what is max number of Catalyst 9300 switch can be stack together?

[Herman2018]

hi, we have 4 new C9300 switches which are already stacked together and work as core switch. Now we have other 2x old C9300, and want to add them to the network. what is the best practice of below two options? 

option 1. upgrade the os of old C9300 to the same version of new C9300, then add them to the existing stack

option 2. just stack 2x old switches and uplink connected them to existing 4x new switch stack using port-channel, this 2x old switches just used for non-important endpoints. 

what is pros and cros of stacking 6 switches together? Can anyone please advise, thanks in advance!

[shambhu.kumar]

Use option 1

- Verify the compatibility and make sure the model is supported to add a stack
- Update firmware with the same software version as the existing stack members. All switches must either be in Install Mode or  Bundle Mode
- Backup configuration
- Connect the new 9300 switch to the stack using the appropriate stacking cables.

- Power on the new switch and verify that it boots up successfully.

- make a note of exiting stack switch numbers and priority and Use the appropriate command on the stack master switch to add the new switch to the stack.

- Once the new switch is added, verify that it has successfully joined the stack. Use the "show switch" command

https://community.cisco.com/t5/switching/c9300-add-new-switch-to-stack/td-p/4386257

[Herman2018]

thanks @shambhu.kumar for your advice. can you pls advise the pros and cons of option 1? why don't use option 2? any disadvantage if too many stack members? thanks. 

[Herman2018]

@shambhu.kumar , all the switches stacked together, I think that there will be one potential issue, if any stack cable faulty, it might cause all the switches to reboot. do you agree? 

[shambhu.kumar]

It has their own advantages and disadvantages
with stack,it provided higher port density, redundancy and provides a single control plane with expand switching capacity with higher Forwarding rate.
you can stack 8 switches with out any performance issues.

Yes agreed with you, In some cases, the unexpected reloads triggered due to bad stack cables, stack adapters, or stack ports. Regardless which software version you run, you can be susceptible to this if the stack parts were not installed properly.

[Joseph W. Doherty]

"you can stack 8 switches with out any performance issues."

BTW

For most real-world usages, very true.

However, stack rings are just that, rings, not switch fabrics.  The inherent capacity/performance issues with rings (or buses) increase as you add nodes.

In the 9300 series, Cisco provides 3 different StackWise technologies, 1T, 480 and 320 (see table 4 in https://www.cisco.com/c/en/us/products/collateral/switches/catalyst-9300-series-switches/white-paper-c11-741468.html).

Basically, ring capacity/performance is potentially additionally stressed as you add stack members.

Again, real-world, you likely won't bump into these (or other) issues even with a max member stack, but as stack member count increases, the odds (slightly) increase you may.

Where this consideration significance's may increase is the usage role of the stack.  For instance, a max member stack might be just fine as an ordinary user edge role device but possibly not so for the core device.

Switches like the 9200s or 9300s have design short comings, beyond stack ring, for roles other than user edge/access switches.

Because OP described existing stack as their core device and additional stack member is noted not being critical to core role, above is more of my thinking why I slightly lean toward not adding them to core stack, but again I think it unlikely that doing so will create an issue.

Also BTW, if these two additional stack members would increase free ports on a core switch stack, that's a plus as if a stack member fails, you might immediately repatch critical ports.

[MHM Cisco World]

All technology like vss vps and stack have one important advantage which is make multi device run one control plane. 

Op2 make two control plane and this need from you config hsrp ecmp etc. 

And in op2 you can not run PO between two stack group and other SW because each stack have it control plane 

So I recommend op1 and max SW as I know is 8 SW so you are safe to stack 6 SW 

MHM

[Leo Laohoo]

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@Herman2018 wrote:
what is max number of Catalyst 9300 switch can be stack together?

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Sixteen

[Joseph W. Doherty]

"what is the best practice of below two options?'

Don't believe there is a best practice.  Choice depends on how you weigh the pros and cons.

As to pros and cons, it boils down to the many advantages vs. the disadvantage of having one device rather than multiple devices.  For example if you needed 36 ports would you prefer two 24 port switches or one 48 port switch?

Usually fewer is considered the better choice.  But the big disadvantage is one device is a single point of failure, even with redundancy within its components.

Adding two switches to your core stack, very, very (very) slightly increases the failure odds for the stack.

You note the 2 additional stack members are not really needed as core ports.  That being the case, and the fact the new device can be its own stack and MEC between stacks, I would lean toward that, but adding to the stack too, I think, is fine too.  It's sort of a coin flip decision.

Other tie breaking criteria might include implementation impact of the two approaches (brownfield are different from greenfield) and expections for down the road network needs.

[Leo Laohoo]

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@Herman2018 wrote:
option 1. upgrade the os of old C9300 to the same version of new C9300, then add them to the existing stack

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On the Stack Master, do the following: 

conf t
 software auto-upgrade enable
end

Connect the two new members to the stack and let the stack master upgrade/downgrade the two new slave switches automatically.

[Giuseppe Larosa]

Hello @Herman2018 ,

you have written that the existing 4 x Cat9300 stack is acting as core switch.

Are there any access layer switches connected to existing stack  ? Either standalone or in other stacks ?

if the answer is yes, probably I would go for option 2 because you could use the two old Cat9300 to build an extra  access layer stack switch to be connected to current core switch.

If your core switch is the only switch in the central office with endpoints connected to it, then  it can be better to add the two Cat9300 to the existing "core switch stack".

Hope to help

Giuseppe

[Herman2018]

Thanks @Giuseppe Larosa @Joseph W. Doherty @MHM Cisco World @Leo Laohoo  for your kind advices! Actually I prefer option2, we have 2x old stack switches C3850, both rebooted recently because of stacking issue ( cisco TAC suspected stack cable issue). Now our 4x new 9300 stacked together which works as the only logical switch for the office, any reboot will cause critical business impact. Here throughput is the not main consideration as there won't be much traffics. The 2x old 9300 were purchased 5 years ago. I have some concern about the stack cables and stack slots on the switches. 

[Joseph W. Doherty]

Thank you for the update, especially your consideration of the age of the older 9300 vs. newer 9300s.  "Little" considerations like those can be a tie breaker.