STP Advanced Features and Variants 2 — Questions and Answers
Question 1: What does MSTP (802.1s) allow that RSTP alone does not?
- Multiple VLANs mapped to different spanning tree instances (Correct answer)
- Faster convergence than RSTP
- Port authentication before STP participation
- Dynamic VLAN assignment based on MAC addresses
Correct answer: Multiple VLANs mapped to different spanning tree instances
MSTP allows multiple VLANs to be mapped to separate spanning tree instances, enabling load balancing across redundant links.
Question 2: In MSTP, what is the Common and Internal Spanning Tree (CIST)?
- A spanning tree that interconnects all MST regions and the CST (Correct answer)
- The spanning tree used only inside a single MST region
- A redundant backup spanning tree for failover
- The spanning tree used exclusively for native VLAN traffic
Correct answer: A spanning tree that interconnects all MST regions and the CST
The CIST is the single spanning tree that connects all MST regions together and interoperates with 802.1D/RSTP bridges.
Question 3: Which STP variant was specifically designed to reduce convergence time to under one second by using a proposal/agreement handshake?
- Classic STP (802.1D)
- PVST+
- RSTP (802.1w) (Correct answer)
- MSTP (802.1s)
Correct answer: RSTP (802.1w)
RSTP uses a proposal/agreement mechanism between adjacent switches to achieve near-instant convergence without relying on timers.
Question 4: What is the function of the MST region in MSTP?
- A group of switches sharing the same MST configuration name, revision, and VLAN-to-instance mapping (Correct answer)
- A group of switches running the same IOS version
- A logical grouping of ports in PortFast mode
- A set of VLANs assigned to the native VLAN trunk
Correct answer: A group of switches sharing the same MST configuration name, revision, and VLAN-to-instance mapping
Switches belong to the same MST region when they share identical configuration name, revision number, and VLAN-to-instance table.
Question 5: Which STP feature prevents a port from becoming a root port by ignoring received superior BPDUs?
- PortFast
- Root Guard (Correct answer)
- BPDU Guard
- Loop Guard
Correct answer: Root Guard
Root Guard enforces the root bridge placement by placing a port into root-inconsistent state if a superior BPDU is received.
Question 6: In Rapid PVST+, what happens when a non-edge port receives a BPDU on a port that was previously in Forwarding state due to Dispute mechanism?
- The port transitions immediately to Blocking
- The port is placed in Discarding state to prevent unidirectional link loops (Correct answer)
- The port sends a TCN to the root bridge
- The port starts a new timer equal to MaxAge
Correct answer: The port is placed in Discarding state to prevent unidirectional link loops
The Dispute mechanism detects unidirectional link failures and places the port in Discarding state to prevent loops.
Question 7: What is the primary advantage of PVST+ over classic 802.1D STP?
- It eliminates the need for a root bridge entirely
- It runs a separate spanning tree instance per VLAN, enabling per-VLAN load balancing (Correct answer)
- It uses only one BPDU for all VLANs to reduce overhead
- It replaces BPDUs with LLDP for topology discovery
Correct answer: It runs a separate spanning tree instance per VLAN, enabling per-VLAN load balancing
PVST+ maintains one STP instance per VLAN, allowing different root bridges per VLAN and load balancing across redundant links.
STP Advanced Features and Variants 2
What does MSTP (802.1s) allow that RSTP alone does not?