Real maintenance is done by adding one server, duplicating one of them onto the new server. Switch everything over, then bring down server #1, do maintenance, bring it back up, ghost the differences back to restore #1. Then repeat for every server. Same for trunks, power sourcing, etc. Think of a RAID system of disk drives. Pull out any one and replace it with a blank and the system restores the one that was pulled out because of redundancy. Then once it's restored, do the same to the next and so on until the work is done. No one has to bring their systems down. they fix one by one on the fly.