3. STP — SPANNING TREE
PROTOCOL Exam questions and
answers Graded A+ 2026/2027
edition
What does STP stand for?
Spanning Tree Protocol.
What problem does STP solve?
Layer 2 switching loops caused by redundant Ethernet links.
Why are Layer 2 loops dangerous?
Ethernet frames do not have a TTL field, so certain frames can circulate
indefinitely, causing broadcast storms, MAC table instability, and
duplicate frames.
What is a broadcast storm?
An uncontrolled multiplication/circulation of broadcast traffic caused
commonly by a Layer 2 loop, potentially consuming nearly all available
bandwidth and switch resources.
What is MAC address table instability?
A condition where a switch repeatedly learns the same source MAC
address on different ports because looping frames arrive from multiple
directions.
What IEEE standard defines classic STP?
IEEE 802.1D.
, What is STP's basic strategy for preventing loops?
STP logically blocks selected redundant paths while leaving them
available as backups.
Does STP physically shut down redundant links?
No. It places selected ports into non-forwarding states logically.
What is a BPDU?
Bridge Protocol Data Unit — an STP control message used by switches
to exchange spanning-tree information.
What does BPDU stand for?
Bridge Protocol Data Unit.
What information is used to elect the STP Root Bridge?
The Bridge ID, or BID.
What is the Bridge ID?
A value used to identify a switch in STP, consisting principally of bridge
priority plus extended system ID/VLAN information and the switch MAC
address.
Which switch becomes the Root Bridge?
The switch with the LOWEST Bridge ID.
What is the first value compared during Root Bridge election?
Bridge priority.
What is the default STP bridge priority on Cisco switches?
32768.
What happens if two switches have the same STP priority?
PROTOCOL Exam questions and
answers Graded A+ 2026/2027
edition
What does STP stand for?
Spanning Tree Protocol.
What problem does STP solve?
Layer 2 switching loops caused by redundant Ethernet links.
Why are Layer 2 loops dangerous?
Ethernet frames do not have a TTL field, so certain frames can circulate
indefinitely, causing broadcast storms, MAC table instability, and
duplicate frames.
What is a broadcast storm?
An uncontrolled multiplication/circulation of broadcast traffic caused
commonly by a Layer 2 loop, potentially consuming nearly all available
bandwidth and switch resources.
What is MAC address table instability?
A condition where a switch repeatedly learns the same source MAC
address on different ports because looping frames arrive from multiple
directions.
What IEEE standard defines classic STP?
IEEE 802.1D.
, What is STP's basic strategy for preventing loops?
STP logically blocks selected redundant paths while leaving them
available as backups.
Does STP physically shut down redundant links?
No. It places selected ports into non-forwarding states logically.
What is a BPDU?
Bridge Protocol Data Unit — an STP control message used by switches
to exchange spanning-tree information.
What does BPDU stand for?
Bridge Protocol Data Unit.
What information is used to elect the STP Root Bridge?
The Bridge ID, or BID.
What is the Bridge ID?
A value used to identify a switch in STP, consisting principally of bridge
priority plus extended system ID/VLAN information and the switch MAC
address.
Which switch becomes the Root Bridge?
The switch with the LOWEST Bridge ID.
What is the first value compared during Root Bridge election?
Bridge priority.
What is the default STP bridge priority on Cisco switches?
32768.
What happens if two switches have the same STP priority?
