Assign an IP address to the Loopback interface for use with modem dial backup using the interface loopback command.
This step allows the Dialer interface to be configured as an IP unnumbered link and still participate in OSPF routing.
Create an access control list with the ip access-control-list command.
The access control list determines which traffic is permitted to use the interface. In this example, access control list 305 is configured to block all traffic other than VoIP signalling traffic. The primary purpose of the access control list is to block bearer traffic from using the Dialer interface. The Dialer interface generally has insufficient bandwidth to support bearer traffic.
Create the Dialer interface using the interface dialer command.
The Dialer interface is created and is available as a backup link for a WAN interface. Only one Dialer interface can be created on the Branch Gateway.
Assign a PPP authentication method with the ppp chap hostname command.
The Dialer interface authenticates its PPP sessions to the remote RAS server using CHAP authentication and a username of area5. The username area5 must be configured on the RAS as a legitimate user.
Assign an initial delay for dialing with the dialer persistent initial delay command.
The initial delay prevents the Dialer from dialing out unnecessarily on reboot. The primary WAN interface often requires a few moments to register itself as up, and during that period, the initial delay prevents the device from activating the Dialer.
Assign a reset delay for the dialer string list using the dialer persistent delay command.
The reset delay determines the amount of time between cycles of call attempts, once all dialer strings have been attempted.
Enter up to five dialer strings using the dialer string command.
When the Dialer interface is activated, the Dialer first attempts to connect to the number associated with dialer string 1. If the connection attempt fails, the Dialer attempts to connect to the number associated with the next dialer string. These strings represent hunt group phone numbers configured on the RAS server in the headquarters data center.
Associate the Dialer interface with its physical port with the dialer modem-interface command.
The Dialer interface must be configured to use a physical interface on the device to which the modem is connected. Modem dial backup is supported on the USB port.
Configure the modem to participate in network routing with the ip unnumbered command.
An unnumbered interface uses the IP address of the interface configured in the command. In this example, the Loopback interface has been created for the Dialer interface to use its IP information. This IP information allows the unnumbered interface to forward and receive IP traffic without actually assigning a static IP address to the Dialer interface.
Assign an access control list to the Dialer interface using the ip access-group command.
All traffic passing through the Dialer interface must meet the conditions of the access control list associated with this access group or be rejected. In this example, the access-group references access control list 305, which is created to block all outgoing traffic across the Dialer interface other than the VoIP signalling traffic between the branch office gateway and the MGC in the headquarters data center.
Configure the USB port to support the modem with the interface usb-modem command.
Assign the Dialer interface to the interface you want to back up with the backup interface dialer command.
For example, interface Dialer 1 is selected as the backup interface to interface FastEthernet 10/2, the primary WAN connection to the headquarters network. The Dialer activates in the event of a failure of the FastEthernet port and all permitted traffic transverses the Dialer interface.
Configure the Loopback interface to participate in the OSPF network using the router ospf command.
For example, a group of branch offices are assigned to OSPF area 5. This configuration allows filtering to take place at the border points and minimizes topology updates on the headquarters data center routers.
