3750 Stackable Switch Trunking

Answered Question
Sep 19th, 2007

Hi Experts,

Would like to check since the 2 x 3750-24 switches are stack together, can we still creat trunk in between the gigabit port to create redundancy?

any problems of looping or i got the concept wrong?

Sorry if this question is silly but trying to make myself clear..:)

Thanks..

I have this problem too.
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Correct Answer by paul.matthews about 9 years 2 months ago

There is no need to trunk at the front between two switches in the same stack - you can't come close to the bandwidth available on stackwise, and it won't pass any traffic.

You can use the ports to trunk to other switches not in the same stack.

Paul.

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Edison Ortiz Wed, 09/19/2007 - 17:33

No, you can't. The port will go into err-disable since it will detect a spanning-tree loop.

The stack hardware provides 2 cables. You have enough redundancy there. Even if one cable goes bad, you can still ride a full backplane connection with just one cable.

cindylee27 Wed, 09/19/2007 - 23:07

Thanks Edison..

But i am stacking it behind to make 2 switches become 1 switch.. So,meaning i can;t trunk the gigabit port in the front ports??

Thanks again.

Correct Answer
paul.matthews Wed, 09/19/2007 - 23:53

There is no need to trunk at the front between two switches in the same stack - you can't come close to the bandwidth available on stackwise, and it won't pass any traffic.

You can use the ports to trunk to other switches not in the same stack.

Paul.

hobbe Wed, 09/19/2007 - 23:55

As I can see there are two different scenarios here

You have installed the 2 3750s and are using the stacking cable, then they are administratively one switch. But lets for ease of explanation call them unit A and B

Scenario one

You have connected A and B to a third Switch not specifically mentioned with two gig lines and want to trunk.

the answer in this situation is YES you can trunk this way, even if you are using one rj45 from A and the other from B

Situation two

You have taken 2 rj45 cables and put one end of them in A and the other in B and want to trunk them.

That would not work.

it would not work on the count that the A and B is administratively the same switch.(due to the stackwise cables and one is master over the other stacked switches.)

What you would get is the same as if you would take one switch and put one end of the cable in port one and the other in port two.

and one cable in port three and four.

ie two loops.

To clarify on the stackwise cables.

together they have a "backplane speed" of 32 Gbps, and If you loose one of the stackwise cables the backplane speed would be reduced to 16Gbps but it would still be fully funktional (except for the speed)

cindylee27 Thu, 09/20/2007 - 00:00

Thanks guys..A bit clearer now..

As I have 1 stackable cable connected from switch A to B, i am just wondering if I would like to ensure more redundancy..

But what if I remove the Stackable cable at the back, does the stackable serves any purposes as i want to trunk this 2 switch together to have redundancy when switch B is down, switch A can take over.

Thanks again..

paul.matthews Thu, 09/20/2007 - 00:15

The stackwise functionality is only available via the stackwise cable, using a trunk at the front makes them two separate switches, two hops in spanning tree etc. If you want more redundancy use both stackwise at the back.

If you don't want to use the stackwise, you would be better saving money and buying 3560s instead.

cindylee27 Thu, 09/20/2007 - 00:28

Thanks Paul for your input..Understand it now.

Well, another question is.. I would like to make this 3750 acts as a core switch.

I understand that 3750 is a layer 3 switch.I have created Interface vlans and put in the IP Address but why i can;t ping to the interface vlan?

it gaves me request timed out. I can ping to loopback address though.

Thanks again.

paul.matthews Thu, 09/20/2007 - 00:41

Is the VLAN "up"? You normally need something in one of the ports in the vlan to bring it up.

This is one of my little bugbears about IOS switches - I wish autostate was configurable...

cindylee27 Thu, 09/20/2007 - 00:51

Paul,

okie..i will try that..guess will plug in to that vlan n try to ping later..

Thanks so much guys..

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