Configuring POTS Dial Peers

As mentioned previously, you can use POTS dial peers to define reachability information for anything connected to your VoIP network from the traditional telephony world. This includes devices connected to FXO, FXS, E&M, and digital BRI/T1/E1 interfaces.

Tip If you are connecting to something without an IP address (such as an analog phone, fax machine, PBX, or the PSTN), it is a POTS dial peer.

The network in Figure 8.6 demonstrates the configuration of POTS dial peers.

Figure 8.6 Dial Peer Configuration Scenario

The configuration will begin with the CME_A router. To create POTS dial peers, you can use the syntax dial-peer voice <tag> pots from global configuration mode. The tag value can be any number you want (from 1 to 2,147,483,647), as long as it is unique on the router. Although this tag does not have any impact on the reachability information you assign to the devices, many administrators have a common practice of relating a dial-peer tag value to the phone number of the device. Example 8.7 assigns the extensions shown in Figure 8.6 to the analog phones attached to the CME_A router's FXS ports.

Example 8.7 Configuring POTS Dial Peers for FXS Ports

CME_A(config)# dial-peer voice ?

<1-2147483647> Voice dial-peer tag CME_A(config)# dial-peer voice 1101 ? mmoip Multi Media Over IP pots Telephony vofr Voice over Frame Relay voip Voice over IP CME_A(config)# dial-peer voice 1101 pots CME_A(config-dial-peer)# destination-pattern ?

WORD A sequence of digits - representing the prefix or full telephone number CME_A(config-dial-peer)# destination-pattern 1101 CME_A(config-dial-peer)# port 0/0/0 CME_A(config-dial-peer)# exit CME_A(config)# dial-peer voice 1102 pots CME_A(config-dial-peer)# destination-pattern 1102 CME_A(config-dial-peer)# port 0/0/1

After you create the dial peer, you can then assign the phone number to the attached device(s) by using the destination-pattern and port commands. After you have entered this configuration, you can place calls between the phones attached to the CME_A router. Before you place any calls, it is always best to verify the dial-peer configuration.

The show dial-peer voice command (without the summary keyword) does show you all the dial peers on your router, but uses about a page of output for each dial peer. Although this information may be useful at times, the summary view, which is shown in Example 8.8, is usually much easier to digest. Notice at the bottom of the output are the dial-peer tags 1101 and 1102, displayed as POTS dial peers with the proper destination pattern and port assignments. The other dial peers listed (with tags 20005-20014) are dial peers created by the CME routers for the ephone-dns configured in Chapter 5, "Basic CME IP Phone Configuration."

Example 8.8 Verifying Dial Peers CME_A# show dial-peer voice summary dial-peer hunt 0 AD

TAG TYPE MIN OPER PREFIX DEST-PATTERN

20005 pots up up 1500$

20006 pots up up 1501$

PRE PASS OUT

FER THRU SESS-TARGET STAT PORT

20007 pots up up

20008 pots up up

20009 pots up up

20010 pots up up

20011 pots up up

20012 pots up up

20013 pots up up

20014 pots up up

1101 pots up up

1102 pots up up

1502$

1503$

1504$

1505$

1506$

1507$

1508$

1509$

1101

1102

50/0/22 50/0/23 50/0/24 50/0/25 50/0/26 50/0/27 50/0/28 50/0/29

You can test the configuration by placing a call between the devices. Because this is a book, you will not actually hear the phones ring, so Example 8.9 shows a useful debug command in which you can see the router process the dialed digits from the phone attached to the FXS port.

Example 8.9 Using the debug voip dialpeer Command to Analyze Digit Processing CME_A# debug voip dialpeer voip dialpeer default debugging is on

.Jul 2 17:16:44.698: //-1/77671F238035/DPM/dpMatchPeersCore:

Calling Number=, Called Number=1, Peer Info Type=DIALPEER_INFO_SPEECH .Jul 2 17:16:44.698: //-1/77671F238035/DPM/dpMatchPeersCore:

Match Rule=DP_MATCH_DEST; Called Number=1 .Jul 2 17:16:44.698: //-1/77671F238035/DPM/dpMatchPeersCore:

Result=Partial Matches(1) after DP_MATCH_DEST .Jul 2 17:16:44.702: //-1/77671F238035/DPM/dpMatchPeersMoreArg:

Result=MORE_DIGITS_NEEDED(1) .Jul 2 17:16:45.114: //-1/77671F238035/DPM/dpMatchPeersCore:

Calling Number=, Called Number=11, Peer Info Type=DIALPEER_INFO_SPEECH .Jul 2 17:16:45.114: //-1/77671F238035/DPM/dpMatchPeersCore:

Match Rule=DP_MATCH_DEST; Called Number=11 .Jul 2 17:16:45.114: //-1/77671F238035/DPM/dpMatchPeersCore:

Result=Partial Matches(1) after DP_MATCH_DEST .Jul 2 17:16:45.114: //-1/77671F238035/DPM/dpMatchPeersMoreArg:

Result=MORE_DIGITS_NEEDED(1) .Jul 2 17:16:45.914: //-1/77671F238035/DPM/dpMatchPeersCore:

Calling Number=, Called Number=110, Peer Info Type=DIALPEER_INFO_SPEECH .Jul 2 17:16:45.914: //-1/77671F238035/DPM/dpMatchPeersCore:

Match Rule=DP_MATCH_DEST; Called Number=110 .Jul 2 17:16:45.914: //-1/77671F238035/DPM/dpMatchPeersCore: Result=Partial Matches(1) after DP_MATCH_DEST

Example 8.9 Using the debug voip dialpeer Command to Analyze Digit Processing continued

.Jul 2 17:16:45.914: //-1/77671F238035/DPM/dpMatchPeersMoreArg:

Result=MORE_DIGITS_NEEDED(1) .Jul 2 17:16:48.426: //-1/77671F238035/DPM/dpMatchPeersCore:

Calling Number=, Called Number=1101, Peer Info Type=DIALPEER_INFO_SPEECH .Jul 2 17:16:48.426: //-1/77671F238035/DPM/dpMatchPeersCore:

Match Rule=DP_MATCH_DEST; Called Number=1101 .Jul 2 17:16:48.426: //-1/77671F238035/DPM/dpMatchPeersCore:

Result=Success(0) after DP_MATCH_DEST .Jul 2 17:16:48.426: //-1/77671F238035/DPM/dpMatchPeersMoreArg:

Result=SUCCESS(0)

List of Matched Outgoing Dial-peer(s):

1: Dial-peer Tag

1=1101

Notice the highlighted output from the debug command in Example 8.9. This shows the router performing digit-by-digit call processing. As the attached phone dials each digit, the router processes that digit and attempts to find a match from among its dial peer configuration. For the first three dialed digits, the result is clear: more digits needed. Once the caller dials the fourth digit, the router matches dial-peer tag 1101 and processes the call.

Now we can turn our attention to the POTS dial-peer configuration on ROUTER_B, which has a T1 PRI connection to a PBX system hosting 2XXX extensions (four-digit extensions beginning with the number 2). In the earlier "Configuring Digital Voice Ports" section of this chapter, the physical characteristics of the T1 VWIC interface were configured to support T1 PRI connectivity (by using the pri-group command). When that command was entered, the router automatically created the voice port 1/0:23, which represented the signaling channel of the T1 PRI connection. Example 8.10 now configures the router to use this T1 PRI port anytime it receives a call for a 2XXX extension.

Example 8.10 Configuring a POTS Dial Peer for a T1 Interface

ROUTER_

B(config)# dial-peer

voice 2000 pots

ROUTER_

_B(config-dial-peer)#

destination-pattern 2...

ROUTER_

_B(config-dial-peer)#

no digit-strip

ROUTER_

_B(config-dial-peer)#

port 1/0:23

It is that simple. Notice that you can use the "." wildcard to represent any dialed digit. This instructs the router to send all 2XXX extensions out port 1/0:23 (the T1 PRI interface). There is one additional command in this example that brings up a big point of discussion: no digit-strip. This command prevents the router from automatically stripping dialed digits from this dial peer. Now, why would the router do that? Because of the POTS dial-peer rule Cisco has programmed into Cisco IOS. Here's the rule:

Digit Stripping Rule of POTS Dial Peers:

The router will automatically strip any explicitly defined digit from a POTS dial ■ Kp^ peer before forwarding the call.

An explicitly defined digit is any non-wildcard digit. In the case of Example 8.10, 2 is an explicitly defined digit. This rule is in place primarily to assist with stripping outside dialing codes before sending calls to the PSTN. For example, organizations commonly require users to dial 9 to access an outside line (often receiving a second dial tone after they have dialed 9). However, if you keep this access digit prepended to the dialed phone number, the PSTN carrier will reject the call. Thus, you could create a POTS dial peer with the destination-pattern 9 command (for seven-digit dialing), and the router will automatically strip the explicitly defined 9 digit before sending the call to the PSTN.

In the case of Example 8.10, stripping the 2 digit before sending the call to the PBX system would not be a desired behavior. Thus, the no digit-strip command prevents this automatic digit stripping process. This concept will be revisited in the upcoming "Understanding Router Call Processing and Digit Manipulation" section.

Note The automatic digit stripping function is specific to POTS dial peers. VoIP dial peers (discussed in the following section) do not automatically strip digits.

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