Optimal modem dial backup configuration is a complex undertaking, dependent on a large number of factors. For an extensive discussion of network design, capacity planning, routing configuration, device configuration, and security considerations, see Application Note - VoIP Network Resiliency. Device and network configuration features that need to be taken into account include:

• The backup interface command allows you to designate the Dialer interface as the backup to an existing WAN interface on the Branch Gateway. When the Branch Gateway reports the primary WAN interface down for a specified period of time, the Dialer interface is automatically activated and the modem dials.

• A Branch Gateway USB port can be used to support a USB modem for dial backup. Thus, the Dialer can use the same USB modem that is used for remote access to the device. Asynchronous dialing and modem recognition options must be set on the USB port to support creation of the Dialer interface.

• The Dialer interface supports PAP and CHAP authentication for PPP connections. In addition, the Dialer interface can be configured to be a member of a VPN, allowing encryption of the modem traffic. Van Jacobsen compression is available for encrypted traffic over the Dialer interface, allowing optimal use of bandwidth.

• It is recommended to filter traffic through the Dialer interface to permit only those packets necessary for continued interaction with the CM server. Filtering can be accomplished using access control lists, which specify traffic permissible through a selected interface.

• Dynamic CAC can be used in conjunction with IGAR to provide a stable backup path for continued IP phone function in the event of a dial backup scenario. Dynamic CAC notifies the CM server that no bandwidth is available for bearer traffic, keeping the dial circuit from becoming fully congested. IGAR provides a path for gateway-to-gateway traffic destined for a remote CM server by forcing voice calls to and from the branch office to use the PSTN for bearer traffic. For more information on configuring IGAR, see Administrator Guide for Avaya Aura® Communication Manager .

• Static IP addressing for the Dialer interface may not be feasible. Dynamic IP addressing is available to enable you to connect to the remote network through an ISP. ISPs commonly provide IP addressing for connected ports on an as-needed basis. IP unnumbered links are available to supply addressing in situations where you wish to run routing over your network link without committing a subnet.

• Object tracking can be used with the Loopback interface to provide an alternative method for activating the Dialer interface when connectivity with the main office is lost. This is useful in configurations where the WAN interface is not connected directly to the Branch Gateway. Use object tracking to configure RTRs to verify connectivity with the main office. If the RTR fails, the object tracker can be configured to change the status of the Loopback interface to down. If the Dialer interface is configured as the backup for the Loopback interface, the Dialer interface will automatically dial when connectivity fails.