STP (Spanning Tree Protocol)

It prevents a network from frame looping by putting some interfaces in forwarding state & some interfaces in blocking state.
Whenever two or more switches are connected with each other for redundancy purpose loop can occur. STP Protocol is used to prevent the loop. STP is layer 2 Protocol & by default it is enabled on switches.

If we don’t use STP, these problems will occur on the network:

(i) Broadcast Storms
(ii) High Processor Utilization
(iii) Mac Table instability
(iv) Multiple Frame Transmission

STP Tasks
1. Elect Root Bridge

2. Elect Designated Port
3. Elect Root Port
Root Bridge- A switch which has best bridge ID (Lower Best)
Bridge ID is a combination of Switch priority and its MAC addresses. It is 8 bytes ID. It contains 2 bytes priority Plus 6 bytes MAC.



Switches by default Priority is 32768.
We can change the priority between 0-65535.
MAC- Each Switch has a supervisor engine. Supervisor engine has a MAC pool, the pool contain 1024 MAC addresses. When a switch wants to create Bridge ID, it borrows MAC from MAC Pool.

Requirements for Root Bridge
1. Lower Bridge Priority
2. Lower Mac Address.
(Note: Rood Bridge Always Generates 0 Cost BPDU.)

Requirements for DP and RP
1. Lower Bridge ID (Only for DP, Between Root & Non Root)
2. Lower Cost | For DP & RP
3. Lower Sender Bridge ID | B/W Root – Non Root
4. Lower Sender Port Priority | Non Root – Non Root
5. Lower Sender Port ID
(Note: All Ports of Root Bridge are DP.
Every non root bridge must have at least one root port.)