Spanning Tree Protocol (STP)

 STP – Points to Remember

1. STP is a layer 2 protocol that runs on switches and bridges, the purpose of STP is to remove

switching loops. By default, STP is enabled on cisco switches.

2. All switches participating in STP exchange info with other switches in the network

Through messages known as BPDUs (Sent out at a frequency of 2 sec on every port)

3. STP port states are Blocked, Listen, Learn, Forward, Disabled

4. The command “show spanning-tree” includes the following info

i. VLAN number

ii. Root bridge priority, MAC address

iii. Bridge timers (Max Age, Hello Time, Forward Delay)

STP Port Roles

1. Root : A bridge can have only one root port. The root port is the port that leads to the root

bridge. All bridges except the root bridge will have a root port. the root port is in the STP

forwarding state.

2. Designated : One designated port is elected per link (segment). The designated port is

the port closest to the root bridge. Each designated port is in the STP forwarding state

3. Alternate : Alternate ports lead to the root bridge, but are not root ports. The alternate

ports maintain the STP blocking state.

4. Backup: This is a special case when two or more ports of the same bridge (switch) are

connected together, directly or through shared media. In this case, one port is designated,

and the remaining ports block. The role for this port is backup