Spanning-Tree Loop and Broadcast Storm in Packet Tracer
If every switch port turns amber, traffic stops and the topology floods, you have a switching loop. Layer 2 frames have no TTL, so a broadcast circulates forever, multiplying at every switch. Spanning Tree exists to prevent exactly this by blocking redundant paths.
What you see
%SPANTREE-2-RECV_1Q_NON_TRUNK: Received 802.1Q BPDU on non trunk %SPANTREE-7-RECV_1Q_NON_TRUNK: Blocking FastEthernet0/2 on VLAN0001. Inconsistent peer vlan.
Why it happens
Ethernet frames have no TTL field. When you connect two switches with two cables — or a user loops a patch lead back into the same switch — a single broadcast is forwarded out every port, arrives back, and is forwarded again. Within seconds the segment saturates and CPU hits 100%. This is a broadcast storm.
Spanning Tree Protocol prevents it by electing a root bridge and blocking any redundant link. In Packet Tracer the usual causes are: STP disabled on a switch, a loop created through a hub, or PortFast configured on a port that is actually connected to another switch — which skips the listening/learning states and forwards immediately, creating a loop before STP can react.
How to confirm it
show spanning-tree show spanning-tree blockedports show interfaces status | include err|amber show processes cpu sorted | exclude 0.00
A healthy topology has at least one port in BLK (blocking). If every port on every switch shows FWD and there is a physical loop, STP is not doing its job.
The fix
! Make sure STP is running (it is on by default — verify it was not disabled) spanning-tree mode rapid-pvst ! ! Never put PortFast on a switch-to-switch link interface FastEthernet0/2 no spanning-tree portfast ! ! Protect access ports so a rogue switch cannot form a loop interface range FastEthernet0/1 - 24 spanning-tree portfast spanning-tree bpduguard enable
PortFast belongs on access ports facing end devices only. Combined with BPDU Guard, a switch plugged into a user port is shut down instead of merging into your topology.
How to stop it happening again
Deploy PortFast + BPDU Guard on every access port, and pin your root bridge deliberately with spanning-tree vlan 1 root primary instead of letting the lowest MAC address decide.
Questions people ask
Why does Packet Tracer go amber on all ports?
Amber means the port is in STP listening/learning or blocking. During a storm you will see ports cycling. It is the visual signature of spanning tree reconverging, or of a loop overwhelming the switches.
Can I just disable spanning tree to stop the errors?
No — that removes the only thing preventing the loop. Disabling STP on a looped topology takes the network down completely. Fix the loop, keep STP on.
Related reading
Spanning Tree Protocol ExplainedWhat Is A Broadcast StormStp Root Bridge ElectionLab Portfast Bpduguard
Reading the fix is not the same as doing it
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