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stephengun
Level 1
Level 1

Cisco switch hardware installation guides have a section on how to connect stack cables. The diagram below is the classic example, taken from the 9300 guide. The guides say this is a recommended configuration, not the recommended configuration

 

 

To me, this method has the drawback that you must have one stack cable long enough to reach from top to bottom. This becomes pretty significant when you have several switches in your stack, and more so if you put cable management between each switch.

 

Let me recommend another configuration.

 

If you connect two switches with stack cables, most people connect the two A ports and the two B ports together. It is tidy:

 

stack-2.PNG

But not as per the recommendation from Cisco. So what if you want to add a third switch to the stack? You can't go from here to the Cisco method without breaking the stack, but you don't need to. You could instead do this:

 

stack-3.PNG

You don't need to break the stack, and can add your third switch without disrupting the network. We can now add a fourth switch, and then a fifth switch in the same manner, never breaking the stack, and never needing a stack cable reach more than two switches away, and no cable even crosses another:

stack-5.PNG

 

Another, and perhaps better, use for this method is creating a single stack of top-of-rack switches across numerous racks using only 3m stack cables:

 

top-of-rack-stack.PNG

 

This is extensible to any number of switches, not withstanding the limitations of the hardware itself. So, why isn't this the recommended configuration?

 

As an interesting aside, this pattern is very similar to the straight-laced boot problem, where the stack cables are equivalent to the lacing inside the boot, and the internal stack bus is equivalent to the external straight-laces.

6 Comments
msmith508
Level 1
Level 1

Any update to how well this works in practice? It looks too great to be true.

kb03
Level 1
Level 1

@msmith508 a bit late, but I can attest that this stack cable layout works great. I run ~500 stacks of C9300-48UXMs this way with zero issues. Stackwise shows 480G full-ring.

 

Sw#/Port#  Port Status  Neighbor/Port  Cable Length   Link OK   Link Active   Sync OK   #Changes to LinkOK  In Loopback 
-------------------------------------------------------------------------------------------------------------------
1/1        OK           2/1            50cm           Yes       Yes           Yes       1                   No           
1/2        OK           3/1            50cm           Yes       Yes           Yes       1                   No           
2/1        OK           1/1            50cm           Yes       Yes           Yes       1                   No           
2/2        OK           4/1            50cm           Yes       Yes           Yes       1                   No           
3/1        OK           1/2            50cm           Yes       Yes           Yes       1                   No           
3/2        OK           5/1            50cm           Yes       Yes           Yes       1                   No           
4/1        OK           2/2            50cm           Yes       Yes           Yes       1                   No           
4/2        OK           6/1            50cm           Yes       Yes           Yes       1                   No           
5/1        OK           3/2            50cm           Yes       Yes           Yes       1                   No           
5/2        OK           7/2            50cm           Yes       Yes           Yes       1                   No           
6/1        OK           4/2            50cm           Yes       Yes           Yes       1                   No           
6/2        OK           7/1            50cm           Yes       Yes           Yes       1                   No           
7/1        OK           6/2            50cm           Yes       Yes           Yes       1                   No           
7/2        OK           5/2            50cm           Yes       Yes           Yes       1                   No   



switch#show switch stack-ring speed 

Stack Ring Speed        : 480G
Stack Ring Configuration: Full
Stack Ring Protocol     : StackWise        

 

Gary Keaney
Level 1
Level 1

I too have used this cable method. Came up with it independently not long after passing my CCNA. Have used it in the field with no issues, at the end of the day you are just making a ring. As long as that holds true any cabling method will work.

Great idea to use that for inter-rack.

sirrobin4ever
Level 1
Level 1

This used to be one of the recommended ways of cabling the stackwise cabling. Not sure why it was removed from the newer documentation, but if you go back in time to the 3750-X hardware installation guide, you'll see exactly what you're illustrating in figure 2-1.

mumbles202
Level 5
Level 5

I've always used a S1-S2 port layout and gone from last switch in the stack back to the first, but I've always had the luxury of stacks residing in the same rack or just a rack over.

Using this scenario w/ a stack that's L3, what happens when you lose say the middle switch in the stack of 5 for instance?  Wouldn't you then have a scenario where both sets of 2 switches try to assume the ip of the SVI that should belong to the master switch?

Bladefox
Level 1
Level 1

Agree with the above, if you lose the middle switch the stack fails

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