flexstack vs no flex stack

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Jan 25th, 2012
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I have two L3 swithcs and one L2 switch.

The first isp router is connecte to one of the L3 switch, the second isp router is connected to the other L3 switch.

There is a OSPF configuration in both switchs and the routers. Each router connection to the switch is via routed port (no switchport).

I want to know what is better between stack both L3 switchs or not. see the attached picture.

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Correct Answer by Joseph W. Doherty about 5 years 6 months ago

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Reza Sharifi wrote:


In addition to Joe's comments, with stacking, you eliminate the use of spanning tree. So, your L2 switch would uplink to both 3750s (stacked) via an Etherchannel.


HTH

To clarify what I think Reza intended, Etherchannel to a stack negates the need for STP for intentional L2 loops (redundancy) but you might still need STP to avoid accidental L2 loops.

Correct Answer by Reza Sharifi about 5 years 6 months ago

In addition to Joe's comments, with stacking, you eliminate the use of spanning tree. So, your L2 switch would uplink to both 3750s (stacked) via an Etherchannel.


HTH

Correct Answer by Joseph W. Doherty about 5 years 6 months ago

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Generally, stacking the devices, if supported, offers many advantages.  The biggest disadvantages might be you can't easily have two versions of software (i.e. different version on the different separate devices) running nor often bounce one device while running traffic through the other.

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Correct Answer
Joseph W. Doherty Wed, 01/25/2012 - 07:12
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Generally, stacking the devices, if supported, offers many advantages.  The biggest disadvantages might be you can't easily have two versions of software (i.e. different version on the different separate devices) running nor often bounce one device while running traffic through the other.

Rafael Jimenez Wed, 01/25/2012 - 07:20
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what about the timeout if the master switch fail?.

what is the impact on the ospf convergence?.

Joseph W. Doherty Wed, 01/25/2012 - 09:57
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rafaeljimenez wrote:


what about the timeout if the master switch fail?.

what is the impact on the ospf convergence?.

There may be some, although NSF/SSO features should minimize master failure's impact.  Separate L3 switches, with p2p links would likely be a little faster yet in your diagram you're also connecting L2 switches.  Good chance a gateway change on separate L3 switches would take longer than Etherchannel (as mentioned by Reza).


I didn't list all the pros and cons, it's possible some con would be so important to you a stack would not be suitable, but for most, the stack's pros usually outweigh their cons.

Correct Answer
Reza Sharifi Wed, 01/25/2012 - 07:24
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In addition to Joe's comments, with stacking, you eliminate the use of spanning tree. So, your L2 switch would uplink to both 3750s (stacked) via an Etherchannel.


HTH

Joseph W. Doherty Wed, 01/25/2012 - 10:05
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Reza Sharifi wrote:


In addition to Joe's comments, with stacking, you eliminate the use of spanning tree. So, your L2 switch would uplink to both 3750s (stacked) via an Etherchannel.


HTH

To clarify what I think Reza intended, Etherchannel to a stack can utilize bandwidth often blocked by spanning tree, but spanning tree generally should not be disabled as it can block accidental L2 loops.  In other words there's still need for spanning tree, but not necessarily to support intentional redundancy as you would without a stack.

Correct Answer
Joseph W. Doherty Wed, 01/25/2012 - 10:10
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The   Author of this posting offers the information contained within this   posting without consideration and with the reader's understanding that   there's no implied or expressed suitability or fitness for any purpose.   Information provided is for informational purposes only and should not   be construed as rendering professional advice of any kind. Usage of  this  posting's information is solely at reader's own risk.


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In   no event shall Author be liable for any damages whatsoever (including,   without limitation, damages for loss of use, data or profit) arising  out  of the use or inability to use the posting's information even if  Author  has been advised of the possibility of such damage.


Posting


Reza Sharifi wrote:


In addition to Joe's comments, with stacking, you eliminate the use of spanning tree. So, your L2 switch would uplink to both 3750s (stacked) via an Etherchannel.


HTH

To clarify what I think Reza intended, Etherchannel to a stack negates the need for STP for intentional L2 loops (redundancy) but you might still need STP to avoid accidental L2 loops.

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