Cisco IOS Dial-Peers in H.323 and SIP Gateways

In this blog, we will explore a North American numbering plan (NANP) PSTN dial plan that can be used on either SIP or H.323 gateways. We will configure the dial plan to allow the following types of calls:

3-digit service codes
7-digit local
10-digit local area code
11-digit long distance
Variable length International calls

The following example configuration is a dial-peer that will route 3 digit service codes to an ISDN primary rate interface (PRI) on port 0/0/0 (channel 23 is the Q.931 signaling channel) and strip out the access code of 9:

Dial-peer voice 200 pots
Destination-pattern 9[2-8]11
Forward-digits 3
Port 0/0/0:23

The destination pattern of this dial peer matches on the 9 and then a numeric range as identified by the [2-8] parameter. The numbers in brackets will match on one single digit even if the bracketed figure calls a range like [12-79]. [12-79] would match on one single digit (1, 2 – 7, or a 9). The default digit consumption rules of POTS dial-peers would properly route 3 digits without the forward digits option I used, but the configuration is much clearer using the forward digits command. The forward digits command identifies numerical values from right to left. POTS dial-peer will forward any exact match digits that occur after a wildcard, but do not let that potentially confuse you... it is far easier to remember that a POTS dial-peer only forwards wildcards by default, while a VOIP dial-peer forwards all digits by default.

The next example is used to steer emergency calls prefix with a 9 access code to the local PRI resource (0/0/0:23):

Dial-peer voice 9911 pots
Destination-pattern 9911
Forward-digits 3
Port 0/0/0:23

The forward-digits 3 command is required because the default digit consumption rules will not forward any digits to the PRI interface because there are no wild cards. The prefix 911 digit manipulation command could be used in place of the forward-digits 3 command. This dial-peer requires users to dial the 9 access code to router emergency phone calls. To support 911 calls without access code 9, a number expansion rule or additional dial-peer could be configured.

The following example is a global number expansion rule that translates 911 to 9911:

Router(config)#num-exp 911 9911

The translated number will then match on dial-peer 9911 and be properly routed. The number expansion rule is not my first preference, but would definitely work. Alternatively, the following dial-peer could be used:

Dial-peer voice 911 pots
Destination-pattern 911
Prefix 911
Port 0/0/0:23

In the next blog, we will continue the discussion of dial-peer configurations for H.323 or SIP controlled gateways.

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