STP Principle

The Problem of Looping

→ broadcast storm

→ MAC table flapping

Calculation Process of Spanning Tree → exchange information and parameters in BPDU

→ Select a bridge as the root bridge among all bridges (based-on bridge id: 2-byte bridge priority [0-65535, default: 32768] + 6-byte MAC address). Smallest id become root bridge

→ Calculate the shortest path from the current bridge to the root bridge

→ For every shared network segment, select the bridge nearest to the root bridge as the designated bridge, responsible for the data forwarding of this network segment

→ For every bridge, select a root port. Based on path cost outgoing port, smaller is better. On VRP, the cost of 100M port is 200. If the path cost is same, compare with identifier of upstream switches. If it's still not elected, the port whose upstream port has the smallest identifier is elected. Port identifier: 1-byte port priority [default: 128] + 1-byte port number.

→ Select the designated port besides the root port. Root path cost → bridge id → port id. On the root bridge, all ports are the designated ports of the connected network segments.

Switch port role :

→ root port : root port is the nearest port to the root switch, it is in forwarding state

→ designated port : it forwards data from network segment which connects to the root switch and data from switch to the network segment it connects to

→ alternate port : backup port, it will not forward any data to the network segment it connects to.

Port status description :

→ disabled : port will not forward data, learn MAC address and calculate spanning tree

→ listening : port will not forward data and learn MAC address, but it will calculation spanning tree, receive and send BPDU

→ blocking : port will not forward data and learn MAC address; it will receive and deal with BPDU but not send BPDU

→ learning : port will not forward data, but it will learn MAC address, calculate spanning tree, receive and send BPDU

→ forwarding : port will forward data, learn MAC address, calculate spanning tree, receive and send BPDU.

After enabled, a port switches to Listening state and begins to calculate the spanning tree. If the port is set to the alternate port, the port state changes to Blocking. If the port is set to the root port or designated port, change from Listening to Learning, wait forward delay, then Learning to Forwarding.