STP question

Hi there,

I was wondering if there was some stp experts out there? Yes? Ok...

When a port is in blocking state, normally it does not send out BPDUs. Right? What can make it change this and step to listenning state? I mean topology changes are ok, but administrative changes (like port cost for instance) change the port state as well?

I am currently preparing for CCNA and this is not very clear to me as it does not clearly appear in the W. Odom's book.

Thank you. Excellent forum.

Cheers
Glenus

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Comments

Netstudent

If you change the default path costs, it can make a switch hear lower cost bipeedoos on some other port in which case this would change a switch's root port. This new root port will then transition to listening for 15 seconds where it listens for BPDU's to see who is the root bridge and if there may be a new root bridge on the network. It does this to ensure that it does not create a loop when it transitions to fwd'ing. Then after listening, it goes to learning for 15 seconds. It learns of the MAC addresses in relation to the new root port. If it kept the MAC addresses associated with the old root port, things would get ugly.

You can also use the spanning-tree vlan1 root primary command to change the root or make that switch the root bridge. This would change the whole topology. Doing this will make the current switch have a priority of 100 less than the current root bridge.

SanKuKaï

Thank you for your reply.

My question was more on blocking state ports. As far as I understand you are talking about forwarding ports which are processing BPDUs.

Let's make it simple: Imagine a switch with 3 ports. Fa0/1 in Fwd'ing, Fa0/2 and Fa0/3 in Blocking. Fa0/1 is the RP. Now I change Fa0/1's cost from 19 to 1 for instance. Do Fa0/2 and Fa0/3 advertise the change? As far as I understand Fa0/2 and Fa0/3 must advertise the change out for the toplogy to change, but how come? They are in blocking state aren't they? A blocking port advertises nothing, am I right?

Cheers
Glenus

Netstudent

Let's make it simple: Imagine a switch with 3 ports. Fa0/1 in Fwd'ing, Fa0/2 and Fa0/3 in Blocking. Fa0/1 is the RP. Now I change Fa0/1's cost from 19 to 1 for instance. Do Fa0/2 and Fa0/3 advertise the change? As far as I understand Fa0/2 and Fa0/3 must advertise the change out for the toplogy to change, but how come? They are in blocking state aren't they? A blocking port advertises nothing, am I right?

Well I thought you asked what can be "administratively" done to chnage a port from blocking to fwd'ing. And that was a couple ways

Okay a blocking port does not fwd BDPU's. However, each segment has a designated port. So even though a port is blocking, it can still hear BPDU's that the designated port puts out on the segment. If a blocking port never hears BPDU's, then how will it know when a lower cost BPDU comes to the switch? It won't.

So to answer your scenario, no fa0/2 and fa0/3 will not advertise that change. They are blocking and will not fwd anything. However, when a BPDU enters the root port which is FA0/1, it will append or ADD the cost(now 1) to the original cost of the BPDU as it fwd's the BPDU to the next switch. So lets say this switch is adjacent to the root bridge which means it is recieveing a BPDU with a cost of 0. When this BPDU gets fwd'ed out the designated port it will have a cost of 1. 0 +1

When a topology change occurs on the switched network, the affected switch(I think) sends out whats called a TCN frame. This TCN frame makes it's way back to the root through a single path in the tree. Then the root bridge floods this TCN out all of it;s ports ensuring that every path in the tree starts propagating this TCN. This tells all switches that a change has occured and that all switches should timeout their MAC table entries for a period defined by the Forward Delay time.

I feel your pain. It was only 2 or 3 weeks ago when I was having my own struggles with STP. I read the ICND STP chapter about 7 times, literally. But I have it now. Now it's on to PVST+ and RSTP YIKES!