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Difficulty: Very hardSpanning Tree Protocol Operations

An enterprise network topology features four switches operating standard IEEE 802.1D Spanning Tree Protocol (STP) with the following bridge configurations:
- Switch-A: Priority 32768, MAC 00:00:0C:11:11:11
- Switch-B: Priority 28672, MAC 00:00:0C:33:33:33
- Switch-C: Priority 28672, MAC 00:00:0C:22:22:22
- Switch-D: Priority 32768, MAC 00:00:0C:44:44:44

The switches are connected in a physical ring topology: Switch-A connects to Switch-B, Switch-B connects to Switch-C, Switch-C connects to Switch-D, and Switch-D connects to Switch-A. All inter-switch links are Gigabit Ethernet (STP path cost = 4), except the link between Switch-B and Switch-C, which is 10-Gigabit Ethernet (STP path cost = 2). Which switch will be elected as the Root Bridge, and what role will Switch-D assign to its port on the link connecting directly to Switch-A?

  1. Switch-C is elected as the Root Bridge, and Switch-D assigns the Designated Port role to its interface connected to Switch-A.Answer
  2. B
    Switch-B is elected as the Root Bridge, and Switch-D assigns the Designated Port role to its interface connected to Switch-A.
  3. C
    Switch-C is elected as the Root Bridge, and Switch-D assigns the Alternate/Blocking Port role to its interface connected to Switch-A.
  4. D
    Switch-B is elected as the Root Bridge, and Switch-D assigns the Root Port role to its interface connected to Switch-A.

Answer

Switch-C is elected as the Root Bridge, and Switch-D assigns the Designated Port role to its interface connected to Switch-A.
Switch-C is elected as the Root Bridge because it ties for the lowest priority (28672) with Switch-B and has a lower MAC address (00:00:0C:22:22:22). For port roles, Switch-D's direct link to Switch-C yields a Root Path Cost of 4. Switch-A reaches Switch-C via Switch-B with a Root Path Cost of 6 (4 + 2). On the segment connecting Switch-A and Switch-D, Switch-D advertises a superior path cost to the Root (4 vs. 6), causing Switch-D to assume the Designated Port role on that link.

Step-by-Step Solution

1
Determine the Root Bridge election result across all four switches.
Switch-B and Switch-C share the lowest numerical bridge priority value of 28672. Comparing their MAC addresses, Switch-C (00:00:0C:22:22:22) is lower than Switch-B (00:00:0C:33:33:33). Therefore, Switch-C is elected Root Bridge.
STP elects the Root Bridge based first on the lowest Bridge Priority, and breaks ties using the lowest MAC address.
2
Calculate the Root Path Cost for Switch-D and identify its Root Port.
Switch-D's direct link to Switch-C has a path cost of 4 (Gigabit Ethernet). Switch-D's alternate path (Switch-D -> Switch-A -> Switch-B -> Switch-C) has a cost of 4 + 4 + 2 = 10. Switch-D designates its direct port to Switch-C as its Root Port with a Root Path Cost of 4.
Each non-root bridge selects the interface offering the lowest cumulative path cost back to the Root Bridge.
3
Calculate the Root Path Cost for Switch-A and evaluate the segment between Switch-A and Switch-D.
Switch-A's best path to Root is via Switch-B (cost 4 to Switch-B + cost 2 to Switch-C = 6). On the link between Switch-A and Switch-D, Switch-D advertises a Root Path Cost of 4, while Switch-A advertises a Root Path Cost of 6. Because 4 < 6, Switch-D's interface on this link becomes the Designated Port.
On any shared segment between two switches, the switch that advertises the lower Root Path Cost wins the Designated Port role for that segment.

Key Concept

Spanning Tree Protocol Root Bridge election and port role calculation based on lowest priority, MAC tie-breaking, and path cost evaluation.
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Topics

Spanning Tree Protocol Operations

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