Cisco CCNA Spanning Tree Protocol (STP) 5 — Questions and Answers
Question 1: Which STP feature prevents a port connected to a non-root switch from becoming a root port and thus prevents root bridge changes?
- Root Guard (Correct answer)
- Loop Guard
- BPDU Guard
- PortFast
Correct answer: Root Guard
Root Guard enforces the root bridge placement by putting a port in root-inconsistent state if a superior BPDU is received on it.
Question 2: How does RSTP achieve faster convergence compared to traditional 802.1D STP?
- RSTP uses a proposal/agreement handshake mechanism between neighboring switches (Correct answer)
- RSTP eliminates the need for BPDUs entirely
- RSTP uses shorter timer values only
- RSTP elects multiple root bridges to distribute convergence
Correct answer: RSTP uses a proposal/agreement handshake mechanism between neighboring switches
RSTP's proposal/agreement mechanism allows ports to synchronize and transition to Forwarding without waiting for timers to expire.
Question 3: What STP port cost is assigned to a 100 Mbps Fast Ethernet link using the original IEEE 802.1D-1998 cost table?
- 19 (Correct answer)
- 10
- 4
- 100
Correct answer: 19
The original 802.1D-1998 standard assigns a cost of 19 to 100 Mbps links; this is the default cost Cisco switches use.
Question 4: In STP, which switch port is elected as the designated port on each network segment?
- The port with the lowest cumulative path cost to the root bridge on that segment (Correct answer)
- The port with the highest bridge priority
- The port with the lowest port ID
- The port on the switch with the most VLANs
Correct answer: The port with the lowest cumulative path cost to the root bridge on that segment
On each segment, the designated port is the one offering the best (lowest cost) path to the root bridge; if tied, the lower bridge ID wins.
Question 5: A switch port is stuck in the STP Blocking state and does not transition even after the network stabilizes. Which feature is most likely responsible?
- Root Guard placed the port in root-inconsistent blocking state (Correct answer)
- PortFast was not enabled on the port
- The switch has too many VLANs configured
- The forward delay timer is set too high
Correct answer: Root Guard placed the port in root-inconsistent blocking state
Root Guard places a port in root-inconsistent (blocking) state when a superior BPDU is received; the port stays blocked until no more superior BPDUs arrive.
Question 6: Which command configures BPDU Guard globally on all PortFast-enabled ports on a Cisco switch?
- spanning-tree portfast bpduguard default (Correct answer)
- spanning-tree bpduguard enable
- interface range fa0/1-24 spanning-tree bpduguard enable
- spanning-tree guard bpdu
Correct answer: spanning-tree portfast bpduguard default
The global command 'spanning-tree portfast bpduguard default' enables BPDU Guard on all ports that have PortFast configured.
Question 7: What is the purpose of the STP max age timer (default 20 seconds)?
- It defines how long a switch stores BPDU information before discarding it as stale (Correct answer)
- It sets the maximum time a port spends in the Forwarding state
- It limits how long a switch waits to elect a root bridge
- It controls the interval between BPDU transmissions
Correct answer: It defines how long a switch stores BPDU information before discarding it as stale
If a switch does not receive a BPDU within the max age period (20s default), it assumes the path has failed and begins STP recalculation.
Cisco CCNA Spanning Tree Protocol (STP) 5
Which STP feature prevents a port connected to a non-root switch from becoming a root port and thus prevents root bridge changes?