ACSP Spanning Tree Protocol (STP) 4 — Questions and Answers
Question 1: Which RSTP proposal-agreement mechanism element must be true for a port to rapidly transition to Forwarding without going through full STP timers?
- The link must be half-duplex
- The link must be point-to-point full-duplex (Correct answer)
- The link must have BPDU Guard enabled
- The link speed must be at least 1 Gbps
Correct answer: The link must be point-to-point full-duplex
RSTP rapid transition via proposal-agreement requires a point-to-point link, which is automatically detected on full-duplex connections.
Question 2: In MSTP, what is the maximum number of MST instances that can be configured (excluding MSTI 0/CIST)?
- 15 (Correct answer)
- 16
- 64
- 4094
Correct answer: 15
IEEE 802.1Q allows up to 64 MST instances total (0-63), with MSTI 0 reserved as the CIST, leaving 63 configurable instances, but most vendors limit this to 15 or 16.
Question 3: What STP feature disables a PortFast-enabled port when it receives a BPDU, protecting against accidental switch connections?
- Root Guard
- Loop Guard
- BPDU Guard (Correct answer)
- BPDU Filter
Correct answer: BPDU Guard
BPDU Guard err-disables a port upon receiving any BPDU, preventing switches from being connected to PortFast ports and disrupting the topology.
Question 4: Which spanning tree mode on Aruba switches is recommended for modern networks with multiple VLANs to provide per-VLAN load balancing?
- STP (802.1D)
- RSTP (802.1w)
- MSTP (802.1s) (Correct answer)
- PVST+
Correct answer: MSTP (802.1s)
MSTP (802.1s) maps multiple VLANs to a smaller number of spanning tree instances, enabling per-group load balancing while reducing control plane overhead.
Question 5: A switch's root port has failed. In RSTP, which port transitions to Forwarding most quickly to restore connectivity?
- The Backup port
- The Alternate port (Correct answer)
- The Designated port
- A newly elected Designated port
Correct answer: The Alternate port
The RSTP Alternate port already holds a pre-computed alternate path to the root and can immediately transition to Forwarding when the root port fails, without waiting for timers.
Question 6: What is the effect of configuring 'spanning-tree port-type admin-edge' on an Aruba switch port?
- It sets the port cost to 0
- It enables PortFast behavior and suppresses TCNs on that port (Correct answer)
- It forces the port into Blocking state permanently
- It disables BPDU processing on the port
Correct answer: It enables PortFast behavior and suppresses TCNs on that port
Configuring a port as admin-edge on Aruba switches enables PortFast-equivalent behavior, causing immediate Forwarding transition and suppressing topology change notifications.
Question 7: Which condition triggers Loop Guard to place an STP port into a loop-inconsistent state?
- A superior BPDU is received on a designated port
- BPDUs stop being received on a root or alternate port (Correct answer)
- A BPDU is received on a PortFast-enabled port
- The root bridge stops sending hello BPDUs on all ports
Correct answer: BPDUs stop being received on a root or alternate port
Loop Guard detects the loss of incoming BPDUs on a non-designated port and places it in loop-inconsistent (Blocking) state to prevent it from erroneously transitioning to Forwarding.
ACSP Spanning Tree Protocol (STP) 4
Which RSTP proposal-agreement mechanism element must be true for a port to rapidly transition to Forwarding without going through full STP timers?