STP Quick Reference

  1. Port Role:
    1. Designated:
      1. one per segment
      2. it is the port closest to the root bridge.
      3. it sends BPDU to the segment to the root or alternate ports.
      4. all ports on the root bridge are in designated role.
      5. It is in forwarding state.
    2. Root:
      1. one per switch, except the root bridge itself.
      2. it is the port that leads to the root bridge.
      3. it receives BPDU from the upstream switch.
      4. It is in forwarding state.
    3. Alternate:
      1. Alternate ports to the root bridge, but are not the root port.
      2. it receives BPDU from upstream switch.
      3. It is in blocking state.
  2. STP Timers:
    1. Max Age:
      1. It is how long a switch would store BPDUs it received on all its ports.
      2. When a switch didn’t receive BPDU on its port up to the Max Age, it will send its BPDU out to the port instead.
      3. Alternate port in blocking mode wait for Max Age and 2 times forward_delay timer to expire before transition into forwarding state
      4. Max Age default is 20 seconds
    2. Delay Timer:
  3. LoopGuard:
    1. When connection to the root bridge is broken, Alternate Port stopped receiving the root BPDU, and will transition to Designated Port and Start Forwarding traffic after Max Age + 2x Delay.
    2. However, sometimes that behavior could create a loop. when there is unidirectional link issue, Alternate Port also did not get BPDU from its upstream designated port. When the Alternate Port starts transitioned to Designated Port and start forwarding traffic, that will create a unidirectional loop in the switch network, in the reverse direction of the failure, because the upstream switch port still can accept packet in its receiving direction.
    3. The is a architecture issue with STP because STP fail open instead of fail close.
    4. LoopGuard tells Alternative Port not to change to forward state until it hears any BPDU from its upstream switch’s designated port.
    5. You want to configure LoopGuard on all Root and Alternate Ports
  4. Backbone Fast:
    1. All a indirect link failure, an Alternate Port could take as long as 50 seconds (Max Age + 2x Delay) before it transitions to forwarding state.
    2. Backbone Fast implements a layer 2 PING liked mechanism. Once enable, switches can send query to the root bridge and root bridge would response.
    3. The switch timeout its Max Age immediately after it receives the response from the root bridge.
    4. The reduce the potential convergence time from 50 seconds to 30 seconds.
    5. Backbone Fast is native to RSTP. So you do not need to enable it manually when running RSTP.
  5. Uplinks Fast:
    1. On classic STP, when a directly connected uplink failed, the switch alternate port would wait max 2x delay timer because it transition to root or designated port and start forwarding traffic. So the max convergence time is 30 seconds.
    2. The convergence time is 2x of the delay timer because an blocking port needs to go through listening state (one delay timer) plus learning state (another delay timer) because it is fully transitioned into forwarding state.
    3. Once enable, enable experience uplink failure on the switch root port, the switch immediately transition the Alternate Port with the lower cost to the root bridge into forwarding state, thus reduce the convergence time from 30 second to less than 1 second.

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