Which access-layer switch port will assume the Discarding role?

[a1111]

Hello,

Can someone please confirm if my reasoning is correct?:

I believe the answer is C because on SW4, both interfaces lead through the SW3 to the root (which is SW1 due to its priority), and F0/11 has the highest number, so F0/2 is preferred over it, resulting in F0/11 being a Discarding port (because the lowest port number wins in case of a tie). What I'm not sure about is why SW3's G0/2 is not a correct answer. Is it because we don't know what kind of interfaces connect SW1 and SW2, and the answer depends on that fact?

In other words, it's possible that SW1 and SW2 are connected with Fastethernet or Ethernet ports, and since SW3 reaches SW1 via a Gigabitethernet link, SW1 advertises the lowest cost Hello onto the LAN segment with SW2, which results in SW3 putting its G0/2 port in Forwarding state.

Or it's also possible that SW1 and SW2 are connected with Tengig (or faster) links, so in this case SW2 would advertise the lowest cost Hello onto the LAN segment with SW3, and as a result SW3 would put its G0/2 into Discarding state. (Assuming in both scenarios default cost values.) Is this the right solution?

Thanks

Attila

[Flavio Miranda · Accepted solution]

Hi

"What I'm not sure about is why SW3's G0/2 is not a correct answer."

  Because Switch 3 has a lower bridge ID than Switch 4 .  They tie in the priority then the next step is the mac address and switch 3 has smalled mac address.

 Furthermore, the question says, " Which Access Layer Switch" and switch 1 and 2 are "Distribution Switches" so they are not part of the question. You dont need to worry about them.

[a1111]

Hi Flavio,

Thanks.

So this idea is correct?: "on SW4, both interfaces lead through the SW3 to the root (which is SW1 due to its priority), and F0/11 has the highest number, so F0/2 is preferred over it, resulting in F0/11 being a Discarding port (because the lowest port number wins in case of a tie)."

[Flavio Miranda]

Exactly.  Answer c is correct.

[a1111]

Thanks. 

You said " Because Switch 3 has a lower bridge ID than Switch 4 .  They tie in the priority then the next step is the mac address and switch 3 has smalled mac address." Are you saying that there will be 2 STP instances running? One between SW1 and SW2, and one between SW3 and SW4? My understanding was that all 4 switches take part in the same STP election process.

[Flavio Miranda]

Your understanding is right, only one STP instance.  But the question is clear about the switches you need to look at. They say access layer. 

  The fact that the switch Sw1 is the root  prove that only one instance exist here. But they are distribution.

In certification exams, this details can make thing easier.

[a1111]

Thanks again. It looks like I didn't read this question with enough attention to detail.
I'd like to explore this question a bit more to check my understanding of STP. I wonder what your thoughts are.
Let's change the question a bit: everything is the same, except the question now asks which ports on SW3 are going to be in Discarding state. So the question is only about SW3.

SW3's G0/1 is in Forwarding state because that's directly connected to the root (SW1), and SW3's other port to SW1 is also a Gigabitethernet port, so the root cost is the lowest via G0/1. So whatever the ports between SW1 and SW2 are, for SW3 going via G0/2 would be more costly as compared to G0/1.

Based on what we know via the topology diagram, we won't know what state SW3's G0/2 is going to be in. The reason being because we don't know what kind of interfaces connect SW1 and SW2, and the answer depends on that fact.

In other words, it's possible that SW1 and SW2 are connected with Fastethernet or Ethernet ports, and since SW3 reaches SW1 via a Gigabitethernet link, SW1 advertises the lowest cost Hello onto the LAN segment with SW2, which results in SW3 putting its G0/2 port in Forwarding state.

Or it's also possible that SW1 and SW2 are connected with Tengig (or faster) links, so in this case SW2 would advertise the lowest cost Hello onto the LAN segment with SW3, and as a result SW3 would put its G0/2 into Discarding state. (Assuming in both scenarios default cost values.)

Is this the right solution when we change the question?
Thank you again for all your help so far.

[a1111]

I've re-read the thread again and would like to ask you again about your comment where you said " Because Switch 3 has a lower bridge ID than Switch 4 .  They tie in the priority then the next step is the mac address and switch 3 has smalled mac address."

Why did we have to consider bridge IDs of SW3 and SW4? They aren't candidates for the STP election. They would only be if the access layer switches had a different STP instance running, which in this example they don't.

Thank you.

[a1111]

Thanks again, I've marked your answer as the solution because you've addressed my original concern. Thanks for the other responses as well, I appreciate it!

[MHM Cisco World]

there are three port BLK in your lab 
the three circile represent the L2 Loop, and in this circle the STP must BLK one port to stop the loop 

[a1111]

Hi.

Thank you very much. On your topology, the port connecting switch IOU3 to switch IOU2 is in Blocking state; but if the port between IOU3 and IOU2, and IOU3 and IOU1 would be a Gigabitethernet port (instead of the current Ethernet port), wouldn't the port connecting switch IOU3 to switch IOU2 be in a Forwarding state? (The ports between IOU1 and IOU2 would still be Ethernet ports in this second scenario.)

Have a nice day.

[MHM Cisco World]

meanig the Cost of path IOU1-IOU3-IOU2 << path IOU1-IOU2 
the Yes the IOU3-IOU2 will be in FWD and path between IOU1-IOU2 will be BLK

[a1111]

Thank you very much for your help! I've marked Flavio's answer as the solution because he addressed my original question, but I'm very grateful to you as well for reinforcing my understanding of STP! If I could, I would mark your answer as the accepted answer as well.

Have a nice weekend.

[MHM Cisco World]

Dont worry for Me it important that you get idea of how STP BLK port. 
Have a nice weekend Too.
MHM