JNCIP-ENT Practice Question: Designing an enterprise network running Multiple Spanning Tree Protocol (MSTP) across…

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

• ✓

  Region name, revision number, and VLAN-to-instance mapping

  Why this is correct

  Switches are in the same MST region only when their MST configuration identifier matches exactly: the region name, the revision number, and a digest of the full VLAN-to-instance mapping table, which is carried in BPDUs. One VLAN mapped differently, even an unused one, puts a switch in another region. On Junos, compare 'show spanning-tree mstp configuration' across switches, checking the name, revision and digest.

• ✗

  VLAN-to-instance mapping, bridge priority, and MAC address

  Why it's wrong here

  Bridge priority and MAC address are per-bridge attributes that legitimately differ between switches, so they cannot define region membership; only the configuration identifier (region name, revision, VLAN-to-instance mapping) must match. It is tempting because bridge priority and MAC address do determine root election, which is a separate process from region identification.

• ✗

  Bridge priority, region name, and maximum age timer

  Why it's wrong here

  Bridge priority is a per-instance setting used to choose root bridges and deliberately differs between switches, and the maximum age timer is a timing parameter, inherited from the CIST root in MSTP. Neither is part of the MST configuration identifier, so differences in them do not split a region. Only the region name, revision number and VLAN-to-instance mapping decide region membership.

• ✗

  Root bridge ID, region name, and revision number

  Why it's wrong here

  The root bridge ID is the result of an election, not something configured to match, so it cannot define region membership. Region name and revision number are two of the three required matching values, which makes this option look close, but the third is the VLAN-to-instance mapping. Without matching mappings, switches with the same name and revision are still in different regions.

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