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Difficulty: MediumSpanning Tree Protocol Operations

Three switches—S1, S2, and S3—are connected in a triangular physical topology using standard IEEE 802.1D Spanning Tree Protocol (STP):

- S1 has a Bridge Priority of 4096 and MAC address 00:11:22:33:44:55.
- S2 has a Bridge Priority of 32768 and MAC address 00:AA:BB:CC:DD:EE.
- S3 has a Bridge Priority of 32768 and MAC address 00:AA:BB:CC:DD:FF.

Link speeds and default STP path costs are configured as follows:
- The link between S1 and S2 is 1 Gbps (STP Cost = 4).
- The link between S2 and S3 is 1 Gbps (STP Cost = 4).
- The link between S1 and S3 is 100 Mbps (STP Cost = 19).

Based on STP operations, which TWO of the following statements accurately describe port role determinations and traffic paths in this network?

  1. Switch S3 selects its interface connected to Switch S2 as its Root Port because the cumulative path cost to the Root Bridge via S2 is 8, which is lower than the direct path cost of 19.Answer
  2. Switch S2's interface on the link connecting S2 and S3 assumes the Designated Port role because Switch S2 advertises a lower Root Path Cost to the segment than Switch S3.Answer
  3. C
    Switch S3 selects its direct interface connected to Switch S1 as its Root Port because a directly connected link is always preferred over a multi-hop path regardless of link bandwidth.
  4. D
    Switch S3's interface connected to Switch S2 is placed into the Blocking state because S3 has a numerically higher MAC address than S2.
  5. E
    A native VLAN mismatch on the trunk between S1 and S3 will prevent S1 from being elected as the Root Bridge for the entire topology.

Answer

The two correct statements are that Switch S3 selects its interface connected to Switch S2 as its Root Port (due to a lower cumulative path cost of 8 versus 19), and that Switch S2's interface on the link to Switch S3 becomes the Designated Port (because S2 has a lower advertised root path cost than S3).
In standard IEEE 802.1D STP, non-root switches elect their Root Port based on the lowest cumulative root path cost to the Root Bridge. Switch S1 is the Root Bridge due to its priority of 4096. Switch S3 evaluates two paths to S1: the direct 100 Mbps link with a cost of 19, and the path through Switch S2 (1 Gbps + 1 Gbps) with a cumulative cost of 4 + 4 = 8. Because 8 is less than 19, S3 selects its interface toward S2 as its Root Port. Furthermore, for the segment connecting S2 and S3, S2 advertises a Root Path Cost of 4 while S3 advertises a Root Path Cost of 8. Since S2 advertises a lower root path cost, S2's interface on that segment is elected as the Designated Port.

Step-by-Step Solution

1
Determine the Root Bridge
Switch S1 is elected Root Bridge because it has the lowest Bridge Priority (4096 < 32768). All active ports on S1 become Designated Ports in the Forwarding state.
STP elects the bridge with the lowest Bridge ID (Priority + MAC) as the Root Bridge.
2
Calculate Root Path Costs for Switch S2 and Switch S3
S2 direct path cost to S1 = 4. S3 path cost via S1 (direct 100 Mbps) = 19; S3 path cost via S2 (1 Gbps + 1 Gbps) = 4 + 4 = 8.
Root path cost is cumulative, adding incoming port path costs along the shortest path back to the Root Bridge.
3
Assign Root Ports for non-root switches
S2's port to S1 becomes S2's Root Port (cost 4). S3's port to S2 becomes S3's Root Port (cost 8 < 19).
Each non-root switch chooses one port with the lowest cumulative root path cost as its Root Port.
4
Determine Designated Port on the S2-S3 segment and remaining port roles
S2 advertises cost 4, S3 advertises cost 8. S2 wins the segment, so S2's port on the S2-S3 link is Designated. S3's port connected directly to S1 goes into Blocking state (non-designated).
The switch advertising the lower root path cost onto a shared segment wins the Designated Port role.

Key Concept

STP Root Port & Designated Port Path Cost Calculations
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Topics

Spanning Tree Protocol Operations

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