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I have a PowerConnect 6248 and 6224P stacked with the 6248 acting as the managing switch. They're connected using the 48Gbps stacking module with 2 cables running between the switches.

Do I need to have STP on for this configuration? I'm not primarily a network guy, and my beginner-level thinking was "well there are 2 connections between the switches, that's a potential loop, therefore I need STP." However, I might be wrong and would like to find a way to disable STP if possible.

(Everything seems to work fine without STP disabled, but I want to verify that I'm not missing something.)

Mike Naylor
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FullTimeCoderPartTimeSysAdmin
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    Why disable STP regardless? It's not going to do any harm if it's enabled but not needed. Here's my modus operandi for `all` switches, whether they're single, stacked or whatever: `Do Not Disable STP`. – joeqwerty Apr 04 '14 at 19:47

2 Answers2

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The stacking protocol should handle loop detection and prevention. It may be using spanning tree, it may be doing something else. You best bet is to read the docs on how Dell's stacking works.

I'm not primarily a network guy, and my beginner-level..

For the love of god if you don't know what you are doing don't turn off spanning tree. There are a few valid reasons to turn off spanning tree, but if you don't know what you are doing don't turn it off you are much more likely to take down the network than anything.

Zypher
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  • I made my edit mentioning that I had tried turning it off just to see what happened before seeing your post. This is exactly why I'm asking the question... turning STP off doesn't immediately bring the network to a halt, but I don't feel comfortable blindly disabling it. Thanks for your advice, I'll do some reading! – FullTimeCoderPartTimeSysAdmin Apr 04 '14 at 19:13
  • Re: my motivation for turning STP off in the first place. We're having issues with our VOIP service. WinMTR shows a lot of packet loss to the first hop (our internal router). This obviously has me very concerned. One of the VOIP people mentioned that STP could be a problem... I know very little about it beyond having to enable portfast mode (?) to get dhcp registration to be faster, hence this post. I'm looking into it further now, thanks. – FullTimeCoderPartTimeSysAdmin Apr 04 '14 at 19:15
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    Sorry if this is blunt, but that guy doesn't know what he is talking about. It's more likely you have an overloaded router, or your connection is saturated. STP is a L2 technology. If you turn it off your chance of accidentally taking down the whole network with a loop at some point now or in the future goes to almost 100% – Zypher Apr 04 '14 at 19:27
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    STP wouldn't be interfering with VOIP. As @Zypher mentioned it's most likely an overload of traffic. I'd recommend looking at Quality of Service and prioritizing VOIP traffic if you are currently having issues. This is venturing into another question from the original though. – Mike Naylor Apr 04 '14 at 19:36
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    [The Configuration Guide](ftp://ftp.dell.com/Manuals/all-products/esuprt_ser_stor_net/esuprt_powerconnect/powerconnect-6224_reference%20guide2_en-us.pdf) has details on Quality of Service starting on page 139 – Mike Naylor Apr 04 '14 at 19:40
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    You know why the VOIP guy told you that? Because he is the VOIP guy and his mission is to make VOIP work, regardless and irrespective of everything else. – joeqwerty Apr 04 '14 at 19:48
  • Thanks for all the info. I have STP enabled and working properly and am zeroing in on what the actual issue with our voip system is. After reading a bit, I'm glad I didn't take the "leave it disabled and see what happens" route. – FullTimeCoderPartTimeSysAdmin Apr 04 '14 at 21:01
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First off, the cables on the stacking module (if stacking is configured correctly) wouldn't be a loop on your network. These are part of the Stacking Module which is/should be separate from your standard TCP/IP network. It may still be that the stack is misconfigured but that is not part of the question so we'll move on.

STP or any variant of STP (MSTP, RSTP...) is designed to keep loops from occurring on your network, as you already know. To go into a little more detail, enabling STP is recommended as it will help prevent someone from creating accidental network loops on your network and causing broadcast storms, which could bring the network down.

Being that you have the 6248 as the primary in the stack I'd assume you'd want to make it your root switch as well. On your 6248 under the STP configuration set the priority of that switch to 0 to make it the root. On the other switch you shouldn't have to change the value (unless it is configured as 0 already then just give it a higher value). By setting the 6248 as 0 this means it becomes the root of the spanning tree. If you leave the 6224P's STP priority as the default as this will not cause an issue. If the switch is not the root and is set to the default value then it calculates the priority based on the lowest MAC address value of the switches that are connected.

Here is a handy article that reviews some common mistakes when configuring STP. Hope this helps clarify some things. I would recommend keeping STP enabled and configured correctly to avoid any issues down the road. The protocol has very little overhead and reduces large headaches later...

Mike Naylor
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