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In VLAN 134, SW-A and SW-B both advertise configured bridge priority 4096. Their MAC addresses are SW-A 00:11:22:00:00:20 and SW-B 00:11:22:00:00:05. No switch advertises a lower priority. Which switch becomes the Rapid PVST+ root and why?
STP compares full bridge IDs: the priority (including VLAN system ID) ranks first; with equal priority, the numerically lower MAC address wins the root election.
When priorities tie, STP selects the smaller MAC address, not the larger one.
A single spanning-tree instance has one root bridge; an ID tie is resolved deterministically.
Port count is not part of the bridge ID comparison.
At an airport terminal, the on-call technician must identify the Rapid PVST+ root for VLAN 144; all listed switches are on the same Layer 2 topology and no superior external BPDU exists. Which switch wins?
SW-2 has bridge priority 4096; root selection compares bridge ID priority first and MAC as tie-breaker. It wins because this is the lowest priority in the presented candidates.
SW-4 has bridge priority 20480; root selection compares bridge ID priority first and MAC as tie-breaker. The winning bridge has lower priority 4096, so this cannot be root despite its MAC value.
SW-1 has bridge priority 16384; root selection compares bridge ID priority first and MAC as tie-breaker. The winning bridge has lower priority 4096, so this cannot be root despite its MAC value.
SW-3 has bridge priority 28672; root selection compares bridge ID priority first and MAC as tie-breaker. The winning bridge has lower priority 4096, so this cannot be root despite its MAC value.
A non-root Rapid PVST+ switch receives equal best root path cost on Gi1/0/1 and Gi1/0/2. Gi1/0/1 receives BPDUs from upstream bridge ID 32768:0011.2233.4455 and Gi1/0/2 from 32768:0011.2233.4400. Which becomes the root port, all else equal?
With equal root ID and total root path cost, spanning tree compares the sender bridge ID next. The lower MAC component of the equal-priority bridge ID wins.
The local port ID tie-break applies only after the superior BPDU comparisons; the upstream bridge IDs already differ.
A non-root bridge elects a single root port for a given STP instance, not two equal root ports.
STP has additional deterministic BPDU tie-break fields, including the upstream bridge ID and port ID.
A pair of distribution switches runs Rapid PVST+ across VLAN 137 and VLAN 138. The administrator wants SW-C to be the preferred root only for VLAN 137, retaining the existing root for VLAN 138. Which approach is appropriate?
Rapid PVST+ computes a separate root bridge per VLAN. Changing only VLAN 137 priority can steer that instance without deliberately changing the root election in VLAN 138.
Management IP addressing is not a bridge ID priority setting for STP election.
Disabling STP removes loop protection and is not how an unchanged root should be preserved.
Host VLAN membership does not replace per-instance bridge priority configuration.
On VLAN 138 a non-root switch receives root BPDUs on Gi1/0/1 and Gi1/0/4 with equal total root-path cost. The BPDU on Gi1/0/1 comes from designated bridge ID 32768:00aa.0000.0009; Gi1/0/4 comes from 32768:00aa.0000.0004. Other tie-breakers are equal. Which interface becomes the root port?
After equal root path cost and the same root bridge, STP compares the sender/designated bridge ID. The lower sending bridge ID on Gi1/0/4 is preferred before considering designated port ID.
STP chooses the lower designated bridge ID, not higher.
A non-root bridge selects one root port per STP instance, subject to special link aggregation mechanisms.
802.1Q native VLAN assignment is unrelated to the received bridge-ID tie-break.
An access switch has a non-designated alternate port that usually receives superior BPDUs. Due to a one-way fiber fault, BPDUs stop arriving while the link remains up. The operator wants to prevent the port from incorrectly transitioning to forwarding and creating a loop. Which safeguard fits?
Loop guard detects unexpected loss of superior BPDUs on root/alternate ports and holds the port loop-inconsistent instead of letting it incorrectly forward while Layer 1 remains up.
The concern is a silent loss of expected BPDUs on a non-designated inter-switch port; BPDU guard is intended for unexpected BPDUs on edge ports.
Root guard responds to received superior BPDUs on designated ports, not to missing BPDUs on alternate/root ports.
PortFast on a redundant switching link can defeat topology protection and does not diagnose unidirectional faults.
A campus switch receives a superior BPDU and the reported root for VLAN 140 changes unexpectedly. The operator must identify the currently elected root and the local port leading toward it. Which single command is most direct?
The VLAN-specific STP display identifies the elected Root ID and the port that provides the selected root path; configuration intent alone does not prove the active topology.
VLAN membership does not identify the STP root bridge or root port.
The STP root is elected by bridge IDs and signaled in BPDUs, not by a host default-gateway IP address.
Route source preference does not report per-VLAN STP root or local port roles.
On VLAN 141, two candidate root ports have the same path cost and the same designated bridge ID. Gi1/0/2 receives a BPDU from designated port ID 128.8; Gi1/0/3 receives designated port ID 128.12. Which root-port tie-break result is expected?
STP compares received BPDU priority vectors: if root path cost and designated bridge ID tie, the lower designated port ID is preferred. Local port ID matters only after those tie-breaks.
A larger designated port identifier is less preferred when preceding tie-breakers match.
STP resolves a tie using sender bridge/port fields and ultimately local port ID.
Native VLAN configuration is not the next tie-break element in this received-BPDU comparison.
Two Cisco switches on the same Rapid PVST+ VLAN advertise bridge priority 4096. SW1 has bridge MAC 00aa.0000.1000 and SW2 has 00aa.0000.0800. No lower-priority bridge exists. Which one becomes root?
Root bridge selection compares bridge IDs: lower priority first, then lower MAC when priorities match. SW2's 0800 component is lower than SW1's 1000.
For STP root election, a lower—not higher—MAC address wins the remaining tie.
Link path cost determines root ports on non-root bridges, not which of two equal-priority bridge IDs wins root election.
Each VLAN tree has one root bridge. Equal numeric priority does not create dual roots; the MAC tie-break resolves it.
A distribution switch has a designated downlink to an access switch that must never become the VLAN 90 root. An unexpected access switch starts advertising superior BPDUs. Which guard best protects the intended root placement while preserving normal operation after superior BPDUs stop?
Root guard places a designated port into a root-inconsistent state when superior BPDUs arrive; it helps enforce the intended upstream root placement on that boundary.
Filtering BPDUs across a redundant switched topology can hide loops; it is not a safe substitute for protecting designated-port root placement.
PortFast is intended for appropriate edge connections and does not enforce bridge-root hierarchy on a switch-to-switch trunk.
BPDU guard on an unrelated host port has no control over the superior BPDUs entering on the distribution downlink.
A switch has two possible root paths for VLAN 144. Path A has total STP cost 8 and path B has cost 4. An engineer raises the interface cost of path B to 12 without modifying other costs. Which selection should be expected after convergence?
STP root-port selection prioritizes the lowest accumulated root path cost. After changing B to 12, A provides the lower-cost path to the same root.
STP recalculates its topology after relevant cost changes and does not lock a root port forever.
Normal STP selects one root port for each per-VLAN instance.
Interface path cost changes do not replace the bridge-ID election of the root bridge.
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