‘boot -L’ will give you the list of boot environments to boot from. So then you can just use the -Z option to boot the corresponding root fs.
Handy when you screw up boot environment activation 🙂
On x86 you can just select the be you want in grub.
{0} ok boot -L
Boot device: /pci@400/pci@0/pci@8/scsi@0/disk@0,0:a File and args: -L
1 Oracle Solaris
2 solaris-1
Select environment to boot: [ 1 - 2 ]: 2
To boot the selected entry, invoke:
boot <root-device> -Z rpool/ROOT/solaris-1
Program terminated
{0} ok boot /pci@400/pci@0/pci@8/scsi@0/disk@0,0:a -Z rpool/ROOT/solaris-1