ACSP Spanning Tree Protocol (STP) 3 — Questions and Answers
Question 1: In an Aruba switch running MSTP, which command correctly maps VLAN 10 and VLAN 20 to MST instance 1?
- spanning-tree mst 1 vlan 10,20 (Correct answer)
- spanning-tree instance 1 vlan 10 20
- spanning-tree mstp vlan 10 20 instance 1
- spanning-tree mst instance 1 add vlan 10-20
Correct answer: spanning-tree mst 1 vlan 10,20
On Aruba switches, the correct syntax to assign VLANs to an MST instance is 'spanning-tree mst <instance> vlan <vlan-list>'.
Question 2: What happens to STP topology when two switches have the same bridge priority and the same MAC address is theoretically used?
- Both switches become root simultaneously
- The switch with the lower MAC address becomes root (Correct answer)
- The switch with the higher MAC address becomes root
- STP fails to converge and loops form
Correct answer: The switch with the lower MAC address becomes root
When bridge priorities are equal, STP uses the MAC address as a tiebreaker, and the numerically lower MAC address wins the root election.
Question 3: Which STP timer controls how long a switch stores BPDU information before considering the topology stale?
- Hello time
- Forward delay
- Max age (Correct answer)
- Topology change notification timer
Correct answer: Max age
Max age (default 20 seconds) is the time a switch retains BPDU information; if no BPDU is received within this period, the port information ages out.
Question 4: Which feature prevents a non-root bridge from becoming root by blocking superior BPDUs received on a designated port?
- BPDU Guard
- Root Guard (Correct answer)
- Loop Guard
- PortFast
Correct answer: Root Guard
Root Guard places a port in root-inconsistent state if a superior BPDU is received, preventing that port from becoming the root port.
Question 5: In RSTP, what is the purpose of the Backup port role?
- It provides an alternate path to the root bridge
- It provides redundancy to a segment where the switch itself is the designated bridge (Correct answer)
- It acts as a standby for the root port
- It connects to non-STP-capable devices
Correct answer: It provides redundancy to a segment where the switch itself is the designated bridge
An RSTP Backup port is a redundant path to a segment for which the local switch is already the designated bridge, typically seen with hubs.
Question 6: A network administrator notices frequent topology changes causing MAC table flushes. Which STP enhancement can reduce unnecessary topology change notifications on access ports?
- BPDU Guard
- PortFast (Correct answer)
- Root Guard
- Uplink Fast
Correct answer: PortFast
PortFast suppresses Topology Change Notifications (TCNs) on edge ports since link flaps on host ports should not trigger network-wide MAC table flushes.
Question 7: What is the STP port cost for a 100 Mbps link using the original IEEE 802.1D short path cost method?
- 4
- 10
- 19 (Correct answer)
- 100
Correct answer: 19
The original IEEE 802.1D short path cost assigns a cost of 19 to a 100 Mbps Fast Ethernet link.
ACSP Spanning Tree Protocol (STP) 3
In an Aruba switch running MSTP, which command correctly maps VLAN 10 and VLAN 20 to MST instance 1?