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Difficulty: MediumRapid PVST+ Spanning Tree Protocol Operations and Roles

In a network running Rapid PVST+ for VLAN 30, Switch-4 has two direct Gigabit Ethernet connections to Switch-2, which is the Root Bridge. Link 1 connects Switch-4 port GigabitEthernet0/1 to Switch-2 port GigabitEthernet0/1. Link 2 connects Switch-4 port GigabitEthernet0/2 to Switch-2 port GigabitEthernet0/2. All interface costs use default values. Which port role and state will port GigabitEthernet0/2 on Switch-4 assume after STP convergence completes?

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

Answer

Port GigabitEthernet0/2 on Switch-4 will become an Alternate port in the Discarding state.
When a non-root switch has multiple parallel links connected to the same upstream Root Bridge, the tie-breakers for selecting a Root Port are: lowest cumulative Root Path Cost, lowest Designated Bridge ID, lowest upstream Designated (Sender) Port ID, and lowest local Port ID. Here, cumulative cost and Bridge ID are equal for both links. The tie is broken by Switch-2's sender Port ID: Gi0/1 has a lower numerical Port ID than Gi0/2. Thus, Switch-4's GigabitEthernet0/1 becomes the Root Port in the Forwarding state, while GigabitEthernet0/2 assumes the Alternate port role in the Discarding state to block switching loops.

Step-by-Step Solution

1
Determine Root Port selection on Switch-4 across redundant paths to the Root Bridge.
Both local ports (Gi0/1 and Gi0/2) connect directly to Root Bridge Switch-2, resulting in identical Root Path Cost (44) and identical Root/Designated Bridge ID.
When Root Path Cost and Designated Bridge ID are tied, Rapid PVST+ evaluates the upstream Designated (Sender) Port ID to break the tie.
2
Compare upstream sender Port IDs from Switch-2.
Switch-2 sends BPDUs out its Gi0/1 port (Port ID 128.1128.1) to Switch-4's Gi0/1, and out its Gi0/2 port (Port ID 128.2128.2) to Switch-4's Gi0/2. Lower port priority/number wins.
Since 128.1<128.2128.1 < 128.2, Switch-4 selects port GigabitEthernet0/1 as its single Root Port.
3
Determine the role and state for the remaining unselected port GigabitEthernet0/2 on Switch-4.
GigabitEthernet0/2 receives superior BPDUs from Switch-2 compared to any BPDUs Switch-4 could send, so Switch-4 places GigabitEthernet0/2 into the Alternate port role, which operates in the Discarding state.
Alternate ports offer an alternate path to the Root Bridge and remain blocked (Discarding) to prevent Layer 2 loops.

Key Concept

Rapid PVST+ Root Port selection tie-breaking via upstream Sender Port ID and Alternate port role assignment.
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Topics

Rapid PVST+ Spanning Tree Protocol Operations and Roles

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