If you have a pair of 3750 switches in a stack. You then split the stack (not from a config point of view, just say for e.g switch two is powered off) Switch one stays powered on and functioning as the master. If a number of config changes are then made to the master, eg.
- Switch port changes
- aaa changes
- Syslog changes
What will happen when the second switch is powered on? Will all those new changes be synced across and overwrite anything on switch two when it boots and joins or will it try and merge the config?
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As Balaji already noted, the current master will "sync" the current config with other stack members joining the stack. BTW, "sync" in this case means the master's config will effectively replace/overwrite the new stack member's config.)
This also applies if, in your example case, the stack member that was powered off was the stack master. Further, even if that stack member has a higher priority than the stack member that kept running, and took over the role of stack master, the former stack master won't preempt the current stack master. If that member loses power, then the former stack master (again in your two member stack example) will take over again as stack master, but also again, using the config it obtained from the then active replacement stack master.
One situation where the configs may not exactly "sync" up is if the switch member that lost power is replaced by another physical switch. In such a case, what happens depends on whether the replacement switch has been provisioned with the stack member number of the switch that failed and assuming it's like hardware, it will come on-line just as the former powered off switch would have.
However, if the switch is not provisioned like the former switch or of like hardware, if will not come on-line exactly as if the prior switch would have. This applies whether you've made additional configuration changes or not.
An example of "unlike" hardware, original switch has 24 ports and replacement has 48, or the converse. Or, original switch has FE ports and replacement has gig ports, or also the converse.
Ye got you thanks. So effectively any switch that is currently master (regardless of priorities) will be the one that syncs the config to members joining. (The 'current' config will always be with the currently active master)
Physically, unsure whether master pushes to member or member pulls from master as member joins stack and when stack insures configs (running and saved) matches what the stack is using. I suspect joining member gets an "immediate" copy of running config and then stack insures the newly joining stack member's saved config matches that of stack's current saved config. (I.e. besides, of course, joining stack member's saved config possibly not matching current stack's current saved config, as with other Cisco devices, running config and saved configs can differ.)
Further, anytime you change running config, running config should sent to all current stack members. Ditto for saving the config, i.e. config should be saved on all current stack member's flash.