From Halopedia, the Halo wiki
Containment Protocols are series of protocols established by the Forerunners for dealing with Flood outbreaks. They help Monitors enforce the containment of the Flood parasite in the various Flood containment facilities.
On the Halo installations, Containment Protocol is normally enforced by Monitors, but there have been cases of Sentinels taking it upon themselves to enforce containment. There are different procedures about enforcing the protocol, judged by the size of the outbreak. If the outbreak is a minor one, the Monitor will deploy a number of Aggressor Sentinels and Constructors to eliminate the Flood present. In the event of a major outbreak, like that seen on Installation 05's Quarantine Zone, Major Sentinels and Enforcer drones will be mass deployed to eradicate all Flood and its vectors. Unburdened communication is crucial in the event of an outbreak; the protocols dictate that immediate reporting on the status of the outbreak be established among local forces. If containment is breached on a larger scale, then more drastic measures must be taken to ensure that the situation returns to stability, the final option being the use of a Halo installation.
Protocol also includes the procedures involved in activating the Halo Array. It requires a Reclaimer to retrieve an Index for activation, that the Monitor store the Index within its own data arrays for transport, and that only the Reclaimer can use the Index to activate the system, because the Monitor is not able to activate Halo by itself.
The leadership of a Monitor is important, but not always necessary. The Sentinels and Enforcers of Delta Halo continued to enforce containment even after their monitor, 2401 Penitent Tangent, was captured by the Gravemind, and despite their lack of a Monitor the Sentinels of Onyx abide by Containment Protocol by neutralizing local infection vectors - the UNSC Marines, Spartan-IIIs and the Covenant forces on or near the planet.
Containment protocols, however, cannot be activated or proceeded if the global Sentinel Defense Network is disabled. When Voridus disabled Installation 00's Sentinel Defense Network, the Sentinels were unable to perform their duties, including conducting containment protocol. The containment protocols can only be reenacted if the global Defense Network is online.
List of procedures
When Flood outbreak appears imminent, the Containment Protocol becomes the principal priority. This is the reason that the monitor makes simulations of this event.
When enforcing this protocol, the Monitor must:
Here, the Containment Protocol presumably finishes and the Halo Array is activated. If the firing is stopped for some reason, (Delta Halo for example) the Monitor must restart the Containment Protocol or activate the rings through the Ark.