RAS Gatekeeper Discovery
Thisis the processes by which H.323 terminals/gateways discover their zone gatekeepers Automatic Gatekeeper Discovery:
- If an H.323 endpoint does not know its gatekeeper, then it can send a Gatekeeper Request (GRQ). This is a UDP datagram addressed to the well-known destination port 1718 and transmitted in the form of an IP multicast with the multicast group address 224.0.1.41.
- One or several gatekeepers can answer the request with either a positive Gatekeeper Confirmation (GCF) message or a negative Gatekeeper Reject (GRJ) message. A reject message contains the reason for the rejection and can optionally return information about alternative gatekeepers. Auto discovery enables an endpoint to discover its gatekeeper through a multicast Gatekeeper Request (GRQ) message. Because endpoints do not have to be statically configured for gatekeepers, this method has less administrative overhead. A gatekeeper replies with a GCF or GRJ message. A gatekeeper can be configured to respond only to certain subnets.
Note: A Cisco IOS gatekeeper always replies to a GRQ with a GCF/GRJ message. It never remains silent.
If a gatekeeper is not available, the gateway periodically attempts to rediscover a gatekeeper. If a gateway discovers the gatekeeper has gone off-line, it ceases to accept new calls and attempts to rediscover a gatekeeper. Active calls are not affected.
This table defines the RAS gatekeeper discovery messages:
| Gatekeeper Discovery | |
|---|---|
| GRQ (Gatekeeper_Request) | A message sent by endpoint to gatekeeper. |
| GCF (Gatekeeper_Confirm) | A reply from gatekeeper to endpoint which indicates the transport address of the gatekeeper RAS channel. |
| GRJ (Gatekeeper_Reject) | A reply from gatekeeper to endpoint that rejects the endpoint's request for registration. Usually due to gateway or gatekeeper configuration error. |
RAS Registration and Unregistration
Registration is the process by which gateways, terminals, and/or MCUs join a zone and inform the gatekeeper of their IP and alias addresses. Registration occurs after the discovery process. Every gateway can register with only one active gatekeeper. There is only one active gatekeeper per zone.
The H.323 gateway registers with an H.323 ID (email ID) or an E.164 address. For example:
- EmailID (H.323 ID): gwy-01@domain.com
- E.164 Address: 5125551212
This table defines the RAS gatekeeper registration and unregistration messages:
| Gatekeeper Discovery | |
|---|---|
| RRQ (Registration_Request) | Sent from an endpoint to a gatekeeper RAS channel address. |
| RCF (Registration_Confirm) | A reply from the gatekeeper that confirms endpoint registration. |
| RRJ (Registration_Reject) | A reply from the gatekeeper that rejects endpoint registration. |
| URQ (Unregister_Request) | Sent from endpoint or gatekeeper to cancel registration. |
| UCF (Unregister_Confirm) | Sent from endpoint or gatekeeper to confirm an unregistration. |
| URJ (Unregister_Reject) | Indicates that the endpoint was not preregistered with the gatekeeper. |
