Spanning tree loop guard

When loop guard enabled, a blocked port on the switch that has been receiving BPDU message suddenly stops receiving BPDU, this port will be put in loop-inconsistent state.

When loop guard is not enabled, the blocked port on the switch that has been receiving and sending BPDU message suddenly stops receiving BPDU, the port will think it is safe to move the status from blocking to listening, learning, and forwarding. In the following case this will bring L2 network loop:

  • That one direction of fiber link is broken, the the other direction of the fiber link is still in operation.

Below is the relationship between port status and BPDU sending/receiving:

Port states:

  • Blocking: State where the switch port can receive BPDU, but can not forwarding user traffic or BPDUs.
  • Listening: State where the switch port can send & receive BPDU, but can not forwarding user traffic.
  • Learning: State where the switch port can learn MAC address, send and receive BPDU, but not forwarding user traffic.
  • Forwarding: State where the switch port can learn MAC address, send and receive BPDU, and forwarding user traffic.

Ports

  • Blocked – Doesn’t send BPDU’s, but is receiving them.
  • Designated – Send BPDU’s and Receives BPDU’s.
  • Root – Doesn’t send BPDU’s, but is receiving them. (Root port can send TCN (topology change) BPDU up to the upper switch.)

 

 

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