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Zorluk: OrtaRapid PVST+ Spanning Tree Protocol Operations and Roles

A non-root Cisco Catalyst switch running Rapid PVST+ receives BPDUs on multiple interfaces connected to neighboring switches for VLAN 20. The switch must determine which of its interfaces will become the single Root Port for VLAN 20. Place the tie-breaking steps evaluated by Rapid PVST+ in the exact sequential order from first step evaluated to last step evaluated.

  1. 1Select the interface with the lowest cumulative Root Path Cost to the Root Bridge.
  2. 2Select the interface connected to the neighboring switch with the lowest Sender Bridge ID (BID).
  3. 3Select the interface connected to the neighbor port sending the lowest Sender Port ID.
  4. 4Select the local receiving interface with the lowest local Port ID.

Cevap

The correct order of tie-breaking evaluation for Rapid PVST+ Root Port selection is: 1. Lowest cumulative Root Path Cost, 2. Lowest Sender Bridge ID (BID), 3. Lowest Sender Port ID, 4. Lowest local Receiver Port ID.
Rapid PVST+ evaluates Root Port selection using a strict four-step tie-breaker hierarchy: 1. Lowest cumulative Root Path Cost, 2. Lowest Sender Bridge ID (BID), 3. Lowest Sender Port ID (neighbor's port priority and index), 4. Lowest Receiver Port ID (local port priority and index).

Adım Adım Çözüm

1
Identify the primary STP metric for Root Port selection.
The switch calculates total path cost by adding link costs to reach the Root Bridge. Lowest cumulative path cost is evaluated first.
STP prioritizes reaching the Root Bridge via the least costly path.
2
Apply the first tie-breaker if path costs are equal.
The switch evaluates the Sender Bridge ID (Bridge Priority + Extended System ID + MAC address) of the upstream neighbor switches.
Preferring the neighbor with the lowest Bridge ID ensures a deterministic loop-free structure.
3
Apply the second tie-breaker if path costs and neighbor BIDs are equal.
The switch checks the Sender Port ID (Port Priority + Port Number) advertised in the BPDU by the neighbor switch.
This differentiates multiple parallel links connecting to the exact same neighbor switch.
4
Apply the final tie-breaker if all upstream BPDU fields are identical.
The switch selects its own local interface with the lowest Port ID.
Local port ID is the final fallback when upstream BPDU data offers no difference.

Anahtar Kavram

Rapid PVST+ Root Port Tie-Breaking Sequence
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Konular

Rapid PVST+ Spanning Tree Protocol Operations and Roles

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