ITPROTV CCNA Cisco Spanning Tree Protocol (STP) Root Bridge Election

 This is how you find a Spanning Tree Protocol Root Bridge Election.

This example is with three switches.

Step 1:

Lowest priority will become root bridge, if equal lowest MAC Address will become root bridge.

Step 1A:

If you want to manually make a switch root bridge, change its priority by multiples of 4096.

Step 2:

All root bridge ports will become forwarded and a designated port. There can only be one designated port per segment.

Step 3:

Find root port (the lowest cost going back to root bridge) root ports can't be blocked. There can only be one designated root port per segment (switch to switch.)

Step 3A:

When another switch has same priority, same MAC address, use the CLI to find root port.

enable <enter>

show spanning-tree <enter>

look at priority.nbr lowest will become root port.

Step 4:

For final designated port look at cost as if forwarding from that port to root bridge. If equal continue to priority, and then MAC Address. Lowest wins designated port.

Step 5:

If you have a root port on a switch complete it, and it must be a blocked alternate port.

enable <enter>

show spanning-tree <enter>

This bridge is the root

Reference: ITVPRO.TV Cisco CCNA - Cisco Switching: STP Operations