Specifications

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Cisco 1751 Router Software Configuration Guide
OL-1070-01
Note You can use a period (.) to represent variables (such as extension numbers) in a telephone
number. A period is similar to a wildcard, which matches any entered digit.
The information included in this example needs to be configured on both Cisco 1751 Router 1 and
Cisco 1751 Router 2. In this configuration, Cisco 1751 Router 1 can call any number string that begins
with the digits 17295552 or 17295553 to connect to Cisco 1751 Router 2. Similarly, Cisco 1751 Router
2 can call any number string that begins with the digits 14085551 to connect to Cisco 1751 Router 1.
Configure Number Expansion
To define how to expand an extension number into a particular destination pattern, use the following
global configuration command:
Router(config)# num-exp extension-number extension-string
Use the show num-exp command to verify that you have mapped the telephone numbers correctly.
After you have configured dial peers and assigned destination patterns to them, use the show dialplan
number command to see how a telephone number maps to a dial peer.
Configure Dial Peers
The key to understanding how VoIP functions is to understand dial peers. All of the voice technologies
use dial peers to define the characteristics associated with a call leg. A call leg is a discrete segment of
a call connection that lies between two points in the connection, as shown in Figure 3 and Figure 4. For
instance, between a telephone and a router, a router and a network, a router and a PBX, or a router and
the PSTN. Each call leg corresponds to a dial peer. An end-to-end call is comprised of four call legs,
two from the perspective of the source router as shown in Figure 3, and two from the perspective of the
destination router as shown in Figure 4. Dial peers are used to apply specific attributes to call legs and
to identify call origin and destination. Attributes applied to a call leg include QoS, CODEC, voice
activity detection (VAD), and fax rate.
Table 1 Sample Number Expansion Table
Extension
Destination
Pattern
Num-Exp
Command Entry Description
1... 14085551... num-exp 1...
14085551...
To expand a four-digit extension
beginning with the numeral 1 by prefixing
1408555 to it
2... 17295552... num-exp 2...
17295552...
To expand a four-digit extension
beginning with the numeral 2 by prefixing
1408555 to it
3... 17295553... num-exp 3...
17295553...
To expand a four-digit extension
beginning with the numeral 3 by prefixing
1408555 to it