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

A network engineer is operating a four-switch topology (Switch-A, Switch-B, Switch-C, and Switch-D) running Rapid PVST+ for VLAN 50. All inter-switch links are 1 Gbps 802.1Q trunks operating with standard IEEE 802.1D short path costs (cost = 4).

The configured base bridge priorities and MAC addresses for VLAN 50 are as follows:
• Switch-A: Base Priority 24576, MAC 0010.1111.1111
• Switch-B: Base Priority 24576, MAC 0010.2222.2222
• Switch-C: Base Priority 28672, MAC 0010.3333.3333
• Switch-D: Base Priority 32768, MAC 0010.4444.4444

Link connections:
��� Switch-C connects to Switch-A via GigabitEthernet0/1.
• Switch-C connects to Switch-B via GigabitEthernet0/2.
• Switch-D connects directly to Switch-A via GigabitEthernet0/1.
• Switch-D connects directly to Switch-B via GigabitEthernet0/2.
• Switch-A and Switch-B are connected together via two parallel 1 Gbps trunks: Trunk-1 (Gi0/1 on both) and Trunk-2 (Gi0/2 on both). On Switch-A, interface Gi0/1 has port priority 112, and Gi0/2 has port priority 128 (default).

Based on Rapid PVST+ operations for VLAN 50, which port role and state are assigned to interface GigabitEthernet0/2 on Switch-C?

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

Answer

Interface GigabitEthernet0/2 on Switch-C is assigned the Alternate port role and is placed in the Discarding state.
Switch-A is elected Root Bridge for VLAN 50 due to having the lowest Bridge ID (Priority 24576+50=2462624576 + 50 = 24626, MAC 0010.1111.1111). Switch-C's interface Gi0/1 connects directly to Switch-A with a cumulative path cost of 4, making Gi0/1 the Root Port. Interface Gi0/2 on Switch-C connects to Switch-B. Both Switch-B and Switch-C have a Root Path Cost of 4 to reach Switch-A. When electing the Designated Port for the segment connecting Switch-B and Switch-C, Switch-B wins because its Bridge ID (24626.0010.2222.222224626.0010.2222.2222) is lower than Switch-C's Bridge ID (28722.0010.3333.333328722.0010.3333.3333). Therefore, Switch-B's interface becomes Designated Forwarding, and Switch-C's interface Gi0/2 becomes an Alternate port in the Discarding state.

Step-by-Step Solution

1
Calculate the Bridge ID for each switch for VLAN 50 by adding the System ID Extension (VLAN ID 50) to the base bridge priority.
Switch-A Bridge ID: 24576+50=2462624576 + 50 = 24626 with MAC 0010.1111.1111.
Switch-B Bridge ID: 24576+50=2462624576 + 50 = 24626 with MAC 0010.2222.2222.
Switch-C Bridge ID: 28672+50=2872228672 + 50 = 28722 with MAC 0010.3333.3333.
Switch-D Bridge ID: 32768+50=3281832768 + 50 = 32818 with MAC 0010.4444.4444.
Rapid PVST+ uses the 12-bit System ID Extension added to the 4-bit priority field to uniquely identify the bridge priority for each VLAN.
2
Determine the Root Bridge for VLAN 50.
Switch-A is elected Root Bridge because Switch-A and Switch-B tie on priority (24626), but Switch-A has the lower MAC address (0010.1111.1111 < 0010.2222.2222).
The switch with the lowest numerical Bridge ID (Priority + Sys-ID + MAC) becomes the Root Bridge.
3
Determine the Root Port for Switch-C.
Switch-C Gi0/1 connects directly to Root Bridge Switch-A with path cost 4. Switch-C Gi0/2 connects to Switch-B, which has a path cost of 4 to Switch-A (via Gi0/1), making the path via Gi0/2 cost 4+4=84 + 4 = 8. Thus, Switch-C Gi0/1 is the Root Port (Forwarding).
Root Port is selected based on the lowest cumulative Root Path Cost.
4
Determine the role of Switch-C GigabitEthernet0/2 on the link connecting Switch-C and Switch-B.
Both Switch-B and Switch-C advertise a Root Path Cost of 4 on this segment. To elect the Designated Port for the link, compare Bridge IDs of the advertising switches: Switch-B (24626.0010.2222.2222) vs Switch-C (28722.0010.3333.3333). Switch-B has the lower Bridge ID, so Switch-B Gi0/1 becomes the Designated Port (Forwarding). Consequently, Switch-C Gi0/2 becomes an Alternate Port and enters the Discarding state.
In Rapid PVST+, a non-root port receiving superior BPDUs from another switch on a point-to-point link becomes an Alternate port in the Discarding state to maintain a loop-free topology.

Key Concept

Rapid PVST+ Root Bridge Election, Path Cost Calculation, and Alternate Port Role Assignment
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Topics

Rapid PVST+ Spanning Tree Protocol Operations and Roles

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