Spanning-Tree Protocol
When booted up, all switches declare themselves as the root bridge. They send Hello packets
every 2 seconds to adjacent switches. Each switch compares the BID in the Hello packets to
their own BID value. The BID is determined by the priority (default 32768). The switch with the
lowest priority becomes the root bridge. If all priorities are the same value, the switch with the
lowest MAC address becomes the root bridge.
Every non-root bridge has one and only one root port which is going toward the root bridge.
A designated port is an active port connected to a root port on a switch. (The opposite end of
every root port is a designated port.) All ports on a root bridge are designated ports.
Why do we need STP?
 To stop loops due to redundancy at layer 2
 Layer 3 uses Time-To-Live counter, but layer 2 doesn’t have anything like that
Bridge ID
 Consists of MAC address and priority
 Default priority is 32768
 Priority is always multiples of 4096
Every switch will have one root port EXCEPT the root bridge. The root bridge will have all
designated ports.
Switch Diameter – number of switches a frame will have to travel through to go between the
two farthest points within a broadcast domain.
Port priority
 0 - 240 (default 128)
 Spanning-tree port-priority priority-number
5 Port States of STP:
 Blocking – nondesignated and does not participate in forwarding
 Listening – nondesignated but sends and receives BPDU’s to determine if it is a better
route than something else
 Learning – preparing to participate in frame forwarding, building MAC address table

Forwarding – part of the active topology and forwards frames, sends and receives
BPDUs
 Disabled – administratively shut down
Timers:
 Hello Time – time between each BPDU frame sent on a port - default 2 seconds
 Forward Delay – time spent in listening and learning states - default is 15 seconds for
BOTH states
 Maximum Age – maximum length of time a switch port saves configuration BPDU
information – default 20 seconds
First compare Bridge IDs to find your root bridge.
 All the ports of the root bridge will be designated ports
Next, find the root ports on the other switches
 Root ports are the closest to the root bridge (closest meaning least-cost path)
 Any port in the path of a root port is designated
 When there are two paths from a switch that have equal cost, the path going through
the switch with the lowest BID will win
Determine which ports are blocked.