Cisco CCNA (200-301)Network AccessMedium
Three Rapid PVST+ switches (SW1, SW2, SW3) form a triangle for VLAN 10. SW1 is elected root. SW2 and SW3 each connect directly to SW1 and also to each other. After convergence, the port on SW3 facing SW2 has a higher root path cost than the port on SW2 facing SW3. Which RSTP role and port state will the SW3-to-SW2 port settle into?
- ABackup port, listening state
- BRoot port, forwarding state
- CDesignated port, forwarding state
- DAlternate port, discarding state
Show answer & explanationAnswer & explanation
Correct answer: D. Alternate port, discarding state
On the shared segment between SW2 and SW3, the port with the lower cost to the root (SW2's port) becomes designated and forwards, while the higher-cost port on SW3 becomes the alternate port and stays in the discarding state to prevent a loop. Backup ports only appear when two ports on the same switch connect to the same segment (e.g., a hub), which is not the case here.
Why the other options are wrong
- A. Backup ports require two ports on the same switch on one segment, not applicable here.
- B. Root ports point toward the root bridge on the shortest path, which for SW3 is its direct link to SW1.
- C. The lower-cost port on SW2, not SW3, becomes designated.
RSTP Alternate Port
In Rapid PVST+/RSTP, the alternate port provides a backup path to the root bridge and remains in the discarding state to prevent loops.
- Root port = lowest cost path to root on a switch
- Designated port = lowest cost port for a segment
- Alternate port = backup path, discarding state
- Backup port = redundant port on same switch/segment (rare, e.g. hub)
Memory trick: Only one door per hallway can be open — the pricier door for that segment stays shut but ready.