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Difficulty: HardSpanning Tree Protocol Operations

A network administrator connects Switch-A (Root Bridge) to Switch-B using two parallel 10 Gbps Ethernet links for path redundancy. The connections are established as follows:
- Link 1 connects Switch-A port Gi1/0/1 to Switch-B port Gi1/0/1.
- Link 2 connects Switch-A port Gi1/0/2 to Switch-B port Gi1/0/2.

On Switch-A, port Gi1/0/1 uses the default port priority of 128, while port Gi1/0/2 is manually configured with a port priority of 64. On Switch-B, port Gi1/0/1 is manually configured with a port priority of 32, while port Gi1/0/2 retains the default port priority of 128.

Assuming all other Spanning Tree Protocol (STP) parameters remain default and both links are operational, which interface on Switch-B will be selected as the Root Port, and why?

  1. Interface Gi1/0/2, because it receives BPDUs from Switch-A with a lower sender port priority.Answer
  2. B
    Interface Gi1/0/1, because it has a lower local port priority configured on Switch-B.
  3. C
    Interface Gi1/0/1, because Switch-A's interface Gi1/0/1 transmits a higher numerical port priority value.
  4. D
    Interface Gi1/0/2, because it has a higher local port priority than interface Gi1/0/1 on Switch-B.

Answer

Interface Gi1/0/2 will be elected as the Root Port on Switch-B because it receives BPDUs containing a lower upstream (Designated) port priority value (64) from Switch-A.
When a non-root switch has multiple equal-cost paths to the Root Bridge originating from the same upstream switch, Root Port selection is determined by evaluating the upstream sender's Port ID (Port Priority + Port Index). Switch-A transmits BPDUs out of Gi1/0/2 with a port priority of 64, which is lower than the default priority of 128 sent out of Gi1/0/1. Switch-B receives this lower value on its Gi1/0/2 interface and selects it as the Root Port.

Step-by-Step Solution

1
Evaluate the Root Port decision criteria sequence for Switch-B.
Switch-B compares incoming BPDUs on Gi1/0/1 and Gi1/0/2 using the standard STP tie-breaker hierarchy.
Non-root switches must select exactly one Root Port facing the Root Bridge.
2
Compare Root Path Cost and Designated Bridge ID.
Both ports receive BPDUs from the same Root Bridge (Switch-A) over identical 10 Gbps links, resulting in a tie for both criteria.
Both links connect directly between Switch-A and Switch-B at identical speed.
3
Evaluate the Designated (sender) Port ID tie-breaker.
Switch-A transmits BPDUs with Port ID 128.1 on link 1 and Port ID 64.2 on link 2.
Port ID consists of Port Priority + Port Number. Switch-A port Gi1/0/2 advertises a priority of 64, which is lower than 128.
4
Select the Root Port based on lowest Designated Port ID.
Switch-B port Gi1/0/2 receives the BPDU with the lower Designated Port ID (64.2) and becomes the Root Port.
Lower numerical values take precedence in STP decision processes.

Key Concept

STP Root Port Selection & Designated Sender Port ID Tie-Breakers
Estimated Time:1m 30s
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Topics

Spanning Tree Protocol Operations

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