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Difficulty: HardRapid PVST+ Spanning Tree Protocol Operations and Roles

A Cisco switch operating with Rapid PVST+ receives multiple BPDUs across several redundant trunk links connecting toward the Root Bridge. To elect its single Root Port for a given VLAN instance, the switch executes a standardized four-step tie-breaking sequence to select the best path.

In what chronological sequence does the switch evaluate these criteria, starting from the first criterion evaluated (highest precedence) to the final criterion evaluated (lowest precedence)?

  1. 1Lowest cumulative Root Path Cost received in the BPDU
  2. 2Lowest Sender Bridge ID (Designated Bridge ID)
  3. 3Lowest Sender Port ID (Designated Port ID)
  4. 4Lowest Local Port ID (Receiver Port ID)

Answer

The correct sequence for electing a Root Port in Rapid PVST+ evaluates: 1) Lowest cumulative Root Path Cost, 2) Lowest Sender Bridge ID, 3) Lowest Sender Port ID, and 4) Lowest Local Port ID.
In Rapid PVST+ (and standard 802.1D/802.1w STP), a non-root switch selects its single Root Port by comparing BPDU criteria in strict order: first, the lowest cumulative Root Path Cost; second, the lowest Sender (Designated) Bridge ID; third, the lowest Sender (Designated) Port ID; and fourth, the lowest Local (Receiver) Port ID.

Step-by-Step Solution

1
Evaluate candidate path metrics toward the elected Root Bridge.
The interface receiving the BPDU with the lowest cumulative Root Path Cost is preferred.
STP path selection prioritizes optimal forwarding efficiency to the Root Bridge.
2
Compare upstream neighbor identities if path costs match.
The interface connected to the neighbor switch possessing the lowest Bridge ID (Priority + MAC address) is chosen.
Sender BID serves as the primary tie-breaker when multiple paths yield identical path costs.
3
Compare upstream neighbor port identifiers if connecting to the same neighbor switch across parallel links.
The interface receiving BPDUs from the lower Sender Port ID (Port Priority + Port Index) is selected.
The upstream switch's Designated Port ID breaks ties when path cost and neighbor BID are identical.
4
Evaluate local receiving interface identifiers as the final tie-breaker.
The local port with the lowest internal Port ID (Port Priority + Port Number) becomes the Root Port.
When all external BPDU parameters match, the local switch uses its own port numerical priority as the final determinant.

Key Concept

Rapid PVST+ Root Port Decision Sequence
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Topics

Rapid PVST+ Spanning Tree Protocol Operations and Roles

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