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I understand how root election works, but can someone explain it in terms of BPDUs? Is a configuration BPDU sent, and does the downstream switch forward the BPDU? What if the downstream switch actually has the lower ID, and how long and how many BPDU does the switch send for election process? If there is a loop in the network, and if the switches keep forwarding the BPDUs, when does it actually stop (I understand that after election process, BPDUs are forwarded only in designated port, so there is no looping is this)? After election is the root bridge the only switch that sends a BPDU unless there is a topology change?

Sorry for the long questions, but all i can find is how STP works.

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  • Removed the off-topic request for resources.
    – Ron Maupin ♦
    Commented Oct 14, 2017 at 23:20
  • I'm not really sure what you are asking. A bridge comes up thinking it is the root bridge until it receives a superior BPDU from another bridge. BPDUs are sent to a link-local multicast address, so they are not sent beyond the link. If you understand how STP works, then you understand BPDUs because they are inherent to STP.
    – Ron Maupin ♦
    Commented Oct 15, 2017 at 0:46
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer.
    – Ron Maupin ♦
    Commented Feb 19, 2018 at 19:43

1 Answer 1

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You have basically asked the election process :p

So, here is what happens. All switches that have STP enabled (which is defaut setting in most switches), send out BPDU's.

What the BPDU mainly contains is this : a. the ID of that switch b. The ID of the root according to that switch (At the beginning of the process every switch thinks it is the root).

As Ron pointed out, This is sent on a link local address which means that it will be sent to all the devices that are directly connected to the switch. (Yes even end devices like desktops can receive this message but they will discard the packet as STP is not enabled on them)

Now as a switch when you receive the packet from all your neighboring switches, here is what you do, you compare their ID with your ID. One of the two things can happen - your ID lower than their ID in that case you still think you are the root or you find out that your ID is not the lowest. So in the next BPDU packet you will replace the root ID with the the ID of that device.

Now this process continues till all the devices unanimously agree as to which device is the root.

At this point the election stops and the root is elected.

I dont fully understand by what you mean by upward / downward packets, but after the root is elected all switches send their BPDU hello packets before the dead time to the root (in RSTP not very sure about this) and the root sends its hello packet (STP and RSTP). if any device fails to reeive the hello packets before the dead interval there is a re election which I am not mentioning in detail here

BPDU's are forwarded on the designated ports

After the election, the non root switches can have one of these links:

a. in designated state - the one which is that is used to connect to the root b. Forwarding - literal meaning , not connected to the root, but forwards packets c. blocked - the link that could potentially cause loops and thus is blocked.(cannot send but can receive packets)

Hope it clears your doubts ..

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  • thanks a lot for the explanation, but when u say untill all devices unanimously agree to which device is the root - when do they actually stop sending bpdu's eg if there are 4 switches abcd connected in circle and if a is the root, a sends to (b,c) and then (b/c) sends to d and by now every switch knows a is the root so do they stop sending bpdu's. my main question is what is the rule for sending bpdu's before/during election process. should u send bpdu to a switch from whom u received a bpdu with a lower bridge id or only on ports where u received a bpdu with higher bridge id.
    – Ram Kumar
    Commented Oct 16, 2017 at 6:50
  • i also read that after the election process only the root bridge sends bpdu's unless there is a topology change is this right?? and last part i think u mixed up states(listening,learning,forwarding) with roles(designated,root,blocking)
    – Ram Kumar
    Commented Oct 16, 2017 at 6:51
  • First if you say a is the root that means the election is already over isnt it ? Second switches do not introduce additional computation like what is the ID of the switch they are sending to , so there is no such thing as sending on ports with lower / higher IDs. and No i didnt mix up states. listening leraning are the states of the election. Designated blocked is the role a port under takes after the election. If you are confused becuase i used forwarding for 'roles' than yes it is valid role
    – john
    Commented Oct 17, 2017 at 7:12

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