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Difficulty: Very hardRapid PVST+ Spanning Tree Protocol Operations and Roles

A Cisco switch running Rapid PVST+ receives BPDUs on multiple interfaces from neighboring switches leading toward the Root Bridge. Place the tie-breaking criteria in the exact sequential order (from first evaluated to last evaluated) that the switch processes to elect its single Root Port.

  1. 1Lowest cumulative Root Path Cost to the Root Bridge
  2. 2Lowest Designated (upstream neighbor) Bridge ID (BID)
  3. 3Lowest Designated (upstream neighbor) Port Priority
  4. 4Lowest Designated (upstream neighbor) Port Number
  5. 5Lowest local (receiving) Port ID on the evaluating switch

Answer

The correct sequential order for Rapid PVST+ Root Port election tie-breakers is: 1. Lowest cumulative Root Path Cost to the Root Bridge, 2. Lowest Designated (upstream neighbor) Bridge ID (BID), 3. Lowest Designated (upstream neighbor) Port Priority, 4. Lowest Designated (upstream neighbor) Port Number, 5. Lowest local (receiving) Port ID on the evaluating switch.
When electing a Root Port, Rapid PVST+ follows a strict hierarchical evaluation of BPDU criteria: 1) Lowest cumulative path cost to the root bridge, 2) Lowest transmitting neighbor Bridge ID, 3) Lowest transmitting neighbor port priority, 4) Lowest transmitting neighbor port number, and 5) Lowest local receiving port ID. Local interface parameters are evaluated last because upstream BPDU parameters take precedence.

Step-by-Step Solution

1
Evaluate the cumulative path cost
The switch calculates the sum of port costs along the path to the Root Bridge. The port receiving the lowest total cost becomes the Root Port candidate.
Path efficiency to the Root Bridge is the primary metric for spanning-tree forwarding path selection.
2
Compare upstream neighbor Bridge IDs
If cumulative path costs are equal, compare the sending switch's Bridge ID (Bridge Priority + System ID Extension + MAC Address).
Paths connected to a superior upstream bridge take precedence.
3
Compare upstream neighbor Port Priority
If BPDUs are received from the same upstream neighbor over parallel links, compare the neighbor's Port Priority value (default 128).
Upstream administrators can influence downstream port selection by modifying port priority on the transmitting end.
4
Compare upstream neighbor Port Number
If neighbor port priorities match, compare the numerical interface index of the neighbor's transmitting interface (e.g., GigabitEthernet0/1 vs GigabitEthernet0/2).
Lower interface numbers on the neighbor take precedence when priorities are equal.
5
Compare local receiving Port ID
If all upstream BPDU fields are identical (such as when connected to an shared medium like an unmanaged hub), evaluate the local receiving switch's Port ID (Priority then Port Number).
The local receiving port ID serves as the absolute final tie-breaker because no upstream BPDU fields can differentiate the paths.

Key Concept

Rapid PVST+ Root Port Election Sequence
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Topics

Rapid PVST+ Spanning Tree Protocol Operations and Roles

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