Cisco H.323 VC-289 - Manual (Page 8)

Configuring H.323 Gatekeepers and Proxies
H.323 Gatekeeper Features
VC-296
Cisco IOS Voice, Video, and Fax Configuration Guide
Note For ease of maintenance, the same prefix type should be used to denote the same gateway type in all
zones under your administration. No more than 50 different technology prefixes should be registered
per zone.
Also, with the gw-type-prefix command, a hop off can be forced to a particular zone. When an endpoint
or gateway makes a call-admission request to its gatekeeper, the gatekeeper determines the destination
address by first looking for the technology prefix. When that is matched, the remaining string is
compared against known zone prefixes. If the address is determined to be a remote zone, the entire
address, including technology and zone prefixes, is sent to the remote gatekeeper in a location request.
That remote gatekeeper then uses the technology prefix to decide on which of its gateways to hop off. In
other words, the zone prefix (defined using the zone prefix command) determines the routing to a zone,
and once there, the technology prefix (defined using the gw-type-prefix command) determines the
gateway to be used in that zone. The zone prefix takes precedence over the technology prefix.
This behavior can be overridden by associating a forced hop-off zone with a particular technology prefix.
Associating a forced hop-off zone with a particular technology prefix forces the call to the specified
zone, regardless of what the zone prefix in the address is. As an example, you are in the 408 area code
and want callers to the 212 area code in New York to use H.323-over-IP and hop off there because it
saves on costs. However, the only H.320 gateway is in Denver. In this example, calls to H.320 endpoints
must be forced to hop off in Denver, even if the destination H.320 endpoint is in the 212 area code. The
forced hop-off zone can be either a local zone (that is, one that is managed by the local gatekeeper) or a
remote zone.
HSRP Support
Cisco routers support Hot Standby Router Protocol (HSRP), which allows one router to serve as a backup
to another router. Cisco gatekeepers can be configured to use HSRP so that when one gatekeeper fails,
the standby gatekeeper assumes its role.
To configure a gatekeeper to use HSRP, perform the following tasks:
• Select one interface on each gatekeeper to serve as the HSRP interface and configure these two
interfaces so that they belong to the same HSRP group but have different priorities. The one with
the higher priority will be the active gatekeeper; the other assumes the standby role. Make a note of
the virtual HSRP IP address shared by both of these interfaces. (For details on HSRP and HSRP
configuration, refer to the Cisco IOS IP Configuration Guide.)
• Configure the gatekeepers so that the HSRP virtual IP address is the RAS address for all local zones.
• Make sure that the gatekeeper-mode configurations on both routers are identical.
• If the endpoints and gateways are configured so that they use a specific gatekeeper address (rather
than multicasting), use the HSRP virtual IP address as the gatekeeper address. You can also let the
endpoints and gateways find the gatekeeper by multicasting. As long as it is on standby status, the
secondary gatekeeper neither receives nor responds to multicast or unicast requests.
As long as both gatekeepers are up, the one with the higher priority on its HSRP interface will be the
active gatekeeper. If this active gatekeeper fails, or if its HSRP interface fails, the standby HSRP
interface assumes the virtual HSRP address and, with it, the active gatekeeper role. When the gatekeeper
with the higher HSRP priority comes back online, it reclaims the HSRP virtual address and the
gatekeeper function, while the secondary gatekeeper goes back to standby status.