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352-001 · Question #24

Which statement correctly describes how MTU mismatches are addressed in the IS-IS neighbor- formation process?

The correct answer is D. IS-IS pads hellos, so neighbor relationships will not be formed on links with mismatched MTUs.. IS-IS pads IIH hello PDUs to the full interface MTU, so when two routers have mismatched MTUs the oversized hellos are silently discarded and adjacency cannot form.

Layer 3 Control Plane

Question

Which statement correctly describes how MTU mismatches are addressed in the IS-IS neighbor- formation process?

Options

  • AIS-IS checks the locally configured MTU against the MTU advertised in neighbor hello packets.
  • BIS-IS checks the locally configured MTU against the MTU advertised in neighbor LSPs.
  • CIS-IS does not check for MTU mismatches when forming a neighbor relationship.
  • DIS-IS pads hellos, so neighbor relationships will not be formed on links with mismatched MTUs.

How the community answered

(34 responses)
  • A
    3% (1)
  • B
    6% (2)
  • C
    3% (1)
  • D
    88% (30)

Why each option

IS-IS pads IIH hello PDUs to the full interface MTU, so when two routers have mismatched MTUs the oversized hellos are silently discarded and adjacency cannot form.

AIS-IS checks the locally configured MTU against the MTU advertised in neighbor hello packets.

IIH PDUs do not carry an explicit MTU field for routers to compare against their local configuration; MTU enforcement is achieved implicitly through the hello-padding mechanism, not through an advertised value in the hello itself.

BIS-IS checks the locally configured MTU against the MTU advertised in neighbor LSPs.

IS-IS LSPs carry link-state and reachability information after adjacency is already established and do not contain MTU data used during the neighbor-formation process.

CIS-IS does not check for MTU mismatches when forming a neighbor relationship.

IS-IS does enforce MTU consistency, but it does so passively by padding hellos rather than performing an explicit MTU comparison step, so stating it 'does not check' misrepresents the actual behavior.

DIS-IS pads hellos, so neighbor relationships will not be formed on links with mismatched MTUs.Correct

IS-IS pads every IIH PDU to the full MTU of the sending interface before transmission. On a link where one side has a larger MTU than the other, that side's padded hellos exceed the receiving interface's MTU and are dropped at the link layer. Because IS-IS adjacency requires receipt of valid hellos from the neighbor, the relationship is never established, effectively enforcing MTU consistency without an explicit negotiation field.

Concept tested: IS-IS hello padding enforcing MTU consistency

Source: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_isis/configuration/xe-16/irs-xe-16-book/irs-overview.html

Topics

#IS-IS MTU#hello padding#neighbor formation#MTU mismatch

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