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A question regarding the spanning-tree protocol. During the root bridge election process, how is the end of the election validated? In other words how does the spanning-tree protocol know when the root bridge election has finished? What is the condition or "trigger" to begin the next step of the spanning-tree protocol? (set ports status)

  • Thanks for your responses ! Could you finally confirm me these 3 statements about spanning tree: 1. When a switch boot or spanning tree is enabled on a switch, the switch ports pass in blocking state during 20 seconds before to pass in listening mode 2. The root bridge election and the root and designated port are chosen during the ports are in listening mode 3. We could consider that the root bridge election and the root and designated port selection are done after the listening state duration (15 seconds). Thanks you. – bdes31 Jan 5 '18 at 19:56
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During the root bridge election process, how is the end of the election validated? In other words how does the spanning-tree protocol know when the root bridge election has finished?

The protocol doesn't "know." Each bridge performs its own calculation. When the received root ID is the same as the advertised root ID, the election is done, as far as that bridge is concerned. When all the bridges agree, we say the network has converged. But each bridge doesn't "know" when that happens.

What is the condition or "trigger" to begin the next step of the spanning-tree protocol? (set ports status)

The expiration of the forwarding_delay timer (default 15 sec).

  • "The expiration of the forwarding_delay timer (default 15 sec).". Not the expiration of the max-age ? My understanding: new election, all ports blocked, when no "better" BPDU is received the switch stay in blocking state during 20 seconds (the max_age), and then it pass its ports in Listening mode ? Is it correct ? – bdes31 Jan 5 '18 at 16:26
  • @bdes31 I would see it as 2xforwarding_delay as this accounts BPDU propagation delay. More info here: cisco.com/c/en/us/support/docs/lan-switching/… – DRP Jan 5 '18 at 18:32
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End election is validated when no more better (lower BID's) are received from neighbor switches.

When the switches first come up, they start the root switch selection process. Each switch transmits a BPDU to the directly connected switch on a per-VLAN basis.

The Configuration BPDU has the BID (bridge priority and unique device identifier) and it used during initial setup to find out who will be the root bridge. Lower is preferable.

Recall the STP states:

  • Blocking
  • Listening
  • Learning
  • Forwarding

They all process BPDU's and have a time associated before moving to the next state, this time allows for the root bridge knowledge convergence to be synch'd.

So basically you are waiting for the best BID to be cascaded to all devices, which will eventually occur, and is considered time costly in today's networks.

  • "They all process BPDU's and have a time associated before moving to the next state, this time allows for the root bridge knowledge convergence to be synch'd." This time, which allows for the root bridge knowledge convergence to be synchronized, is what we call the max_age, of 20 seconds ? – bdes31 Jan 5 '18 at 16:11
  • @bdes31 max_age, of 20 seconds is used to detect any change in the topology, more info here: learningnetwork.cisco.com/thread/50291 – DRP Jan 5 '18 at 18:01

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