Chapter 3: Routing Calls over Analog Voice Ports

Cisco Press

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Configuring Destination Pattern Options

The destination pattern you configure is used to match dialed digits to a dial peer. The dial peer is then used to complete the call.

When a router receives voice data, it compares the called number (the full E.164 telephone number) in the packet header with the number configured as the destination pattern for the voice-telephony peer. It also determines the dialed digits the router collects and forwards to the remote telephony interface, such as a PBX, Cisco Unified Communications Manager, or the PSTN.

Note - In the case of POTS dial peers, the router strips out the left-justified numbers that explicitly match the destination pattern. If you have configured a prefix (using the prefix digits command), the prefix is appended to the front of the remaining numbers, creating a dial string, which the router then dials. If all numbers in the destination pattern are stripped out, the user receives a dial tone.

To specify either the prefix or the full E.164 telephone number to be used for a dial peer, use the destination-pattern command in dial peer configuration mode, which has the following syntax:

destination-pattern [+] string [T]

Destination-pattern options include the following:

  • Plus sign (+): An optional character that indicates an E.164 standard number. E.164 is the International Telecommunication Union Telecommunication Standardization sector (ITU-T) recommendation for the international public telecommunication numbering plan. The plus sign in front of a destination-pattern string specifies that the string must conform to E.164.

  • string: A series of digits specifying the E.164 or private dial-plan telephone number. The following examples show the use of special characters often found in destination pattern strings:

    • Asterisk (*) and pound sign (#): An asterisk (*) and pound sign (#) appear on standard touch-tone dial pads. These characters might need to be used when passing a call to an automated application that requires these characters to signal the use of a special feature. For example, when calling an interactive voice response (IVR) system that requires a code for access, the number dialed might be 5551212888#, which would initially dial the telephone number 5551212 and input a code of 888 followed by the pound key to terminate the IVR input query.

    • Comma (,): A comma (,) inserts a one-second pause between digits. The comma can be used, for example, where a 9 is dialed to signal a PBX that the call should be processed by the PSTN. The 9 is followed by a comma to give the PBX time to open a call path to the PSTN, after which the remaining digits are played out. An example of this string is 9,5551212.

    • Period (.): A period (.) matches any single entered digit from 0 to 9 and is used as a wildcard. The wildcard can be used to specify a group of numbers that might be accessible via a single destination router, gateway, PBX, or Cisco Unified Communications Manager. A pattern of 200. allows for ten uniquely addressed devices, whereas a pattern of 20.. can point to 100 devices. If one site has the numbers 2000 through 2049 and another site has the numbers 2050 through 2099, a bracket notation would be more efficient, as described next.

    • Brackets ([ ]): Brackets ([ ]) indicate a range. A range is a sequence of characters enclosed in the brackets. Only single numeric characters from 0 through 9 are allowed in the range. In the previous example, the bracket notation could be used to specify exactly which range of numbers is accessible through each dial peer. For example, the pattern of 20[0–4]. would be used for the first site, and a pattern of 20[5–9]. would be used for the second site. Note that in both cases, a dot is used in the last digit position to represent any single digit from 0 through 9. The bracket notation offers much more flexibility in how numbers can be assigned.

  • T: An optional control character indicating that the destination-pattern value is a variable-length dial string. In cases where callers might be dialing local, national, or international numbers, the destination pattern must provide for a variable-length dial plan. If a particular voice gateway has access to the PSTN for local calls and access to a transatlantic connection for international calls, calls being routed to that gateway have a varying number of dialed digits. A single dial peer with a destination pattern of .T could support the different call types. The interdigit timeout determines when a string of dialed digits is complete. The router continues to collect digits until there is an interdigit pause longer than the configured value, which by default is 10 seconds.

  • However, the calling party can immediately terminate the interdigit timeout by entering the pound character (#), which is the default termination character. Because the default interdigit timer is set to 10 seconds, users might experience a long call-setup delay.

Note - Cisco IOS Software does not check the validity of the E.164 telephone number. It accepts any series of digits as a valid number.

Table 3-8 demonstrates the use of various destination pattern wildcards, including the period, brackets, and the .T wildcards.

Table 3-8  Destination Pattern Options

Destination Pattern

Matching Telephone Numbers


Matches one telephone number exactly, 5550124.

This is typically used when a single device, such as a telephone or fax, is connected to a voice port.


Matches a seven-digit telephone number where the first five digits are 55501. The sixth digit can be a 1, 2, or 3, and the last digit can be any valid digit.

This type of destination pattern is used when telephone number ranges are assigned to specific sites. In this example, the destination pattern is used in a small site that does not need more than 30 numbers assigned.


Matches any telephone number that has at least one digit and can vary in length from 1 through 32 digits total.

This destination pattern is used for a dial peer that services a variable-length dial plan, such as local, national, and international calls. It can also be used as a default destination pattern so any calls that do not match a more specific pattern will match this pattern and can be directed to an operator.

Matching Inbound Dial Peers

When determining how inbound dial peers are matched on a router, it is important to note whether the inbound call leg is matched to a POTS or VoIP dial peer. Matching occurs in the following manner:

  • Inbound POTS dial peers are associated with the incoming POTS call legs of the originating router or gateway.

  • Inbound VoIP dial peers are associated with the incoming VoIP call legs of the terminating router or gateway.

Three information elements sent in the call setup message are matched against four configurable dial-peer command attributes. Table 3-9 describes the three call setup information elements.

Table 3-9  Call Setup Information Elements

Call Setup Element


Called number dialed number identification service

This is the call-destination dial string, and it is derived from the ISDN setup message or channel associated signaling (CAS) DNIS.

Calling number automatic number identification

This is a number string that represents the origin, and it is derived from the ISDN setup message or CAS ANI. The ANI is also referred to as the calling line ID (CLID).

Voice port

This represents the POTS physical voice port.

The four configurable dial-peer command attributes are detailed in Table 3-10.

Table 3-10  Command Attributes for the dial-peer Command

dial-peer Command Attribute


incoming called-number

Defines the called number or DNIS string.


Defines the originating calling number or ANI string.


Uses the calling number (originating or ANI string) to match the incoming call leg to an inbound dial peer.


Attempts to match the configured dial peer port to the voice port associated with the incoming call (POTS dial peers only).

When the Cisco IOS router or gateway receives a call setup request, it looks for a dial-peer match for the incoming call. This is not digit-by-digit matching. Instead, the router uses the full digit string received in the setup request for matching against the configured dial peers.

The router or gateway matches call setup element parameters in the following order:

  1. The router or gateway attempts to match the called number of the call setup request with the configured incoming called-number of each dial peer.
  2. If a match is not found, the router or gateway attempts to match the calling number of the call setup request with the answer-address of each dial peer.
  3. If a match is not found, the router or gateway attempts to match the calling number of the call setup request to the destination-pattern of each dial peer.
  4. The voice port uses the voice port number associated with the incoming call setup request to match the inbound call leg to the configured dial peer port parameter.
  5. If multiple dial peers have the same port configured, the router or gateway matches the first dial peer added to the configuration.
  6. If a match is not found in the previous steps, dial peer 0 is matched.

Because call setups always include DNIS information, you should use the incoming called-number command for inbound dial peer matching. Configuring incoming called-number is useful for a company that has a central call center providing support for a number of different products. Purchasers of each product get a unique toll-free number to call for support. All support calls are routed to the same trunk group destined for the call center. When a call comes in, the computer telephony system uses the DNIS to flash the appropriate message on the computer screen of the agent to whom the call is routed. The agent will then know how to customize the greeting when answering the call.

The calling number ANI with answer-address is useful when you want to match calls based on the originating calling number. For example, when a company has international customers who require foreign-language-speaking agents to answer the call, the call can be routed to the appropriate agent based on the country of call origin.

You must use the calling number ANI with destination-pattern when the dial peers are set up for two-way calling. In a corporate environment, the head office and remote sites must be connected. As long as each site has a VoIP dial peer configured to point to each site, inbound calls from each remote site will match against that dial peer.

Characteristics of the Default Dial Peer

When a matching inbound dial peer is not found, the router resorts to a virtual dial peer called the default dial peer. The default dial peer is often referred to as dial peer 0.

Note - Default dial peers are used for inbound matches only. They are not used to match outbound calls that do not have a dial peer configured.

Dial peer 0 for inbound VoIP peers has the following characteristics:

  • Any codec

  • IP precedence 0

  • VAD enabled

  • No RSVP support

  • fax-rate service

For inbound POTS peers, dial peer 0 is configured with the no ivr application command.

You cannot change the default configuration for dial peer 0. Default dial peer 0 fails to negotiate nondefault capabilities or services. When the default dial peer is matched on a VoIP call, the call leg that is set up in the inbound direction uses any supported codec for voice compression that is based on the requested codec capability coming from the source router. When a default dial peer is matched, the voice path in one direction might have different parameters from the voice path in the return direction. This might cause one side of the connection to report good quality voice while the other side reports poor quality voice. For example, the outbound dial peer has VAD disabled, but the inbound call leg is matched against the default dial peer, which has VAD enabled. VAD would be on in one direction and off in the return direction.

When the default dial peer is matched on an inbound POTS call leg, there is no default IVR application with the port. As a result, the user gets a dial tone and proceeds with dialed digits. Interestingly, the default dial peer cannot be viewed using show commands.

In Figure 3-30, only one-way dialing is configured. Example 3-11 and Example 3-12 illustrate the configuration for this topology. The caller at extension 7777 can call extension 8888 because a VoIP dial peer is configured on Router 1 to route the call across the network. However, no VoIP dial peer is configured on Router 2 to point calls across the network toward Router 1. Therefore, no dial peer exists on Router 2 that will match the calling number of extension 7777 on the inbound call leg. If no incoming dial peer matches the calling number, the inbound call leg automatically matches to a default dial peer (POTS or VoIP).

Figure 3-30

Default Dial Peer 0

Example 3-11  Router 1 Configuration

Router1(config)#dial-peer voice 1 potsRouter1(config-dial-peer)#destination-pattern 7777Router1(config-dial-peer)#port 1/0/0Router1(config-dial-peer)#exitRouter1(config)#dial-peer voice 2 voipRouter1(config-dial-peer)#destination-pattern 8888Router1(config-dial-peer)#session target ipv4:

Example 3-12  Router 2 Configuration

Router2(config)#dial-peer voice 3 potsRouter2(config-dial-peer)#destination-pattern 8888Router2(config-dial-peer)#port 1/1/0

Matching Outbound Dial Peers

Outbound dial-peer matching is completed on a digit-by-digit basis. Therefore, the router or gateway checks for dial-peer matches after receiving each digit and then routes the call when a full match is made.

The router or gateway matches outbound dial peers in the following order:

Step 1. The router or gateway uses the dial peer destination-pattern command to determine how to route the call.

Step 2. The destination-pattern command routes the call in the following manner:

  • On POTS dial peers, the port command forwards the call.

  • On VoIP dial peers, the session target command forwards the call.

Step 3. Use the show dialplan number string command to determine which dial peer is matched to a specific dialed string. This command displays all matching dial peers in the order that they are used.

In Example 3-13, dial peer 1 matches any digit string that does not match the other dial peers more specifically. Dial peer 2 matches any seven-digit number in the 30 and 40 range of numbers starting with 55501. Dial peer 3 matches any seven-digit number in the 20 range of numbers starting with 55501. Dial peer 4 matches the specific number 5550124 only. When the number 5550124 is dialed, dial peers 1, 3, and 4 all match that number, but dial peer 4 places that call because it contains the most specific destination pattern.

Example 3-13  Matching Outbound Dial Peers

Router(config)#dial-peer voice 1 voipRouter(config-dial-peer)#destination-pattern .TRouter(config-dial-peer)#session target ipv4: voice 2 voipRouter(config-dial-peer)#destination-pattern 55501[3-4].Router(config-dial-peer)#session target ipv4: voice 3 voipRouter(config-dial-peer)#destination-pattern 555012.Router(config-dial-peer)#session target ipv4: voice 4 voipRouter(config-dial-peer)#destination-pattern 5550124Router(config-dial-peer)#session target ipv4:


The main topics covered in this chapter are the following:

  • A VoIP network has seven typical call types.

  • A local call is handled entirely by the router and does not travel over an external network.

  • On-net calls can be routed through one or more voice-enabled routers, but the calls remain on the same network.

  • An off-net call occurs when a user dials an access code (such as 9) from a telephone directly connected to a voice-enabled router or PBX to gain access to the PSTN.

  • Voice port call types include local, on-net, off-net, PLAR, PBX to PBX, intercluster trunk, and on-net to off-net calls.

  • Voice ports on routers and access servers emulate physical telephony switch connections.

  • Analog voice port interfaces connect routers in packet-based networks to analog two-wire or four-wire analog circuits in telephony networks.

  • FXS, FXO, and E&M ports have several configuration parameters.

  • CAMA is used for 911 and E911 services.

  • DID service enables callers to dial an extension directly on a PBX or packet voice system.

  • You can set a number of timers and timing parameters for fine-tuning a voice port.

  • The show, debug, and test commands are used for monitoring and troubleshooting voice functions in the network.

  • Dial peers are used to identify call source and destination endpoints and to define the characteristics applied to each call leg in the call connection.

  • An end-to-end voice call consists of four call legs.

  • A dial peer is an addressable call endpoint.

  • POTS dial peers retain the characteristics of a traditional telephony network connection.

  • When a matching inbound dial peer is not found, the router resorts to the default dial peer.

  • The destination pattern associates a telephone number with a given dial peer.

  • When determining how inbound dial peers are matched on a router, it is important to note whether the inbound call leg is matched to a POTS or VoIP dial peer.

  • Outbound dial-peer matching is completed on a digit-by-digit basis.

Chapter Review Questions

The answers to these review questions are in the appendix.

  1. If a client picked up a customer service handset and was automatically connected to a customer service representative without dialing any digits, what kind of call would it be?
    1. Intercluster trunk call
    2. PBX-to-PBX call
    3. On-net call
    4. PLAR call
  2. Which configuration parameter would you change to set the dial tone, busy tone, and ringback tone on an FXS port?
    1. Cptone
    2. Ring frequency
    3. Ring cadence
    4. Description
    5. Signal
    6. PSQM
  3. What is the default (and most commonly used) method of access signaling used on E&M voice ports?
    1. Immediate-start
    2. Wink-start
    3. Delay-start
    4. Loop-start
  4. Which situation most likely requires changes to the FXS port default settings?
    1. The caller and the called party are in different parts of the country.
    2. The caller and the called party are in different countries.
    3. The connection is a trunk to a PBX.
    4. The FXS port configuration does not match the local PSTN switch configuration.
  5. Which two conditions can be checked by using the show voice port port command for an FXS port? (Choose 2.)
    1. Whether the port is using ground-start or loop-start signaling
    2. The ring frequency configured for the port
    3. The E&M signaling type configured for the port
    4. The number of rings after which the port will answer
  6. When an end-to-end call is established across a VoIP network, how many inbound call legs are associated with the call?
    1. One
    2. Two
    3. Three
    4. Four
  7. A POTS dial peer performs which of the following two functions? (Choose 2.)
    1. Provides a phone number for the edge network or device
    2. Provides a destination address for the edge device located across the network
    3. Routes a call across a network
    4. Identifies the specific voice port that connects the edge network or device
  8. When configuring a VoIP dial peer, which command is used to specify the address of the terminating router or gateway?
    1. destination-port
    2. destination-pattern
    3. session target
    4. destination address
    5. dial-peer terminal
  9. What happens if there is no matching dial peer for an outbound call?
    1. The default dial peer is used.
    2. Dial peer 0 is used.
    3. The POTS dial peer is used.
    4. The call is dropped.
  10. Which dial-peer configuration command attempts to match the calling number (that is, the ANI string)?
    1. destination-pattern
    2. port
    3. answer-address
    4. incoming called-number

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