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birdslanger
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STP BPDUs

Mon Jul 16, 2012 8:10 pm

I've been sharpening up on my STP, and I came across a section in the CCNA curriculum where it talks about TCN (topology change notification) BPDUs. There was a sentence that got my attention and I wanted to come on here to clarify to make sure I understand correctly. The sentence that triggered this is below:

"In normal STP operation, a switch keeps receiving configuration BPDU frames from the root bridge on its root port. However, it never sends out a BPDU toward the root bridge. To achieve that, a special BPDU called the topology change notification (TCN) BPDU was introduced. " - Cisco Networking Academy - Lan Switching/Wireless Curriculum.

Now to my clarification question:
So once the root bridge is elected and everthing is converged within the STP broadcast domain, the switches now, will only communicate with the root bridge, as where before the election all switches communicate with neighbor switches on their segments.

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davidrothera
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Re: STP BPDUs

Tue Jul 17, 2012 1:32 am

Yes, but if everything is stable then they won't communicate with the root bridge, they will just carry on receiving BPDU's from the root bridge.

The only time that will change is if there is a change in the topology and only then will it send a TCN to the root bridge.
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mellowd
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Re: STP BPDUs

Tue Jul 17, 2012 1:33 am

No. BPDU's are still sent between all switches. Only TCN's get sent to the root bridge. The root bridge then floods that to the rest of the domain.

This behaviour is changes in RSTP though. In RSTP/MSTP any switch can originate a TCN (IT'll always be the switches who actually have interfaces go down)

birdslanger
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Re: STP BPDUs

Tue Jul 17, 2012 2:33 pm

OK cool.

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dlots
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Re: STP BPDUs

Tue Jul 17, 2012 3:32 pm

I thought in plane old STP once the root bridge was elected only it generated BPDUs and the other switches just forwarded them (All switches would have to change TCN though I would assume)

I knew in RSTP all the switches generated them though

This correct?
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totaluser
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Re: STP BPDUs

Tue Jul 17, 2012 3:38 pm

dlots wrote:I thought in plane old STP once the root bridge was elected only it generated BPDUs and the other switches just forwarded them (All switches would have to change TCN though I would assume)

I knew in RSTP all the switches generated them though

This correct?

sounds right

in 802.1D only the root bridge originates / sends Configuration BPDUs
non-root bridges only forward those Configuration BPDUs but can originate TC BPDUs

in RSTP any bridge can originate both types of BPDUs

'

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