Chapter 2: Spanning Tree Protocol Notes

Three classes of problems caused by not using STP in redundant LANs
  • Broadcast Storms- the forwarding of frames repeatedly on the same links
  • Mac table instability - the continual updating of a switche's mac address table with incorrect entries, in reaction to looping frames, resulting in frames being sent to the wrong locations.
  • Multiple frame transmission - a side effect of looping frames in which muliple copies of the frame are delivered to the intended host, confusing the host.
What IEEE 802.1d Spanning Tree does

STP prevents loops by placing the bridge/switch port in either forwarding state or blocking state. Interfaces in the forwarding state act normal and forward \ receive frames. The interfaces in the blocking state do no process frames except STP messages. 

STP convergence means the process by which the switches collectively realize that a change occurred on the land and how to react to that change.

How Spanning Tree Works

STP uses a algorithm called STA or spanning tree algorithm to determine which interfaces on a switch should be placed in forwarding or blocking state.

STP uses three things to chose to put a port into forwarding state
  • stp elects a root switch. stp will put all interfaces on the root switch in a forwarding state.
  • each nonroot switch considers on of its ports to have the least administrative cost to the root and places that port into the forwarding state. This port is also called the RP root port.
  • many switches can be attached to a Ethernet segment, but the one with the lowest administrative cost to the root bridge is considered the designated bridge and the port connected to the root bridge is called the designated port DP
BPDU - bridge protocol data unit used by bridges and switches to exchange information

Fields in the STP Hello BPDU: Root bridge ID, Sender's Bridge ID, cost to the root, timer values on the root switch

Election process - lower bridge priority wins if there is a tie the lower mac address wins

bridge ID (BID) - is an 8-byte value unique to each switch. The bridge id contains 2-bytes for the priority field and 6-bytes for the MAC address. Example 32,769:0200.0002.0002

root port - is the switches interface through which it has the least STP cost to reach the root switch.

Switches are the only devices that send out BPDUs

Default priority is 32768

Easiest way to make a switch the root bridge is to lower the priority

Every port is assigned a cost relative to it's speed, the higher the speed the lower the cost.


STP port states

Blocking- frames are not forwarded, but BPDUs are accepted.

Listening: Frames are not forwarded, and the MAC address table is not yet being built.

Learning: frames are not forwarded, mac addresses table is being built

Forwarding: frames are forwarded and mac address are still being learned.

Disabled - frames are not forwarded, administratively shutdown port.

Only root switches actually originates BPDUs


Hello Timer: default 2 seconds
Max Age: 20 seconds
Forward Delay: 15 seconds