Simplified Spanning Tree Protocol

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Spanning Tree Protocol is a Layer 2 loop-prevention mechanism.
In order to prevent the loop, the following election have formed.

Spanning Tree must select

• 1 root bridge
• 1 root port per non-root bridge
• 1 designated port per network segment
• The remaining are blocking port

Spanning Tree Election Criteria

• Lowest root bridge ID (BID)
• Lowest path cost to the root
• Lowest sender bridge ID
• Lowest sender port ID (PID)

Things to take notes:

• Bridge ID = Bridge priority : Bridge MAC address.
• Extended Bridge ID = Bridge priority + vlan ID : Bridge MAC address.
• Default priority = 32768
• Port ID = Port priority : port number.
• Port priority = 6 bit value, 0 - 63, default 32.
• Path cost = value of the cost of each link between the bridge and the root.
• 10   mb/s = 100
• 100 mb/s = 19
• 1     gb/s  =  4
• 10   gb/s  =  2

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