CCNP Practice Question: An engineer is designing a Layer 2 network with…

Answer analysis

Option-by-option breakdown

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

• ✓

  The VLAN-to-instance mapping is not consistent across all switches in Region 1.

  Why this is correct

  The IEEE 802.1s MST region is identified by a computed configuration digest, which is derived from the VLAN-to-instance mapping, the region name, and the revision number. If one switch in Region 1 uses a different mapping (for example, VLAN 10 assigned to instance 2 instead of instance 1), its digest differs even if the name and revision match. Switches with different digests cannot distinguish internal MST BPDUs from external ones, so they treat each other as separate regions, causing an incorrect top-level tree and per-instance topology. This is exactly the failure symptom described in the scenario.

• ✗

  The root bridge for each MST instance is not configured correctly.

  Why it's wrong here

  Setting the root bridge for each MST instance is a separate administrative action that determines the root switch within an already-formed region. MST region formation does not depend on which switch is elected root; it depends solely on the configuration digest, which is independent of BPDU priorities. Even if the wrong switch is configured as root or a root is missing, all switches with identical mapping, name, and revision will still agree on the region boundaries. Therefore, root placement cannot produce this region mismatch symptom.

• ✗

  BPDU Guard is enabled on the inter-switch links, preventing BPDU exchange.

  Why it's wrong here

  BPDU Guard is a port security feature that disables a port when a BPDU is received, and it is normally enabled only on PortFast-configured access ports facing hosts. On inter-switch trunk links, enabling BPDU Guard would shut down the link upon receipt of an MST BPDU, which would cause a complete loss of connectivity and a topology error, not a 'region mismatch'. The MST region itself is formed by the exchange of configuration digests, and BPDU Guard does not alter the content or mapping information in those BPDUs. Moreover, if BPDU Guard were active, the port would go into errdisable state rather than allowing the switches to compare regional parameters.

• ✗

  PortFast is enabled on the inter-switch links, causing the switches to ignore BPDUs.

  Why it's wrong here

  PortFast places a port into the forwarding state immediately without waiting for BPDUs, but it does not stop the switch from sending or receiving spanning-tree BPDUs. If PortFast were enabled on an inter-switch trunk, the port would still transmit and process MST BPDUs, so the switches would still exchange configuration digests and could still determine whether they share a region. PortFast is intended for access ports connected to end hosts, and using it on trunk links could create loops, but it has no effect on MST region membership. Thus, PortFast cannot cause the VLAN-to-instance mapping inconsistency described.