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Difficulty: MediumRapid PVST+ Spanning Tree Protocol Operations and Roles

A enterprise network topology consists of three Cisco Catalyst switches (SwitchA, SwitchB, and SwitchC) interconnected in a redundant ring running Rapid PVST+ for VLAN 10. SwitchA is configured as the Root Bridge with a base priority of 4096. SwitchB has a base priority of 28672, and SwitchC has a base priority of 32768. All inter-switch links operate at 1 Gbps GigabitEthernet speeds (STP path cost = 4). SwitchC connects directly to SwitchA via interface GigabitEthernet0/1 and to SwitchB via interface GigabitEthernet0/2. Which Rapid PVST+ port role and state will interface GigabitEthernet0/2 assume on SwitchC for VLAN 10 after STP convergence completes?

  1. Alternate port in the Discarding stateAnswer
  2. B
    Backup port in the Discarding state
  3. C
    Designated port in the Forwarding state
  4. D
    Root port in the Forwarding state

Answer

Interface GigabitEthernet0/2 on SwitchC will become an Alternate port in the Discarding state.
SwitchC selects GigabitEthernet0/1 as its Root Port because its direct path to SwitchA has a cumulative path cost of 4, compared to a cost of 8 through SwitchB. For the link between SwitchB and SwitchC, both switches have an equal cost to the Root Bridge (4), so the switch with the lower Bridge ID (SwitchB at 28682 vs SwitchC at 32778) claims the Designated Port role. As a result, SwitchC's GigabitEthernet0/2 interface becomes an Alternate port and enters the Discarding state.

Step-by-Step Solution

1
Calculate effective Bridge IDs including the System ID Extension (VLAN 10).
SwitchA BID = 4096 + 10 = 4106 (Root Bridge). SwitchB BID = 28672 + 10 = 28682. SwitchC BID = 32768 + 10 = 32778.
Rapid PVST+ appends the 12-bit VLAN ID to the base priority to form the bridge priority.
2
Evaluate root path costs for SwitchC to select its Root Port.
Path via Gi0/1 (direct to SwitchA) cost = 4. Path via Gi0/2 (to SwitchB to SwitchA) cost = 4 + 4 = 8. Gi0/1 becomes Root Port (Forwarding).
Each non-root switch selects the single port with the lowest cumulative cost toward the Root Bridge.
3
Determine Designated Port role on the segment between SwitchB and SwitchC.
Both SwitchB and SwitchC have a root path cost of 4 to SwitchA. SwitchB has a lower BID (28682) than SwitchC (32778), so SwitchB's interface becomes the Designated Port (Forwarding).
When root path costs on a segment are tied, the switch with the lowest Bridge ID wins the Designated Port role.
4
Assign the remaining port role for SwitchC's GigabitEthernet0/2 interface.
GigabitEthernet0/2 receives superior BPDUs from SwitchB and is assigned the Alternate port role in the Discarding state.
In Rapid PVST+ (802.1w), a port receiving BPDUs from another switch on a non-designated link serves as an Alternate port to provide a backup path to the root.

Key Concept

Rapid PVST+ Port Role and State Determination
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Topics

Rapid PVST+ Spanning Tree Protocol Operations and Roles

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