STP Convergence

daveba123

Hello folks,

I'm new here and am looking forward to using this forum.

So, I've been studying STP and I'm a bit puzzled.

The whole purpose of STP is to prevent loops that cause broadcast storms, but what makes STP's BPDUs special and able to compete with the flooded traffic to block the loop?

This is how I think the scenario would go:

1) Loop is brought into the L2 topology

2) Broadcast storm begins

3) Switches are overwhelmed by the broadcast storm and cannot break the loop

Is STP able to break the loop before the storm happens? If so, how?

Please advise.

Thanks!
David

awitt11

STP is on by default. When a new device is introduced into a topology, an election is forced. Traffic isn't allowed across a link unless it becomes the root or a designated port. There are ways to disable this behavior, such as portfast, which will send traffic once a physical connection is made. So for your scenario:

1) Loop is brought into L2 topology
2) Election is forced
3) Switches forward traffic based on port state

powmia

blocking, listening, learning, forwarding (or discarding, learning, forwarding for 802.1w). Notice that forwarding is last on the list? A switch will block all frames (ie, not propagating a broadcast storm) except for BPDUs until that switch has determined which ports need to be blocked to prevent loops... prior to allowing any user traffic across its trunks.

daveba123

Ah, I see. Thanks for the explanation!

binaryhat

This video helped me--->https://www.youtube.com/watch?v=6AxDqaG9M7I