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Difficulty: MediumSpanning Tree Protocol Operations

In an enterprise network running standard IEEE 802.1D Spanning Tree Protocol (STP), Switch-1 serves as the Root Bridge. Two non-root switches, Switch-2 and Switch-3, are directly connected to each other over a point-to-point Gigabit Ethernet link via port FastEthernet 0/1 on both switches. Both Switch-2 and Switch-3 have an identical Root Path Cost of 19. Switch-2 has a Bridge ID of 32768:00-11-22-AA-BB-CC, while Switch-3 has a Bridge ID of 32768:00-11-22-11-22-33. Which switch interface will be elected as the Designated Port for this inter-switch link segment and remain in the Forwarding state?

  1. Switch-3 FastEthernet 0/1, because both switches have equal Root Path Costs and Switch-3 has the lower MAC address in its Bridge ID.Answer
  2. B
    Switch-2 FastEthernet 0/1, because Switch-2 has a higher numerical MAC address value which takes precedence during STP tiebreaker evaluation.
  3. C
    Switch-2 FastEthernet 0/1, because it blocks untagged frames when a native VLAN mismatch occurs across the link segment.
  4. D
    Both interfaces enter the Blocking state automatically to isolate traffic and prevent a potential Layer 2 loop on the segment.

Answer

Switch-3 FastEthernet 0/1 is elected as the Designated Port because both switches have equal Root Path Costs (1919) and Switch-3 has the lower MAC address (00-11-22-11-22-33<00-11-22-AA-BB-CC00\text{-}11\text{-}22\text{-}11\text{-}22\text{-}33 < 00\text{-}11\text{-}22\text{-}\text{AA}\text{-}\text{BB}\text{-}\text{CC}).
For every network segment, STP must elect one Designated Port that forwards traffic. The election process follows a strict hierarchy: 1) Lowest Root Path Cost, 2) Lowest Sender Bridge ID (Priority + MAC), 3) Lowest Sender Port ID. Since both switches have a Root Path Cost of 19 and identical Bridge Priorities (32768), STP evaluates the MAC addresses. Switch-3's MAC address (00-11-22-11-22-33) is lower than Switch-2's MAC address (00-11-22-AA-BB-CC), making Switch-3's interface the Designated Port in the Forwarding state.

Step-by-Step Solution

1
Evaluate the Root Path Cost for both switches connected to the network segment.
Both Switch-2 and Switch-3 have an identical Root Path Cost of 19.
STP first compares Root Path Costs to elect the Designated Port on a network segment.
2
Compare the Sender Bridge IDs of Switch-2 and Switch-3 since the Root Path Costs are equal.
Bridge Priorities are identical (3276832768). Comparing MAC addresses shows 00-11-22-11-22-3300\text{-}11\text{-}22\text{-}11\text{-}22\text{-}33 (Switch-3) is lower than 00-11-22-AA-BB-CC00\text{-}11\text{-}22\text{-}\text{AA}\text{-}\text{BB}\text{-}\text{CC} (Switch-2).
When Root Path Costs tie, the switch with the lowest Bridge ID (Priority + MAC) wins the election.
3
Determine the port roles on the link segment.
Switch-3 FastEthernet 0/1 becomes the Designated Port (Forwarding state), and Switch-2 FastEthernet 0/1 becomes the Non-Designated/Alternate Port (Blocking state).
Each active segment must have exactly one Designated Port to forward frames while preventing Layer 2 loops.

Key Concept

STP Designated Port Election and Tiebreaking Rules
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Topics

Spanning Tree Protocol Operations

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