Examkin

Question

Difficulty: HardRapid PVST+ Spanning Tree Protocol Operations and Roles

Four Cisco Catalyst switches (SW1, SW2, SW3, and SW4) are deployed in an enterprise network running Rapid PVST+ for VLAN 10. SW1 is elected as the Root Bridge. SW4 connects to SW2 via interface GigabitEthernet0/1 and to SW3 via interface GigabitEthernet0/2. Both paths yield an identical cumulative root path cost of 4. SW2 has a lower Bridge ID than SW3. In addition, interfaces GigabitEthernet0/3 and GigabitEthernet0/4 on SW4 are both connected to the same unmanaged hub on a single shared collision domain. Which two statements correctly describe the Rapid PVST+ port roles and state operations on SW4 for VLAN 10? (Select two.)

  1. GigabitEthernet0/2 is assigned the Alternate port role and operates in the Discarding state.Answer
  2. GigabitEthernet0/4 is assigned the Backup port role and operates in the Discarding state.Answer
  3. C
    GigabitEthernet0/2 is assigned the Backup port role because SW3 lost the root path cost tie-breaker to SW2.
  4. D
    GigabitEthernet0/4 transitions through the Listening state for 15 seconds before moving to the Learning state.

Answer

The correct statements are that GigabitEthernet0/2 is assigned the Alternate port role operating in the Discarding state, and GigabitEthernet0/4 is assigned the Backup port role operating in the Discarding state.
In Rapid PVST+ (802.1w), port roles are defined based on path location and the source of BPDUs. GigabitEthernet0/1 is the Root Port because SW2 has a lower Bridge ID than SW3. GigabitEthernet0/2 receives BPDUs from another switch (SW3) offering a secondary root path, making it an Alternate port operating in the Discarding state. For GigabitEthernet0/3 and GigabitEthernet0/4 connected to the same hub, SW4 receives its own BPDUs; the port with the higher Port ID (GigabitEthernet0/4) becomes a Backup port operating in the Discarding state.

Step-by-Step Solution

1
Determine Root Port on SW4
GigabitEthernet0/1 selected as Root Port
Both SW2 and SW3 offer an equal root path cost of 4. The tie-breaker evaluates the neighbor Bridge ID. Because SW2 has a lower Bridge ID than SW3, the path through SW2 (GigabitEthernet0/1) is chosen as the Root Port.
2
Determine role for non-root path to another switch (GigabitEthernet0/2)
GigabitEthernet0/2 becomes an Alternate Port in Discarding state
An interface that receives BPDUs offering an alternate path to the Root Bridge from a neighboring switch acts as an Alternate port. In Rapid PVST+, Alternate ports are placed in the Discarding state.
3
Determine roles for interfaces connected to the shared hub (GigabitEthernet0/3 and GigabitEthernet0/4)
GigabitEthernet0/3 becomes Designated; GigabitEthernet0/4 becomes Backup in Discarding state
When multiple interfaces on the same switch connect to the same collision domain/hub, the switch receives its own BPDUs. The interface with the lower Port ID (GigabitEthernet0/3) becomes Designated, while the higher Port ID (GigabitEthernet0/4) takes the Backup port role in the Discarding state.

Key Concept

Rapid PVST+ Port Roles (Root, Designated, Alternate, Backup) and States (Discarding, Learning, Forwarding)
Rate this question

Topics

Rapid PVST+ Spanning Tree Protocol Operations and Roles

More questions from this topic