An enterprise network uses Rapid PVST+ for spanning tree management across four Catalyst s… — Cisco CCNA

An enterprise network uses Rapid PVST+ for spanning tree management across four Catalyst switches (SW-A, SW-B, SW-C, and SW-D) servicing VLAN 25. The bridge priorities for VLAN 25 are configured as follows:
- SW-A: priority 4096
- SW-B: priority 8192
- SW-C: priority 16384
- SW-D: priority 32768

SW-C has two direct 1 Gbps links (STP cost 4) connected to SW-B: interface Gi0/1 connects to SW-B interface Gi0/1, and interface Gi0/2 connects to SW-B interface Gi0/2. SW-C also has a direct 10 Gbps link (STP cost 2) on interface Gi0/3 connected directly to SW-A. SW-B connects directly to SW-A via a 1 Gbps link on interface Gi0/24.

If all switch MAC addresses follow standard numbering based on their switch letter (e.g., SW-A has the lowest MAC address) and default port priorities (128) are used throughout, which port role and state will interface Gi0/1 on SW-C adopt for VLAN 25?

Alternate port in the Discarding stateAnswer
B
Root port in the Forwarding state
C
Backup port in the Discarding state
D
Designated port in the Forwarding state

Answer

Interface Gi0/1 on SW-C assumes the Alternate port role in the Discarding state.

In Rapid PVST+, SW-A is elected Root Bridge because it has the lowest Bridge ID (

4096 + 25 = 4121

). SW-C compares its path costs to SW-A: interface Gi0/3 has a 10 Gbps link (STP cost 2), giving a total cost of 2 to reach the Root Bridge. Interfaces Gi0/1 and Gi0/2 connect to SW-B (cost 4 to SW-B + cost 4 from SW-B to SW-A = total cost 8). Therefore, Gi0/3 is elected as SW-C's Root Port. On the links between SW-B and SW-C, SW-B is the Designated bridge because it advertises a lower Root Path Cost (4) than SW-C's path cost via SW-B. As a result, SW-C places its non-root interfaces connected to SW-B (Gi0/1 and Gi0/2) into the Alternate port role in the Discarding state.

Step-by-Step Solution

Determine the Root Bridge for VLAN 25.

SW-A has the lowest Bridge ID (

4096 + 25 = 4121

) and is elected as the Root Bridge.

STP elects the switch with the lowest Bridge ID (Priority + System ID Extension + MAC Address) as Root Bridge.

Calculate cumulative path costs from SW-C to Root Bridge (SW-A) to elect the Root Port.

Path via Gi0/3 (10 Gbps) has a cost of

2

. Path via Gi0/1 or Gi0/2 through SW-B has a cost of

4 \text{ (SW-C to SW-B)} + 4 \text{ (SW-B to SW-A)} = 8

. SW-C selects Gi0/3 as its Root Port in the Forwarding state.

The non-root bridge selects the interface with the lowest cumulative cost to the Root Bridge as its Root Port.

Determine Designated Bridge selection between SW-B and SW-C for the links on Gi0/1 and Gi0/2.

SW-B's Root Path Cost is

4

and Bridge ID is

8217

, whereas SW-C's Root Path Cost on those links would be higher. Additionally, SW-B has a superior Bridge ID (

8217 < 16409

). SW-B becomes the Designated switch for both links.

On each segment, the switch advertising the lowest Root Path Cost (or lowest Bridge ID as a tie-breaker) becomes the Designated Bridge.

Assign the port role and state to interface Gi0/1 on SW-C.

Because SW-C receives superior BPDUs on Gi0/1 from SW-B and the port is not its Root Port, Gi0/1 is designated as an Alternate port and placed in the Discarding state.

In Rapid PVST+, non-root ports that receive superior BPDUs from another switch are designated as Alternate ports and held in the Discarding state to prevent loops.

Key Concept

Rapid PVST+ Port Role and State Elections in Multi-Switch Topologies

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Topics

More questions from this topic