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Difficulty: HardSpanning Tree Protocol Operations

Two enterprise switches, Switch-1 and Switch-2, run Rapid Spanning Tree Protocol (IEEE 802.1w). Switch-1 is the elected Root Bridge. Switch-2 connects to Switch-1 via a direct full-duplex Ethernet link. Additionally, Switch-2 has two redundant physical links connected directly to the same downstream shared hub. Which TWO of the following statements correctly describe the RSTP port roles and operational behaviors in this scenario?

  1. The full-duplex link between Switch-1 and Switch-2 allows Switch-2 to rapidly transition its Root Port to the Forwarding state via the sync and Proposal/Agreement handshake without relying on legacy timers.Answer
  2. B
    Switch-2's Root Port on the point-to-point link must remain in the Listening state for 15 seconds and Learning state for 15 seconds before transitioning to Forwarding.
  3. On the dual connections to the shared hub, one port on Switch-2 is elected as the Designated Port while the second port on Switch-2 becomes a Backup Port.Answer
  4. D
    Both interfaces on Switch-2 connected to the shared hub will be assigned the Alternate Port role to provide backup connectivity to the Root Bridge.
  5. E
    To force Switch-2 to become the Root Bridge instead of Switch-1, an administrator should increase Switch-2's bridge priority value to 61440.

Answer

The correct statements are that the full-duplex point-to-point link enables rapid Root Port forwarding via the Proposal/Agreement handshake, and that connecting two interfaces from the same switch to a shared hub results in one Designated Port and one Backup Port.
The statement regarding full-duplex rapid transitions is correct because RSTP replaces timer-based 802.1D transitions on point-to-point links with an active sync and Proposal/Agreement handshake. The statement regarding the shared hub is correct because when a switch receives its own BPDUs on a secondary link connected to a common hub, the primary link becomes the Designated Port and the secondary link becomes a Backup Port.

Step-by-Step Solution

1
Analyze point-to-point link convergence mechanisms under IEEE 802.1w (RSTP).
RSTP uses full-duplex link state detection to execute an explicit Proposal/Agreement handshake, allowing fast convergence to the Forwarding state without waiting for legacy 15-second Listening and Learning forward delay timers.
Point-to-point links guarantee that only two switches exist on the segment, enabling direct negotiation.
2
Differentiate between RSTP Alternate and Backup port roles on Switch-2.
An Alternate Port receives superior BPDUs from another switch (providing a backup path to the Root Bridge). A Backup Port receives superior BPDUs from its own switch on a shared segment (hub).
Because both links connect to the same hub, Switch-2 hears its own BPDUs. The port with the lower Port ID becomes Designated, and the redundant port on the same switch becomes Backup.
3
Evaluate the Root Bridge priority selection mechanics.
Lower numerical bridge priority values are preferred (e.g., 4096 is preferred over 32768). Raising the priority to 61440 makes it least likely to become Root.
STP root elections always choose the minimum Bridge Identifier value.

Key Concept

RSTP Port Roles (Alternate vs. Backup) and Fast Convergence Mechanisms
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Topics

Spanning Tree Protocol Operations

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