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I need to add a C3850 switch to an existing switch stack.

I know I can unplug a stackwise cable from the stack without problems, but is it the same with stackpower cables ?

All switches have at least 1 power supply. Would I be able to unplug my stackpower cable from switch 3, and connect it to my newly added switch 4, and also add one more cable between switch 3 and 4 ?

I want to avoid any downtime / reboot.

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    Yes, but it may still cause a problem or reboot the device. You need to plan it for a time when you may experience downtime. You should have a maintenance widow to perform such things.
    – Ron Maupin
    May 25 at 17:31

3 Answers 3

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Never tried to hot move stack cables and I think it's not the way to go. You may disable stack power ports then you can move cables, at the end enable ports again: stack-power switch x port y disable Better to check if power supplies are working before disabling power stack: show environment powe

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  • I don't have the command to disable a port... Doesn't look like it's possible.
    – pHeoz
    May 27 at 7:37
  • My mistake, i have the command. It's in enable mode, but not in (config) mode. I'll try these commands this coming Friday and report back on the result.
    – pHeoz
    May 31 at 8:39
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I have performed the intervention of adding a new switch to my switch stack (stackwise+stackpower) like this:

  • Connected my Stackwise cables to my (turned off) new switch (switch 4)
  • Disabled the Stackpower ports of Switch 1 and Switch 3

# stack-power switch 1 port 1 disable

# stack-power switch 3 port 2 disable

This did not cause any downtime.

  • Connected the stackpower cables of my new switch (switch 4) to switch 1 and switch 3
  • Turned on my new switch via its power supply. It joined the Stackwise stack

At this point when running "show stack-power details", I had 2 Stackpower configuration : Powerstack-1 (containing my initial stack of 3 switches), and Powerstack-4 (containing my switch 4).

I tried adding my switch 4 in my Powerstack-1 configuration with :

(config)# stack-power switch 4

(config)# stack-power name Powerstack-1

.. but it didn't work and ended up renaming the name of switch 4's powerstack configuration to "Powerstack-1-4". I renamed it back to Powerstack-4.

At this point I re-enabled the ports with :

# stack-power switch 1 port 1 enable

# stack-power switch 3 port 2 enable

After the first command, the two stack-power configuration merged automatically and I was left with just the "Powerstack-4" configuration. The ring was complete and everything was seen as 'Connected'. I never had any downtime.

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Note that the switch ships from the factory in install mode, vs bundle mode which is essentially an inefficient use of switch resources. If the switch is used (configured already by someone), first step is to put it back in its default install mode.

In terms of adding a fourth switch to the stack, in terms of power, there is be no impact on the existing switches because you're never unplugging power to any existing switch. The key is that the new switch is powered OFF when you introduce it to the stack so that it joins as a member and not as master, which would cause a stack reset. Also, prior to adding the new switch, enable software auto-upgrade enable so that the new switch auto upgrades to the same version IOS as the existing stack members.

The Catalyst 3850 Switch Hardware Installation Guide indicates:

Power-sharing mode is the recommended configuration for Cisco Catalyst 3850 Series Switches

Note the Chapter: Power Supply Installation says:

  • The power supply is hot-swappable. In some configurations, such as full PoE+ or power sharing mode, removing a power supply causes powered devices to shut down until the power budget matches the input power of a single power supply. To minimize network interruption, hot swap the power supply under these circumstances:

    • The switch is in StackPower mode and sufficient power is available.

    • The switch is powered by other switches in a power stack, and no active backup is in progress.

  • For the switch commands that display available power budget, see the software configuration guide.

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  • software auto-upgrade enable only works on switches in INSTALL mode, and not in BUNDLE mode.
    – pHeoz
    Jun 3 at 20:16
  • the switch is running INSTALL mode unless someone changed the default settings. Note BUNDLE mode is essentially an inefficient use of Switch resources. Jun 5 at 21:46

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