FAQ-STP# INTERVIEW PREP GUIDE ABOUT SPANNING TREE PROTOCOL
'Practical term used in professional training'
🔹 What is STP ?
STP (IEEE 802.1D) is a Layer 2 protocol that prevents network loops by creating a loop-free logical topology in switched networks.
🔹 Why is STP required ?
To prevent broadcast storms, MAC table instability, and multiple frame copies caused by switching loops..
🔹 What happens if STP is not enabled ?
Broadcast storms occur, CPU utilization increases, MAC tables keep changing, and the network may crash.
🔹 At which OSI layer does STP operate ?
Layer 2 (Data Link Layer).
🔹 What type of frames does STP use ?
BPDU (Bridge Protocol Data Units).
🔹 What is a Root Bridge ?
The central reference switch in an STP topology from which all path calculations are made.
🔹 How is the Root Bridge elected ?
Based on the lowest Bridge ID (Bridge Priority + MAC Address).
🔹 Default STP bridge priority value ?
32768.
🔹 How can you make a switch the Root Bridge ?
By lowering its bridge priority.
🔹 What happens if two switches have the same priority ?
The switch with the lower MAC address becomes Root Bridge.
🔹 What are the STP port roles ?
Root Port, Designated Port, and Blocking (Non-Designated) Port.
🔹 What is a Root Port ?
The port with the lowest path cost to the Root Bridge on a non-root switch.
🔹 How many Root Ports can a switch have ?
Only one.
🔹 What is PVST+ ?
Cisco STP implementation running one STP instance per VLAN.
🔹 PVST+ vs Rapid PVST+ vs MST
PVST+: Slow convergence
Rapid PVST+: Faster (RSTP)
MST: Groups VLANs into instances.
🔹 What is a Designated Port ?
The port that forwards traffic on a LAN segment toward the Root Bridge.
🔹 Can the Root Bridge have a Root Port ?
No, all ports on the Root Bridge are Designated Ports.
🔹 What are the classic STP port states ?
1. Blocking (Port does not forward traffic or learn MAC addresses),
2. Listening (Port listens for BPDUs but does not forward traffic),
3. Learning (Port learns MAC addresses but does not forward traffic),
4. Forwarding (Port forwards traffic and learns MAC addresses),
5. Disabled (After dicision, shutdown the port)
🔹 What are the three STP timers and control it ?
1. Hello Time, 2. Forward Delay, 3. Max Age and Root bridge control the STP timers.
🔹 What is STP Path Cost ?
A metric used to determine the shortest path to the Root Bridge.
🔹 Who sends BPDUs ?
Root Bridge sends BPDUs in every two seconds (Default); other switches forward them.
🔹 What information is inside a BPDU ?
Root ID, Bridge ID, Path Cost, Port ID, Timers.
🔹 What are the types of STP ?
STP (802.1D), RSTP (802.1w), MSTP (802.1s), PVST+, Rapid PVST+.
🔹 What is RSTP ?
Rapid STP provides faster convergence than classic STP.
🔹 What is the convergence time of STP vs RSTP ?
STP ≈ 50 seconds, RSTP ≈ 5–10 seconds.
🔹 What is PVST+ ?
Cisco proprietary STP running per VLAN.
🔹 What is MSTP ?
Multiple Spanning Tree Protocol maps multiple VLANs to one STP instance.
🔹 What is PortFast
Allows immediate forwarding for access ports (skips listening & learning).
🔹 Where should PortFast be enabled ?
Only on access ports connected to end devices.
🔹 What is BPDU Guard ?
Disables a port if a BPDU is received (protects PortFast ports).
🔹 What is BPDU Filter ?
Stops sending or receiving BPDUs on a port.
🔹 What is Root Guard ?
Prevents a port from becoming a Root Port.
# Loop Protection & Troubleshooting
🔹 What is Loop Guard ?
Prevents loops caused by missing BPDUs.
🔹 What is UplinkFast ?
Cisco feature for faster failover of uplinks (legacy).
🔹 What is BackboneFast ?
Speeds up STP convergence for indirect link failures.
🔹 What happens during a topology change ?
MAC address aging time is reduced temporarily.
🔹 How do you change STP priority ?
spanning-tree vlan 'id' priority 'value'
🔹 Can STP prevent Layer 3 loops ?
No, STP only works at Layer 2.
🔹 What is the main disadvantage of STP ?
Blocked links waste bandwidth (one active path only).
|| Alway be study right sight ||
