If you want to boot from the optical drive there are three options:
With a bootable volume in the optical drive, boot while holding down the C key (this does not change the default boot drive)
Boot while holding down the Option key then select the volume you wish to boot from the list provided and click the right pointing arrow (this does not change the default boot drive)
In System Preferences > Startup drive select the bootable volume in the optical drive. (this changes the default boot volume)
Normally you only want to boot from the optical drive in very limited circumstances such as running Disk Utility. When you boot from a read only media there is no virtual memory and that will adversely effect system performance.