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

A network administrator is evaluating Rapid PVST+ spanning-tree behavior across a multi-switch enterprise topology containing VLAN 10 and VLAN 20. Switch-A is configured with `spanning-tree vlan 10 priority 4096`, while Switch-B is configured with `spanning-tree vlan 20 priority 8192`. Switch-C connects to both Switch-A and Switch-B via 802.1Q trunk links with default Rapid PVST+ settings. Which two statements correctly describe the resulting Rapid PVST+ bridge operations, port roles, and configuration rules? (Select two choices.)

  1. For VLAN 10, Switch-A advertises a total Bridge ID priority value of 4106 in its outbound BPDUs due to the addition of the 12-bit System ID Extension.Answer
  2. On Switch-C, an interface assigned the Alternate port role remains in the Discarding state while receiving superior BPDUs from a designated switch.Answer
  3. C
    Executing `spanning-tree vlan 10 priority 4100` on Switch-A is accepted by Cisco IOS, which automatically rounds the value up to the nearest System ID Extension increment.
  4. D
    Enabling PortFast on the trunk link between Switch-A and Switch-C allows the link to bypass BPDU transmission and transition directly into the Learning state.

Answer

The correct statements are that Switch-A advertises a total priority value of 4106 for VLAN 10 due to the System ID Extension, and an Alternate port on Switch-C remains in the Discarding state while receiving superior BPDUs.
In Rapid PVST+, the Bridge ID priority calculation incorporates the 12-bit System ID Extension, resulting in an advertised priority of 4096+10=41064096 + 10 = 4106 for VLAN 10 on Switch-A. Additionally, an Alternate port provides a backup root path and resides in the Discarding state while continuing to process superior BPDUs from the designated bridge.

Step-by-Step Solution

1
Calculate the total Bridge ID priority advertised by Switch-A for VLAN 10.
Total Priority = Base Priority (4096) + System ID Extension (VLAN 10) = 4106.
Rapid PVST+ dynamically includes the VLAN ID inside the 12-bit System ID Extension field of the 16-bit Bridge Priority structure.
2
Verify Cisco IOS syntax rules for configuring STP bridge priority.
Configuring a value such as 4100 is rejected by Cisco IOS.
STP base priority values must be exact multiples of 4096 (0,4096,8192,…,614400, 4096, 8192, \dots, 61440).
3
Determine the state and behavior of an Alternate port in Rapid PVST+.
The Alternate port is placed in the Discarding state.
An Alternate port offers an alternate path to the root bridge but remains blocked (Discarding state) as long as it receives superior BPDUs from another switch.
4
Evaluate the operational mechanics of PortFast on switch links.
PortFast transitions ports directly to Forwarding, not Learning, and does not disable BPDU traffic.
PortFast is intended exclusively for single-host end devices to skip STP listening/learning phases. Applying it to trunk links connecting switches threatens network stability.

Key Concept

Rapid PVST+ Bridge ID Structure, Port Roles, and Port States
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Topics

Rapid PVST+ Spanning Tree Protocol Operations and Roles

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