Spanning Tree Topology (STP) Convergence

Once the Spanning Tree Topology (STP) is established, STP continues to work until some changes occurs. Manual changes that Network Engineer can apply are configuration of Bridge ID and port costs. Also there are other changes like the addition of switch or failure of port of an existing switch. The reaction of STP when some changes occurs by reconfiguration of status (forwarding/blocking) of various port/interface is called STP Convergence. 
Port Costs according to IEEE :

Ethernet Speed	IEEE Cost
10 Mbps	2, 000, 000
100 Mbps	200, 000
1 Gbps	20, 000
10 Gbps	2, 000
100 Gbps	200
1 Tbps	20

Steady State Operation of STP :

• A Hello BPDU is created and send from all working interface of Root switch having root cost of 0.
• All Non-root switch receive Hello BPDU by adding their BID as sender’s BID and root cost. Now they send Hello BPDU from their designated ports.
• Above two processes repeats until some changes occurs.

Reaction of Switches to changes in STP topology : Before we proceed to how switches react, it is important to know some timers used by Spanning Tree Protocol. STP Timers :

Timer	Default Value	Description
Hello	2 seconds	Time between two consecutive Hello BPDU created by root switch.
MaxAge	10*Hello time	Max time a switch must wait if no Hello received before proceeding to do change.
Forward Delay	15 seconds	Time taken by switch to change status of ports from forwarding to blocking and vice-versa.

The reaction of switches are :

• When a switch does not get expected Hello within Hello time, it continues to work as normal and wait until MaxAge time.
• Switch waits for MaxAge time if Hello BPDU is received within MaxAge time, switch consider it as some delay in network and continues as before. If not then switch and start changing their topology.
• Now the status of different ports may change according to new topology adopted by STP. Ports may change their status from forwarding to blocking and vice-versa. But this port status process changing does not do immediately, it goes through two intermediate states i.e, Listening and Learning states.
• The change of state from blocking to forwarding along with intermediate listening and learning state is bounded by the time of Forward Delay.

Spanning Tree States :

State	Data frame forwarding	Learning MAC-Address of received frame	Nature of State
Blocking	No	No	Stable
Listening	No	No	Transitory
Learning	No	Yes	Transitory
Forwarding	Yes	Yes	Stable
Disabled	No	No	Stable