An enterprise topology runs Rapid PVST+ across four switches for VLAN 40. Switch SW1 serve… — Cisco CCNA

An enterprise topology runs Rapid PVST+ across four switches for VLAN 40. Switch SW1 serves as the Root Bridge with a priority of $4096$ (total priority $4136$ with System ID Extension). Switches SW2 and SW3 are distribution switches, both configured with a base bridge priority of $28672$ ( $28712$ total). Switch SW4 is an access switch connected to SW2 via interface GigabitEthernet1/0/1 and to SW3 via interface GigabitEthernet1/0/2. All interconnecting links between switches are 10-Gigabit Ethernet ( $10\text{ Gbps}$ , cost $2$ ).

The MAC addresses of the distribution switches are:
- SW2 MAC: `0011.2233.4455`
- SW3 MAC: `0011.2233.6677`

Both SW2 and SW3 have direct single-link $10\text{ Gbps}$ connections to SW1. Which statement correctly identifies the Rapid PVST+ port role and state for interface GigabitEthernet1/0/2 on Switch SW4, along with the precise technical reason?

Interface GigabitEthernet1/0/2 becomes an Alternate port in the Discarding state because SW4 receives a superior BPDU from SW2 due to SW2's lower MAC address.Answer
B
Interface GigabitEthernet1/0/2 becomes an Alternate port in the Blocking state because legacy 802.1D Spanning Tree port states govern non-root redundant paths.
C
Interface GigabitEthernet1/0/2 becomes a Backup port in the Discarding state because it provides a redundant backup path to the same physical switch chassis.
D
Interface GigabitEthernet1/0/2 becomes the Root Port in the Forwarding state because SW4 prefers its highest local interface index when path costs match.

Answer

Interface GigabitEthernet1/0/2 becomes an Alternate port in the Discarding state because SW4 receives a superior BPDU from SW2 due to SW2's lower MAC address.

The correct answer identifies that interface GigabitEthernet1/0/2 becomes an Alternate port in the Discarding state. SW4 evaluates two potential paths to the Root Bridge (SW1). Both paths have equal cumulative root path costs of 4 (

2 + 2

). Comparing the upstream switch Bridge IDs, both SW2 and SW3 have identical bridge priorities of 28712 (

28672 \text{ base} + 40 \text{ VLAN ID}

). The tie is broken by MAC address comparison, where SW2 (`0011.2233.4455`) is lower than SW3 (`0011.2233.6677`). Thus, SW2 provides superior BPDUs, causing SW4 to select GigabitEthernet1/0/1 as its Root Port and GigabitEthernet1/0/2 as an Alternate port in the Rapid PVST+ Discarding state.

Step-by-Step Solution

Calculate Cumulative Root Path Cost for each path on SW4

Path via SW2 =

2\text{ (SW4-SW2)} + 2\text{ (SW2-SW1)} = 4

. Path via SW3 =

2\text{ (SW4-SW3)} + 2\text{ (SW3-SW1)} = 4

. Costs are equal.

Rapid PVST+ first compares cumulative root path cost to elect the Root Port.

Evaluate Designated Bridge ID (Sender Bridge Priority + MAC)

SW2 Priority =

28672 + 40 = 28712

. SW3 Priority =

28672 + 40 = 28712

. Priorities are equal, so compare MAC addresses: `0011.2233.4455` (SW2) vs `0011.2233.6677` (SW3). SW2 has the lower MAC address.

When root path costs are equal, the switch evaluates the sender's Designated Bridge ID.

Determine Port Role and State for SW4's interfaces

GigabitEthernet1/0/1 (connected to SW2) is elected as Root Port (Forwarding state). GigabitEthernet1/0/2 (connected to SW3) becomes an Alternate port (Discarding state).

Rapid PVST+ assigns Alternate port role to non-root interfaces receiving inferior BPDUs from alternate upstream switches and places them in the Discarding state.

Key Concept

Rapid PVST+ Root Port Election and Alternate Port State Operations

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