ACSP Spanning Tree Protocol (STP) 5 — Questions and Answers
Question 1: In MSTP, what term describes the spanning tree instance that interconnects all MSTP regions and any 802.1D or RSTP bridges in the network?
- IST
- MST Region
- CIST
- CST (Correct answer)
Correct answer: CST
The CST (Common Spanning Tree) is the single spanning tree that connects all MST regions and legacy bridges from an external perspective.
Question 2: What determines MSTP region membership when two switches are being compared?
- Bridge priority and VLAN assignments only
- Configuration name, revision number, and VLAN-to-instance mapping table (Correct answer)
- MAC address ranges and port priorities
- Hello time and max age timer values
Correct answer: Configuration name, revision number, and VLAN-to-instance mapping table
Switches belong to the same MSTP region only if their configuration name, revision number, and VLAN-to-instance mapping are all identical.
Question 3: Which STP enhancement allows a switch to immediately use an alternate uplink when the primary uplink fails, without waiting for full STP convergence?
- BackboneFast
- UplinkFast (Correct answer)
- PortFast
- Loop Guard
Correct answer: UplinkFast
UplinkFast enables access-layer switches to immediately promote an alternate root port to Forwarding when the current root port fails, bypassing Listening and Learning states.
Question 4: A port is in RSTP Discarding state and receives a superior BPDU from the designated bridge on that segment. Which role does this port take?
- Root port
- Alternate port (Correct answer)
- Backup port
- Designated port
Correct answer: Alternate port
A Discarding port that receives superior BPDUs from the designated bridge on its segment becomes an Alternate port, holding a backup path to the root.
Question 5: What is the purpose of BPDU Filter when applied globally (not per-port) on an Aruba switch?
- It drops all BPDUs on all ports including uplinks
- It suppresses BPDUs only on PortFast-enabled ports that have not yet received a BPDU (Correct answer)
- It enables PortFast on all edge ports automatically
- It prevents the switch from sending any BPDUs
Correct answer: It suppresses BPDUs only on PortFast-enabled ports that have not yet received a BPDU
Global BPDU Filter suppresses BPDU transmission on PortFast-enabled ports, but if a BPDU is received on such a port, PortFast is disabled and normal STP resumes.
Question 6: Which STP enhancement allows a switch to immediately begin using a non-root path when an indirect link failure causes the root port to lose connectivity?
- UplinkFast
- BackboneFast (Correct answer)
- PortFast
- Root Guard
Correct answer: BackboneFast
BackboneFast detects indirect failures by processing inferior BPDUs and allows the switch to expire the Max Age timer immediately, accelerating convergence after indirect failures.
Question 7: On an Aruba switch, which spanning-tree configuration command sets the bridge priority to make it the preferred root bridge for MST instance 1?
- spanning-tree mst 1 priority 0 (Correct answer)
- spanning-tree priority 0 instance 1
- spanning-tree mst 1 root primary
- spanning-tree mst 1 bridge-priority 4096
Correct answer: spanning-tree mst 1 priority 0
The command 'spanning-tree mst <instance> priority <value>' sets the bridge priority for a specific MST instance; a priority of 0 makes it the most preferred root.
ACSP Spanning Tree Protocol (STP) 5
In MSTP, what term describes the spanning tree instance that interconnects all MSTP regions and any 802.1D or RSTP bridges in the network?