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Difficulty: MediumRapid PVST+ Spanning Tree Protocol Operations and Roles

An enterprise switch, Switch-A, is connected to neighboring switches via 802.1Q802.1\text{Q} trunk links running Rapid PVST+. An administrator executes the command `spanning-tree vlan 10 priority 24576` on Switch-A, while all other switches in the network retain default STP settings. Which two statements correctly describe the Rapid PVST+ operations and port roles for VLAN 1010 on Switch-A?

  1. Switch-A advertises a total Bridge ID priority of 2458624586 for VLAN 1010 by combining the configured priority with the System ID Extension.Answer
  2. All active non-edge switch ports on Switch-A participating in VLAN 1010 assume the Designated Port role in the Forwarding state.Answer
  3. C
    Switch-A advertises a total Bridge ID priority of 2457624576 for VLAN 1010 without modifying the value for the VLAN identifier.
  4. D
    Switch-A selects the root path with the lowest cumulative cost and transitions one of its trunk ports to the Root Port role.

Answer

Switch-A advertises a total Bridge ID priority of 2458624586 for VLAN 1010 by combining the configured priority with the System ID Extension, and all active non-edge switch ports on Switch-A participating in VLAN 1010 assume the Designated Port role in the Forwarding state.
Rapid PVST+ calculates the total bridge priority by adding the 12-bit System ID Extension (VLAN ID 1010) to the 4-bit configured priority value (2457624576), producing an advertised priority of 2458624586. Because default switch priority is 32768+10=3277832768 + 10 = 32778, Switch-A wins the Root Bridge election for VLAN 1010. On the Root Bridge, every active forwarding port is assigned the Designated Port role.

Step-by-Step Solution

1
Calculate the total Bridge ID priority for Switch-A in VLAN 1010.
Configured base priority (2457624576) + System ID Extension (1010) = 2458624586.
Rapid PVST+ appends the 12-bit VLAN ID (System ID Extension) to the 4-bit priority field (in steps of 40964096) to form the 16-bit Bridge Priority.
2
Determine Root Bridge election outcome for VLAN 1010.
Switch-A has the lowest Bridge ID (24586<3277824586 < 32778) and becomes the Root Bridge for VLAN 1010.
Spanning tree elects the switch with the lowest numeric Bridge ID (Priority + MAC address) as the Root Bridge.
3
Determine port roles for Switch-A as the Root Bridge.
All active interfaces on Switch-A for VLAN 1010 become Designated Ports in the Forwarding state.
The Root Bridge is the central point of the spanning tree topology; none of its ports point toward another root, so it has no Root Ports, and all its active ports serve down-tree segments as Designated Ports.

Key Concept

Rapid PVST+ Bridge Priority Calculation and Root Bridge Port Roles
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Topics

Rapid PVST+ Spanning Tree Protocol Operations and Roles

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