Gatekeeper Discovery
Endpoints attempt to discover a gatekeeper, and consequently, the zone of which they are
members, by using the RAS message protocol. The protocol supports a discovery message
that can be sent via multicast or unicast, as depicted in Figure 8-3.
The initial signaling from a gateway to a gatekeeper is done through H.225 RAS.
Gateways can discover their gatekeepers through one of these two processes:
■ Unicast discovery:
■ Uses UDP port 1718.
■ In this process, endpoints are configured with the gatekeeper IP address and can
attempt registration immediately.
■ The gatekeeper replies with a GCF or GRJ message.
Figure 8-3 Gatekeeper Discovery
■ Multicast discovery:
■ Uses UDP multicast address 224.0.1.41.
■ Auto discovery enables an endpoint to discover its gatekeeper through a multicast
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.
Chapter 8: Configuring H.323 Gatekeepers 451
V V
Gateway A Gateway B
Gatekeeper
GCF GCF
GRQ
(Multicast)
GRQ
(Unicast)
Note A Cisco IOS gatekeeper always replies to a GRQ with a GCF or GRJ message. It
never remains silent.
■ A gatekeeper can be configured to respond to specific subnets.
The GRQ message requests any gatekeeper receiving it to respond with a GCF message
granting it permission to register. The GRJ message is a rejection of this request, indicating
that the requesting endpoint should seek another gatekeeper.
If a gateway requests an explicit gatekeeper name, only that one will respond. If not, the
first gatekeeper to respond will become the gatekeeper of that gateway. If a gatekeeper is
not available, the gateway will periodically attempt to rediscover a gatekeeper. If the
gateway-discovered gatekeeper has gone offline, it will stop accepting new calls, and the
gateway will attempt to rediscover a gatekeeper. Active calls are not affected by this
process because the RTP streams are directly between the phones.