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?
- 2
- 4 (Correct answer)
- 19
- 100
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.
ACSP Spanning Tree Protocol (STP) 2
Which STP port state forwards BPDUs but discards user data frames and does not learn MAC addresses?