermediate-system-is-is/13796-is-is-ospf-compare.html

Topics

#IS-IS#OSPF#MTU mismatch#MPLS traffic engineering

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