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Difficulty: HardRapid PVST+ Spanning Tree Protocol Operations and Roles

An enterprise network topology running Rapid PVST+ consists of three switches: Switch-1, Switch-2, and Switch-3.

- Switch-1 is configured as the Root Bridge for VLAN 20 with a priority of 4096.
- Switch-2 connects to Switch-1 via a 10 Gigabit Ethernet link (10 Gbps10\text{ Gbps}, STP cost = 2).
- Switch-3 connects directly to Switch-1 via a 1 Gigabit Ethernet link (Gi0/1, STP cost = 4).
- Switch-3 also connects to Switch-2 via two parallel 1 Gigabit Ethernet links: Gi0/2 (connected to Switch-2 Gi0/1) and Gi0/3 (connected to Switch-2 Gi0/2).
- All interface STP costs and port priorities use default Cisco values.

Which port role and operational state will interface Gi0/2 on Switch-3 assume for VLAN 20?

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

Answer

Interface Gi0/2 on Switch-3 assumes the Alternate port role in the Discarding state.
Switch-3 evaluates all available paths to Root Bridge Switch-1. The direct 1 Gbps link on interface Gi0/1 provides a path cost of 4, whereas traversing Switch-2 via interface Gi0/2 yields a path cost of 6 (cost 2 from Switch-2 to Switch-1 plus cost 4 from Switch-3 to Switch-2). Consequently, Gi0/1 is elected as Switch-3's Root Port. On the segments connecting Switch-2 and Switch-3, Switch-2 advertises a lower root path cost (2) than Switch-3 (4), electing Switch-2 as the designated bridge. Interface Gi0/2 on Switch-3 receives BPDUs from Switch-2 and is designated as an Alternate port in the Discarding state.

Step-by-Step Solution

1
Determine the Root Bridge for VLAN 20
Switch-1 is the Root Bridge due to its configured priority of 4096.
The switch with the lowest Bridge ID (Priority + System ID Extension + MAC) becomes the Root Bridge.
2
Calculate cumulative path costs from Switch-3 to the Root Bridge
Direct path via Gi0/1 = 4. Path via Switch-2 (Gi0/2 or Gi0/3) = 2 (Switch-2 to Switch-1) + 4 (Switch-3 to Switch-2) = 6.
Standard Rapid PVST+ path cost for 10 Gbps10\text{ Gbps} is 2 and for 1 Gbps1\text{ Gbps} is 4.
3
Select Switch-3's Root Port
Gi0/1 is selected as the Root Port on Switch-3 and transitions to the Forwarding state.
The interface with the lowest cumulative path cost to the Root Bridge is elected as the Root Port.
4
Determine port roles for Switch-3's remaining interfaces (Gi0/2 and Gi0/3)
Switch-2's root path cost (2) is less than Switch-3's root path cost (4), making Switch-2 the Designated Bridge on the inter-switch links between Switch-2 and Switch-3. Gi0/2 on Switch-3 receives superior BPDUs from Switch-2 and is placed into the Alternate role in the Discarding state.
Non-root ports receiving superior BPDUs from another switch become Alternate ports in Discarding state under Rapid PVST+.

Key Concept

Rapid PVST+ Root Port and Alternate Port election rules based on cumulative root path costs and designated bridge tie-breakers.
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Topics

Rapid PVST+ Spanning Tree Protocol Operations and Roles

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