ACSP Spanning Tree Protocol (STP) 2 — Questions and Answers
Question 1: Which STP port state forwards BPDUs but discards user data frames and does not learn MAC addresses?
- Blocking
- Listening (Correct answer)
- Learning
- Forwarding
Correct answer: Listening
In the Listening state, a port sends and receives BPDUs to determine topology but does not forward data frames or learn MAC addresses.
Question 2: What is the default STP port cost for a 10 Gbps link using IEEE 802.1D-2004 long path cost values?
Correct answer: 4
IEEE 802.1D-2004 assigns a long path cost of 4 to a 10 Gbps link, replacing the original short values.
Question 3: In MSTP, what is the name of the instance that all VLANs belong to by default before being mapped to an MST instance?
- IST (Internal Spanning Tree)
- CST (Common Spanning Tree)
- CIST (Common and Internal Spanning Tree) (Correct answer)
- MSTI 0
Correct answer: CIST (Common and Internal Spanning Tree)
The CIST is the spanning tree that interconnects all MST regions and the CST, and all VLANs default to MSTI 0 within it.
Question 4: Which RSTP port role replaces the STP Blocked port and provides an alternate path to the root bridge?
- Backup port
- Alternate port (Correct answer)
- Designated port
- Edge port
Correct answer: Alternate port
The RSTP Alternate port provides a redundant path to the root and can rapidly transition to Forwarding if the root port fails.
Question 5: What STP feature causes a switch port to immediately enter Forwarding state when connected to an end device, bypassing Listening and Learning states?
- BPDU Guard
- Root Guard
- PortFast (Correct answer)
- Loop Guard
Correct answer: PortFast
PortFast enables edge ports to skip the Listening and Learning states and go directly to Forwarding, reducing connectivity delays for end hosts.
Question 6: A switch receives a BPDU with a lower bridge ID than its own on a designated port. What action does STP take?
- The port transitions to Blocking state (Correct answer)
- The port transitions to Forwarding state immediately
- The switch becomes the new root bridge
- The BPDU is discarded and the topology remains unchanged
Correct answer: The port transitions to Blocking state
When a designated port receives a superior BPDU, it yields and transitions to Blocking (or Discarding in RSTP), losing its designated role.
Question 7: Which RSTP link type enables rapid transition to Forwarding without waiting for a proposal-agreement handshake?
- Point-to-point full-duplex
- Shared half-duplex
- Edge port with PortFast (Correct answer)
- Trunk port with BPDU Filter
Correct answer: Edge port with PortFast
Edge ports configured with PortFast in RSTP immediately enter Forwarding state because they are not connected to other bridges, requiring no handshake.
