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Note that the WesternGK has been configured with 2 dial-peers. The preference command
assigns the dial-peer order. This command is generally used for failover purposes when you
have the same destination pattern assigned to multiple dial peers.
Dial peer 1 first sends an ARQ message to the WesternGK to determine if it knows the
called number’s terminating gateway address. If the WesternGK does not know this, the
WesternGK will send an ARJ message to the gateway. The second dial peer will then
append a 2# technology prefix to the called number and again try an ARQ message to the
WesternGK. This time, the 2# matches the gw-type-prefix 2# command to hop off to the
DGK. The DGK also recognizes the 2# and matches its gw-type-prefix 2# command to hop
off to the hopoffGK. Note that the 2# gets propagated with the called number.
You can enter the hopoff keyword and gatekeeper ID multiple times in the same command
to define a group of gatekeepers that will service a given technology prefix. Only one of the
gatekeepers in the hopoff list can be local.
If the technology prefix does not have any forced zone attribute, the gatekeeper uses zone
prefix matching to determine the zone. If the matching zone prefix is associated with a
remote zone, an LRQ message is sent to the remote gatekeeper. The LRQ message contains
the entire called number, including the previously stripped technology prefix. If the
matching prefix is for a local zone, that zone is used to satisfy the request.
If no zone prefix match is found, the default behavior is to attempt to use a local zone for
hopoff rather than to fail the call. However, this might not be the desired behavior. You
might prefer that an ARJ message be returned to the gateway so that it can fall through to
an alternate dial peer (for example, one that specifies that the next hop is through a special-
rate PSTN). To override the default behavior, use the arq reject-unknown-prefix
gatekeeper command.
Example: Use of Translation Rules, Technology
Prefixes, and Dial-Peer Failover
This example demonstrates the use of Cisco IOS tools and features to provide better call
routing control with hierarchical design and to minimize dial-peer configuration.
Figure 5-16 illustrates the topology of the example network.
Example: Use of Translation Rules, Technology Prefixes, and Dial-Peer Failover 207
Figure 5-16 Example network using failover scenarios.
Business Case
In this example, the service provider has two gateways that serve both the 408555* and the
408777* numbering plan area (NPA) NXX zones. The gatekeeper has two local zones
twilight and hopoff.
Calls from GWA to GWB should be made through the gatekeeper VoIP network. However,
if GWB is unavailable because of failure or a resource allocation issue (RAI), you should
make the following provisions:
Calls to 408555* should be hairpinned back through the PSTN (GWA POTS) and
completed to the destination. These calls through the PSTN do not incur any intra-
LATA toll charges.
Calls to 408777* should be sent through to the hopoff zone, not to the PSTN. An intra-
LATA toll charge is associated with these calls, so the customer wants to redirect these
calls to the hopoff zone, which has a better rate.
GK
Intra-LATA
GWA
V
PSTN
408555*
408777*
GWB
V
PSTN
408555*
408777*
V
Carrier
Hopoff GW
Zone
hopoff
Zone
twilight
V
V
V
V
V
V
208 Chapter 5: Designing Static Dial Plans for Large VoIP Networks
Applying Cisco IOS Tools
The following tools are used in this example:
Translation rules
preference command
Technology prefixes
Hopoff zone
Translation Rules
Use translation rules to strip or add a 1 to the calling number. This will be used to allow
additional call-routing control in the gateway selection order.
preference Command
Use the preference command on dial peers to allow a dial peer selection order. For
instance, the gateway will match first on dial peer 1. If the gateway receives a location reject
(LRJ) message from the gatekeeper, the next preferred dial peer will be used. This will
allow for failover scenarios and greater call control.
Technology Prefixes
Use technology prefixes to allow certain dial peers to use a hopoff zone technology prefix
(that is, 27#). When dial-peer failover occurs, the 27# technology prefix will force the call
to go to the hopoff zone.
Hopoff Zone
Create a hopoff zone and a hopoff gateway within the network. This zone has a special
negotiated rate for VoIP calls, so the calls cost less than those going through the PSTN.
Example Solution and Configurations
Examples 5-12 through 5-15 show the configurations for the use of translation rules,
technology prefixes, and dial-peer failover.

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