Hunt Groups

One of the most straightforward ways to control call route selection on a gateway is to create hunt groups. A hunt group is a set of dial peers, all referencing the same destination pattern. You can use hunt groups to provide load balancing over multiple ports or to provide redundancy if a primary port is unusable.

You can use hunt groups with both POTS and VOIP interfaces. This section shows how to configure hunt groups for those gateway protocols that use dial peers: H.323 and SIP. You must configure hunt groups on the call agent when using MGCP gateways.

You can mix POTS and VoIP dial peers in the same hunt group. By default, a router selects which dial peer in a hunt group to use by the following criteria:

1.	Longest match This is the destination pattern that matches the most number of digits.
2.	Preference value Dial peers are chosen in order of preference.
3.	Random selection If all dial peers have the same preference, they are used in random order.

The dial-peer hunt value command gives you a fourth optionleast recently used dial peerand a way to alter the default order of the choice selection criteria. Example 11-1 shows the options that are available with the dial-peer hunt command.

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Shanghai-GW(config)#dial-peer hunt ? <0-7> Dial-peer hunting choices, listed in hunting order within each choice: 0 - Longest match in phone number, explicit preference, random selection. 1 - Longest match in phone number, explicit preference, least recent use. 2 - Explicit preference, longest match in phone number, random selection. 3 - Explicit preference, longest match in phone number, least recent use. 4 - Least recent use, longest match in phone number, explicit preference. 5 - Least recent use, explicit preference, longest match in phone number. 6 - Random selection. 7 - Least recent use.  

 

Using the preference Command

You create a hunt group by using the preference command under the dial peer configuration mode, as shown in Example 11-2. Preference values can range from 0 to 10, with the lowest number being the most preferred. Thus, a dial peer with a preference value of 1 would be used before a dial peer with a preference value of 2, all other things being equal. The default preference for dial peers is 0, which is the most preferred, or highest priority, value. If the most preferred dial peer is not available, the router tries the dial peer with the next higher preference value, and so on.

If the same preference value and destination pattern are configured on a POTS and a VoIP dial peer, the POTS dial peer is used.

!The primary dial peer uses the default preference value of 0
dial-peer voice 3400 voip
 destination-pattern 34..
 session target ipv4:10.6.2.10
!
!The dial peer to use if the primary is unavailable
dial-peer voice 3401 pots
 destination-pattern 34..
 preference 1
 port 1/0:23

 

 

Using the huntstop Command

Dial peer hunting is enabled by default. If a call fails on one dial peer in a hunt group, the gateway continues hunting. The dial peer huntstop command stops the gateway from searching for other matches if the call fails on that dial peer. One use for this is a gateway with dial peers using an explicit destination pattern (such as 221.) pointing to one set of ports, and a dial peer using a less explicit destination pattern (such as 2...) pointing to another port. If you do not want the router to use the less explicit dial peer, add the huntstop command under the dial peer where you want the router to stop hunting, as shown in Example 11-3. Note the huntstop command under dial peer 2212. To re-enable hunting, use no huntstop.

!The preferred dial peer for calls to 221x
dial-peer voice 2211 pots
 preference 1 
 destination-pattern 221.
 forward-digits all
 port 0/0/0
!
!The secondary dial peer for calls to 221x
dial-peer voice 2212 pots
preference 2 
 huntstop 
destination-pattern 221.
 forward-digits all
 port 0/0/1
!
!A less explicit dial peer that should not be used for calls to 221x
dial-peer voice 2200 pots
 destination-pattern 2...
 forward-digits all
 preference 3
 port 1/0:23

 

 

Using Digit Manipulation

When you are using alternate dial peers, you might have to adjust the call information that is sent. Various digit manipulation techniques are available to do this. See Chapter 10, "Digit Manipulation," for detailed information on digit manipulation techniques and commands.

Figure 11-1 illustrates some typical situations in which you might need digit manipulation. The figure shows a simple network with two sites connected over an IP network. If the IP WAN is unavailable, each gateway is configured to reroute calls over the public switched telephone network (PSTN). The remote site on each gateway has a primary VoIP and a secondary POTS dial peer.

Figure 11-1. Using Digit Manipulation with Hunt Groups

Potential problems with this voice network include the following issues. Example 11-4 shows a configuration that uses digit manipulation to solve these problems.

• A VoIP dial peer transmits all digits by default, but a POTS dial peer truncates digits that match the destination pattern. If a POTS dial peer serves as backup to a VoIP dial peer, you need to specify the number of digits to send. In networks in which the entire phone number is dialed, this can be as simple as coding forward-digits all under the POTS dial peer. In Figure 11-1, a site code of 3 is used for Boise. This should not be sent to the terminating gateway. In Example 11-4, the VoIP dial peer 1112 of the Miami router has a translation profile that allows just the last four digits of the dialed number to be transmitted.
• VoIP dial peers usually connect internal networks, so users usually do not dial the entire destination telephone number. If the alternate dial peer routes the call over the PSTN, you need to add information that the PSTN needs to route the call, such as long-distance code, area code, and prefix. In Figure 11-1, the POTS dial peer strips all but the last two digits, so a prefix command is coded on the Miami router under POTS dial peer 202 to add the long-distance access number (1), the Boise area code (208), and the next five digits of the phone number (55501). The site code of 3 is not sent. Similarly, on the POTS dial peer 202 of the Boise router, the long-distance access code (1), the Miami area code (301), and the next five digits of the phone number (55501) are prefixed. The site code of 2 is stripped.
• You might need to manipulate the called number on the terminating gateway. The Boise gateway expects to route calls to its LAN phones based on a four-digit DNIS. If the PSTN sends more digits, you need to strip some digits from the incoming called number. You might also want to manipulate the calling number to provide the caller ID that users expect to see. In Figure 11-1, the PSTN sends a ten-digit dialed number identification service (DNIS) to Boise. Example 11-4 shows that the Boise gateway translates that to just the last four digits under POTS dial peer 202.

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!Miami Router Configuration !Creates a translation rule to remove the site code voice translation-rule 3 rule 1 /^301/ /01/ ! !Creates the translation profile associated with Rule 3 voice translation-profile BOISE-VOIP translate called 3 ! !Primary dial peer, pointing to Boise router, site code is removed, four digits sent dial-peer voice 1112 voip translation-profile outgoing BOISE-VOIP preference 1 destination-pattern 301.. session target ipv4:10.20.25.1 ! !Backup dial peer, sends calls to PSTN and adds necessary digits dial-peer voice 202 pots preference 2 destination-pattern 301,, direct-inward-dial prefix 120855501 port 1/0:23  

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!Boise Router Configuration ! !Creates a translation rule to remove the site code voice translation-rule 2 rule 1 /^201/ /01/ ! !Creates a translation rule to remove the extra digits sent by the PSTN voice translation-rule 22 rule 1 /^208555/ // ! !Creates a translation profile associated with Rule 2 voice translation-profile MIAMI-VOIP translate called 2 ! ! !Creates a translation profile associated with Rule 22 voice translation-profile MIAMI-POTS translate called 22 ! !Primary dial peer, pointing to Miami router, site code is removed, four digits sent dial-peer voice 1112 voip translation-profile outgoing MIAMI-VOIP preference 1 destination-pattern 201.. session target ipv4:10.10.25.1 ! !Backup dial peer, sends call to PSTN, adds necessary digits to outbound calls dial-peer voice 202 pots !Translation profile removes all but four digits on incoming calls from Miami translation-profile incoming MIAMI-POTS preference 2 destination-pattern 201,, direct-inward-dial prefix 130155501 port 1/0:23