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Difficulty: HardSpanning Tree Protocol Operations

An enterprise layer 2 network topology consists of four switches (Switch-1, Switch-2, Switch-3, and Switch-4) connected in a full mesh loop redundancy layout running IEEE 802.1D Spanning Tree Protocol (STP). The switches have the following Bridge ID configurations:

- Switch-1: Priority `32768`, MAC `00-0A-00-00-00-01`
- Switch-2: Priority `8192`, MAC `00-0B-00-00-00-01`
- Switch-3: Priority `32768`, MAC `00-0C-00-00-00-01`
- Switch-4: Priority `4096`, MAC `00-0D-00-00-00-01`

Link speeds and path costs between the switches are established as follows:
- Switch-4 to Switch-1: 1 Gbps link (Cost = 44)
- Switch-4 to Switch-2: 10 Gbps link (Cost = 22)
- Switch-2 to Switch-3: 10 Gbps link (Cost = 22)
- Switch-1 to Switch-3: 1 Gbps link (Cost = 44)
- Switch-1 to Switch-2: 1 Gbps link (Cost = 44)

Based on STP operational rules after convergence, which of the following statements are correct regarding root bridge election and port role calculations? (Select TWO.)

  1. Switch-4 is elected as the STP Root Bridge for the topology because it possesses the lowest numerical Bridge Priority value.Answer
  2. The interface on Switch-3 connected directly to Switch-1 will transition to the Blocking state.Answer
  3. C
    Switch-2 is elected as the Root Bridge because it has the highest bandwidth link (10 Gbps) directly attached to the network backbone.
  4. D
    Switch-3 selects its interface toward Switch-1 as its Root Port because Switch-1 has a lower MAC address than Switch-2.
  5. E
    Native VLAN traffic on trunk links will bypass STP port state restrictions and remain forwarding on all paths.

Answer

The two correct statements are: Switch-4 is elected as the STP Root Bridge due to having the lowest numerical Bridge Priority (4096), and Switch-3's port connected to Switch-1 enters the Blocking state after path cost and Designated Bridge calculations.
Switch-4 wins the root election because its priority value of 4096 is strictly lower than any other switch on the network. Furthermore, Switch-3 calculates a lower cost path to the root through Switch-2 (cost of 4) than through Switch-1 (cost of 8). On the link between Switch-1 and Switch-3, both switches have an equal root path cost of 4. Switch-1 breaks the tie because of its lower MAC address (00-0A... vs 00-0C...), claiming the Designated Port role and placing Switch-3's connected port into the Blocking state.

Step-by-Step Solution

1
Determine the Root Bridge
Switch-4 has Priority 4096, Switch-2 has 8192, Switch-1 and Switch-3 have 32768. The lowest Bridge Priority wins.
STP elects the switch with the lowest Bridge ID (Priority concatenated with MAC address) as the Root Bridge.
2
Calculate Root Path Costs for each switch
Switch-2 path to Root = Cost 2 (Direct 10 Gbps link). Switch-1 path to Root = Cost 4 (Direct 1 Gbps link). Switch-3 path via Switch-2 = Cost 2 + 2 = 4; path via Switch-1 = Cost 4 + 4 = 8. Switch-3 chooses interface toward Switch-2 as its Root Port.
Switches select the path with the lowest cumulative cost to reach the Root Bridge.
3
Determine Designated and Blocking ports on the Switch-1 to Switch-3 segment
Switch-1 Root Cost = 4. Switch-3 Root Cost = 4. Cost is tied. Switch-1 Bridge ID (32768:00-0A...) is lower than Switch-3 Bridge ID (32768:00-0C...). Switch-1 port becomes Designated (Forwarding), and Switch-3 port becomes Non-Designated (Blocking).
When Root Path Cost on a shared segment is tied, the switch with the lower Bridge ID wins the Designated Port role, forcing the opposite end into Blocking.

Key Concept

STP Root Bridge Election and Port Role Determinism
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Topics

Spanning Tree Protocol Operations

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