Enhancing and Extending an Existing Plan to Accommodate VoIP
You can enhance and extend an existing numbering plan in many ways to accommodate a
VoIP network. All require careful planning and consideration. In large-scale service
provider designs, you might rely on the GW to perform digit manipulation, whereby the
GW takes a calling (or called) number and strips or adds (prefixes) digits before sending
the number to its destination. The process of formatting the number to a predefined pattern
is called number normalization.
Number Normalization Example
Consider the topology example in Figure 6-13. When site E (703555....) dials
7275550199, the full 10-digit dial string is passed through the Frame Relay cloud to the
router at site D. RouterD matches the destination pattern 7275550199 and forwards the
10-digit dial string to RouterA. RouterA matches the destination pattern 727555...., strips
off the matching 727555, and forwards the remaining four-digit dial string to the PBX.
The PBX matches the correct station and completes the call to the proper extension.
Chapter 6: Identifying Dial Plan Characteristics 355
Site E
RouterA Site A
727555....
813555....
305555....
Site B
Site C
Site D
RouterD
703555.... 202555....
10 Digits
10 Digits
10.10.10.4
10.10.10.1
10 Digits 4 Digits
Site F
Frame
Relay
4 Digits
4 Digits
10 Digits
V
V
V
V
Figure 6-13 Enhancing and Extending an Existing Dial Plan to Accommodate VoIP
Calls in the reverse direction are handled similarly. However, because a Centrex service
requires a full 10-digit dial string to complete calls, the POTS dial peer at RouterD is configured
with digit stripping disabled. An alternative solution involves enabling digit stripping
and configuring the dial peer with a six-digit prefix (in this case, 703555), which
results in forwarding the full dial string to the Centrex service.
Examples 6-22 and 6-23 show the dial-peer configurations for the routers at Site A and at
Site D.
Example 6-22 Dial-Peer Configuration for RouterA
356 Authorized Self-Study Guide: Cisco Voice over IP (CVOICE)
RouterA#show run
*** OUTPUT OMITTED ***
dial-peer voice 1 pots
destination-pattern 727555….
port 1/0:1
!
dial-peer voice 4 voip
destination-pattern 703555….
session target ipv4:10.10.10.4
!
dial-peer voice 5 voip
destination-pattern 202555….
session target ipv4:10.10.10.4
*** OUTPUT OMITTED ***
Example 6-23 Dial-Peer Configuration for RouterD
RouterD#show run
*** OUTPUT OMITTED ***
dial-peer voice 4 pots
destination-pattern 703555….
no digit-strip
port 1/0:1
!
dial-peer voice 5 pots
destination-pattern 202555….
no digit-strip
port 1/0:1
!
dial-peer voice 1 voip
destination-pattern 727555….
session target ipv4:10.10.10.1
*** OUTPUT OMITTED ***