A senior engineer at a rural cooperative wants to know how far back a Junos router's built-in configuration history can go before older committed configurations simply age out of the rollback list. What should the engineer understand about that limit?
Select an answer to reveal the explanation.
Short Explanation
Junos keeps a running stack of the last several dozen commits, not just the most recent one, so recovery can reach further back than 'undo the last change.' Rollback 0 is always just a name for whatever is active right now.
Full Explanation
Each successful commit pushes the previous configuration one slot deeper into Junos's stored rollback history, up to 49 prior configurations, addressable as rollback 1 through rollback 49, with rollback 0 always referring to the currently active configuration; once that list is full, the oldest stored configuration is discarded to make room for the newest commit. Treating the history as unlimited until storage fills up overstates the mechanism — the count is a fixed structural limit, independent of how much free space the router has. Limiting rollback to only the single most recent commit describes a much shallower history than Junos actually keeps and would make any multi-step recovery, like undoing two or three bad changes in sequence, impossible. The rollback list is stored in the router's local file system and persists across a reboot; it is not wiped on restart the way volatile session state would be. Caveat: rollback numbering shifts with every new commit, so a number identified during one troubleshooting session may point to a different configuration an hour later once more commits have happened. Check: run show system commit to see the live, current numbered list before choosing a rollback target.