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Cisco 3750Es - making a stack

amigan_99 asked
Medium Priority
Last Modified: 2020-05-12
I ran into an odd thing - two Cisco 3750E switches each with 48 ports - but with configuration for 96 ports 1/0/1 - 48 2/0/1 - 48. They IP addresses were the same and in troubleshooting I found DUP IP errors. It looks as though perhaps at some point in the past they were a stack w two members.

I am sending out a couple of stacking cables. But I can't manage the switches because the IP conflict keeps bumping me off SSH.

My question - since I can't lab this out - should attaching stack port 1 on SW1 to stack port 2 on SW2 cause the config from SW1 to sync with SW2? Or is there some config that would need to be added to SW1 or SW2 for stacking to do its thing? In the end I want one config, one set of IP addresses.

Thank you!
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Distinguished Expert 2018
If stack cables are connected switches may or may not do proper master election. Minimum requirements for stacking must be present (matching IOS main train version, compatible/incompatible device model, or stack member configured in config).

One of the options to access at least one switch in the stack would be to statically configure arp for management IP address on upstream device(s), so you would be always connected to the same stack member and should not be kicked to other device.

After establishing what is root cause for stacking issue you can do changes if needed (as upgrade/downgrade IOS on stack member to match other member, or fixing stack memebers (order, model etc)).

After successfully fixing stack please remember to save configuration. Have in mind that configuration from current master will be saved on both devices, so if "wrong stack master" is elected, issuing wr mem from wrong device will overwrite your desired configuration).

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Andy RansomCustomer Support Engineer III

First, I would verify licensing is correct for stacking.  Even if they were a stack in the past, licensing could of course be changed.  Unless Cisco has changed it in the last few years, a switch running IPBase license can stack with anything other than LANBase - which can only stack with another LANBase licensed switch.

As long as the licensing is right, the stacking will happen automatically, as will selection of stack master.  It will typically automatically select the stack master; the other will possibly reload, if needed, to apply configuration from the master.

IOS version should also be the same on both, to ensure full compatibility. Major version number differences are not compatible, and Minor version number differences are considered partially compatible.

I recommend either backing up the config you want as primary, or manually setting a the master to be highest stack priority before adding the other switch, to be sure automatic selection does not pick the wrong one and lose some needed settings. The official Cisco recommendation is to set the priority setting.

A good document related to stacking 3750 including setting stack priority is at https://www.cisco.com/c/en/us/support/docs/switches/catalyst-3750-series-switches/71925-cat3750-create-switch-stks.html

Because you already have a config on both devices, be sure to see the section "Add a provisioned Switch to a Switch Stack"; the direct link for this section is https://www.cisco.com/c/en/us/support/docs/switches/catalyst-3750-series-switches/71925-cat3750-create-switch-stks.html#anc19
amigan_99Network Engineer


That's very clever - hard coding the ARP entry on the WAN router. I have the two Mac addresses now and I'll try that late in the day when folks have gone home.
amigan_99Network Engineer


While I can't SSH to the two switches. I am able to pull information by going through the web interface. But looking at which mac address is active at the time of execution then I know which switch I'm on. Both members perceive themself as master. SW1 has the lower mac address and the highest priority. Their hardware and firmware versions match. So I think just cabling these puppies up will get me to a better state. Just need to disconnect the trunk between the two. Can't believe consultants left these in this state.


 Switch/Stack Mac Address : b0fa.eb12.6a00
                                           H/W   Current
Switch#  Role   Mac Address     Priority Version  State
*1       Master b0fa.eb12.6a00     10     3       Ready              
 2       Member 0000.0000.0000     0      0       Provisioned  
Switch/Stack Mac Address : b0fa.eb29.b400
                                           H/W   Current
Switch#  Role   Mac Address     Priority Version  State
 1       Member 0000.0000.0000     0      0       Provisioned        
*2       Master b0fa.eb29.b400     1      3       Ready      
Switch/Stack Mac Address : b0fa.eb29.b400
Customer Support Engineer III
From your output, it looks like you were probably right;  they were stacked at one point, but the stack was split later with a duplicated IP configured.

With no stack cable connected at the moment, it make sense that both would identify as stack master.  But yes, the priority setting shows switch 1 should auto-elect as stack master once it's re-cabled correctly.

The trunk line between the two may even be able to stay, but there's really not much point to leaving it in place, as I'm sure you're aware.
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