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Difficulty: Very hardSpanning Tree Protocol Operations

An enterprise network topology consists of four switches operating standard IEEE 802.1w Rapid Spanning Tree Protocol (RSTP) using default path costs (1 Gbps=41\text{ Gbps} = 4, 100 Mbps=19100\text{ Mbps} = 19).

- Switch-R is configured with a bridge priority of 4096 (MAC address `00:11:22:33:44:55`).
- Switch-X, Switch-Y, and Switch-Z all maintain the default bridge priority of 32768.
- Switch-X connects to Switch-R via a 1 Gbps1\text{ Gbps} trunk link.
- Switch-Y connects to Switch-R via a 100 Mbps100\text{ Mbps} trunk link.
- Switch-Y also connects to Switch-X via a 1 Gbps1\text{ Gbps} trunk link.

Assuming all links are operational and STP convergence is complete, which statement accurately describes the Root Port selection and port status on Switch-Y?

  1. Switch-Y selects its interface connected to Switch-X as its Root Port, while its direct interface connected to Switch-R enters the Alternate (Discarding) state.Answer
  2. B
    Switch-Y selects its direct interface connected to Switch-R as its Root Port because direct physical connections to the Root Bridge take precedence over indirect paths regardless of cost.
  3. C
    Switch-Y selects its direct interface connected to Switch-R as its Root Port with a path cost of 19, while placing its interface connected to Switch-X into the Backup state.
  4. D
    Switch-Y designates both interfaces connected to Switch-R and Switch-X as active Root Ports to achieve Layer 2 load balancing toward the Root Bridge.

Answer

Switch-Y selects its interface connected to Switch-X as its Root Port, while its direct interface connected to Switch-R enters the Alternate (Discarding) state.
Root Port election on a non-root switch prioritizes the path with the lowest cumulative Root Path Cost to the Root Bridge. Switch-Y has two paths to Switch-R: a direct 100 Mbps100\text{ Mbps} link (cost 1919) and an indirect path via Switch-X (1 Gbps+1 Gbps1\text{ Gbps} + 1\text{ Gbps}, cost 4+4=84 + 4 = 8). Since 8<198 < 19, Switch-Y selects the link toward Switch-X as its Root Port. On the direct link between Switch-R and Switch-Y, Switch-R is the Designated Switch because it advertises a Root Path Cost of 00. Consequently, Switch-Y's direct port to Switch-R assumes the Alternate role and enters the Discarding state.

Step-by-Step Solution

1
Determine the Root Bridge of the topology.
Switch-R is elected Root Bridge because it has the lowest Bridge Priority (4096 vs 32768).
The switch with the lowest Bridge ID (Priority + MAC address) becomes the Root Bridge.
2
Calculate the cumulative Root Path Cost (RPC) for all candidate paths from Switch-Y to Switch-R.
Direct path via 100 Mbps100\text{ Mbps} link: RPC=19\text{RPC} = 19.
Indirect path via Switch-X (1 Gbps+1 Gbps1\text{ Gbps} + 1\text{ Gbps}): RPC=4+4=8\text{RPC} = 4 + 4 = 8.
Root Path Cost is the sum of path costs along the path to the Root Bridge.
3
Select the Root Port for Switch-Y based on the lowest Root Path Cost.
The interface connecting Switch-Y to Switch-X is elected as Switch-Y's Root Port because its path cost (88) is strictly less than the direct path cost (1919).
Each non-root switch selects a single Root Port corresponding to the path with the lowest cumulative cost to the Root Bridge.
4
Determine the role and state of Switch-Y's direct interface connected to Switch-R.
On the Switch-R to Switch-Y link, Switch-R is the Designated Bridge (advertising cost 00). Switch-Y's port on this segment receives superior BPDUs from Switch-R, so Switch-Y's local port becomes an Alternate Port in the Discarding state.
An interface that is neither a Root Port nor a Designated Port transitions to the Alternate role (Discarding/Blocking state) to prevent Layer 2 loops.

Key Concept

Spanning Tree Protocol Root Port Election and Root Path Cost Evaluation
Estimated Time:2m 0s
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Topics

Spanning Tree Protocol Operations

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