Points to note, its configuration mode command and hidden too (you have to type it completely)
root@SRX# set chassis cluster redundancy-mode active-backup
root@SRX# commit
This requires a reboot of both nodes simultaneously (or at least staggered in a way that both chassis' will be offline for a moment).
You can check if it is working by using the following hidden command (well hidden extension to it ).
show chassis cluster information
- Redundancy mode:
Configured mode: active-backup
Operational mode: active-backup