Spanning Tree Protocol Review

You already covered this in CCNA.
Quick recap before ENCOR pushes into RSTP and MST.

The Redundant Path

SW1 connects to both SW2 and SW3, and SW2 and SW3 also connect directly to each other.

Figure 1 – Redundant topology: two physical paths from PC1 to the server

That gives PC1 more than one path to the server.

The Broadcast Spreads

Watch what happens when PC1 needs the server's MAC address: it sends an ARP broadcast.

Figure 2 – PC1 sends an ARP broadcast

SW1 floods it out every other port, duplicating the frame toward both SW2 and SW3.

Figure 3 – SW1 floods the broadcast toward SW2 and SW3

The Loop Forms

SW2 and SW3 flood it again, and the same frame starts circulating back and forth between all three switches: a switching loop.

Figure 4 – SW2 and SW3 flood it again, the loop begins

Each pass through the loop duplicates the frame again. CPU usage spikes, MAC address tables flap between interfaces, and the network eventually stops responding under a broadcast storm.

Figure 5 – CPU spikes as the loop multiplies frames

STP solves the loop problem by picking one active path between any two switches and putting every redundant port into a blocking state. The port stays physically up but forwards no user traffic.

Blocking the Redundant Port

One port on SW3 gets picked to sit in blocking state while every other port keeps forwarding.

Figure 6 – STP blocks the redundant path

Verifying the Block

Verify it on SW3: