3750 Stack vs. Spanning Tree Design

Unanswered Question
May 15th, 2008


I'd like to get everyone's thoughts on a design type question.

I currently have two 3750s acting as core switches aggregating 15-20 other access switches throughout the domain. These are NOT stacked today...they have an etherchannel between them. This is a single flat vlan today.

Within the next few days, I'll be segmenting the newtork, enabling dot1q trunking on all uplinks, etc.

My main question is around HA, STP, and uplink configuration.

Do I want to remain having two separate 3750s having one of the uplinks blocked by STP OR configure the switches for Stacking and take advantage of the active/active uplinks withi cross stack EC, OR stack but with STP still blocking one of the uplinks. I suppose it comes down to what level of redunancy is better? Two physically separate 3750s, two stacked, or are they the same in your perspective. Any software defects when stacking that could cause problems?

Any insight is greatly appreciated.

I have this problem too.
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jaye15394 Thu, 05/15/2008 - 05:42

Understood. I just started working here :)

I think of this question as the same as a 6500 with dual sups or two chassis with a single sup each. When in the Core of network, I'd always go for box level redunancy.

So for 3750, when stacked, I kind of think of that as losing box level redundancy, but I suppose it's not quite the same comparison b/c they are still two separate switches?

MUHAMMAD SHAHEEN Thu, 05/15/2008 - 05:52


If both switches are in the same Rack then there is no justification to Uplink them. By Stcking you will get much more control and easier to manage. If one of them fails the secind one takes over with minimum intruption. Further more say If you are currently uplinking all 4 Gigabit ports using Ether-Channels, you are getting 4 Gig aggr but if you use stacking instead you will get 32G backplane. On top of this you have no STP between them as both switches work like a single switch when you stack them.

Here is the full guide


and check this too to compare with other models




jaye15394 Thu, 05/15/2008 - 06:19

Thanks for the replies, but the only reason I'm posiing this question is I've seen problems affect an entire stack with other vendors solutions. I am familiar with the bw, stp, and other benefits...just want to make sure I'm not essentially creating a single point of failure.

ralphcarter Thu, 05/15/2008 - 19:11

Jaye, be careful. If you Stack the switches together you will be forced to use cross-stack etherchannel into the stack. The 3750s are not designed to aggregate this much etherchannel uplink. I know this from experience. I once tried to aggregate 20 access switches into a 6 stack 3750g Distribution switch and the performance was lousy. Some etherchannels would just not form and caused weird errors. I notified Cisco TAc and they werent able to fix the problem stating that I need to upgrade to 6500's which I did and problems were solved.

Just be aware!

jaye15394 Fri, 05/16/2008 - 09:53

That's not really what I wanted to hear, but thanks for the feedback.

I will then migrate them in phases. Will keep STP in place to block the ports I will not create etherchanels for.



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