Spanning Tree and the path data will take

May 11th, 2010

When Studying for my CCNA exam I had a pretty good understanding of STP but something never really clicked. Now I am studying for my CCNP Switch exam and now I am trying to make sure I have a much deeper grasp on things not only for the exam but for getting better at my job. I want to make sure I understand the path that data will take if STP is not setup in the best way. I have attached a diagram and here is my question.

Let’s say based on the diagram that the 10th floor switch (Switch D) is the Root bridge for all the VLAN’s, if a user on the 12th floor wanted to get to the server VLAN 3 it would take the path in red, correct? Or would it take the path in Blue? I think it is the path in Red but I just needed to clarify.

Thanks,

Mike

Overall Rating: 5 (4 ratings)

Replies

burleyman Tue, 05/11/2010 - 08:35

Forgot to attach the diagram

Mike

Attachment:
Jon Marshall Tue, 05/11/2010 - 10:00
`burleyman wrote:When Studying for my CCNA exam I had a pretty good understanding of STP but something never really clicked. Now I am studying for my CCNP Switch exam and now I am trying to make sure I have a much deeper grasp on things not only for the exam but for getting better at my job. I want to make sure I understand the path that data will take if STP is not setup in the best way. I have attached a diagram and here is my question.Let’s say based on the diagram that the 10th floor switch (Switch D) is the Root bridge for all the VLAN’s, if a user on the 12th floor wanted to get to the server VLAN 3 it would take the path in red, correct? Or would it take the path in Blue? I think it is the path in Red but I just needed to clarify.Thanks,Mike`

Mike

Need a bit more info. Is the user in a different vlan than the server ?

Where is the HSRP active gateway for the user vlan and the server vlan ?

Basically STP forms a loop free topology. This does not mean that all data must go via the root bridge, it's not like an HSRP active gateway in that sense. So from your diagram it's not possible to say because of the above questions.

Jon

burleyman Tue, 05/11/2010 - 10:12

Sorry, I did not realize I left the VLANS off the drawing

Switch A is the VTP Server and all others are Clients all in the same VTP domain.

All Users connected as follows

Switch B is on the 12th floor and VLAN 112

Switch C is on the 11th floor and VLAN 111

Switch D is on the 10th Floor and VLAN 110

Thanks, Mike

Jon Marshall Tue, 05/11/2010 - 10:15

Mike

And which switch is responsible for inter-vlan routing ?

Jon

burleyman Tue, 05/11/2010 - 10:27

Boy I am losing it...

Switch A - It is a layer 3 switch.

Mike

Jon Marshall Tue, 05/11/2010 - 10:31

Mike

No problem

It will take the blue path because the active gateway is on switch A so the packet goes to switch A which then routes the packet onto vlan 3 to get to the server.

Like i say traffic does not have to go to the root bridge for it's vlan. STP is about setting up a loop free path and that's all.

Jon

burleyman Tue, 05/11/2010 - 10:59

Ok. I must have been reading too much into it. In my case it should be good. It would be better if the Core switch were the root, correct?

Mike

Jon Marshall Tue, 05/11/2010 - 11:05
`burleyman wrote:Ok. I must have been reading too much into it. In my case it should be good. It would be better if the Core switch were the root, correct?Mike`

Yes, generally the core switch is set to root. Couple of reasons -

1) with redundant paths you can actually get suboptimal paths if the root switch ends up being one of the access-layer switches.

2) core switches tend to have the most horsepower so it's a good idea to use those. Last thing you want is an old access-layer switch running very high CPU also be root STP because if it gets too busy BPDUs can go missing and then STP convergence kicks in.

Jon

Jon Marshall Tue, 05/11/2010 - 11:01

Mike

Sorry, had to rush off so my answer was a bit brief.

As you know STP is concerned with making sure there are no loops in the L2 network. So if your network has redundant paths between switches then some of the paths will have to be blocked. In your diagram there are no redundant paths so all switch interconnects are forwarding. So once STP has done it's job of creating a loop free topology your data paths simply use the active forwarding links. As i say it's not like HSRP where you must go to your active gateway switch.

So there is no need for the traffic from switch B to go to switch D. All the traffic from the client needs to do is get to it's active HSRP gateway or L3 vlan interface if you are not running HSRP. The quickest path is to go directly to switch A. Once it gets to switch A there is no need to forward the traffic to the root bridge because you have already reached the switch with the active default-gateway. So it can simply be routed onto the server vlan and sent to the server.

Jon

burleyman Tue, 05/11/2010 - 11:13

Thanks Jon.....After reading your explaination I think it clicked for me. I keep getting mixed up because we have all layer 3 switches and when I go over something that are layer 2 I get myself messed up by over thinking things. Thanks for you help as always.

Mike

Jon Marshall Tue, 05/11/2010 - 11:20
`burleyman wrote:Thanks Jon.....After reading your explaination I think it clicked for me. I keep getting mixed up because we have all layer 3 switches and when I go over something that are layer 2 I get myself messed up by over thinking things. Thanks for you help as always. Mike`

Mike

No problem. To be honest i think L2 is not as intuitive as L3 and i often have to sit back and think about it. Trouble with L2, especially ethernet, is that it generally just works with minimal config so it's not something we generally pay much attention to.

I guess what i was trying to say in summary is that once a L2 loop free path has been worked out the user data will always take the shortest path it can using the active links.

Always glad to help and thanks for the ratings.

Jon

burleyman Tue, 05/11/2010 - 11:23

Jon,

I am feeling really brain dead here.... I am trying to mark this question as answered and I do not see it anywhere...am I missing something?

Mike

Jon Marshall Tue, 05/11/2010 - 11:30

Mike

That's the problem when you pretty much only post responses like i do, i have no idea how to mark a question as resolved. From memory i think there is a correct answer box you click within the post but i could be wrong.

Don't worry about it, not an issue.

Jon

burleyman Tue, 05/11/2010 - 11:34

That is what I was looking for....I blame Dan....he took mine away...

Either way thanks.

I figured you would know where it was but I did not think that it would be rare for you to ask a question..... Knower of All....

Mike

Jon Marshall Tue, 05/11/2010 - 11:57

I figured you would know where it was but I did not think that it would be rare for you to ask a question..... Knower of All....

You can pack that in right away, knower of all my ****

You should see me on VOIP ie. useless !!!

Jon

burleyman Tue, 05/11/2010 - 12:33

Maybe it should have been..... Knower of Almost....

Mike

Jon Marshall Wed, 05/12/2010 - 14:03