STP Question

Just want to clarify something, when STP put's a port into block state, does it do so on incoming traffic, outgoing traffic or both?

I was kind of thinking that the block states were determined after the broadcast had been sent out and would only block on incoming traffic to prevent the constant loops.

From the way the book implies a broadcast is sent out ALL ports except for the one in which it came in on. Which led me to believe that STP only blocks after a broadcast has been sent to prevent that broadcast from coming back through in which case the redundant link would send the broadcast back but that port would then block it.

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Comments

steveyeung

a block port in stp means no in or out traffic.

FloOz

A blocked stp port does however receive bpdu's.

Zartanasaurus

Draw a 3 switch diagram and trace what happens if the STP blocked port still forwarded egress broadcast traffic.

bbarrick

Zartanasaurus wrote: »

Draw a 3 switch diagram and trace what happens if the STP blocked port still forwarded egress broadcast traffic.

Well, logically it would continue to loop the traffic. Which makes sense in that aspect now. I was thinking the blocks were set up on a per broadcast basis and that maybe a switch's logic would know when to block a particular broadcast message from coming back in through a redundant port but would still allow traffic to exit that same port.

smcclenaghan

Zartanasaurus wrote: »

Draw a 3 switch diagram and trace what happens if the STP blocked port still forwarded egress broadcast traffic.

I love your replies Zartanasaurus. I'm still at the level where I have to give the answer rather than give a question which will lead someone to the answer like you do. But I really like the way that your responses make us think.