6.4 Scaling Dial-on-Demand Routing
6.4.7 Configuring central site access router interfaces
This section describes how the configuration of the central-site access routers implements authentication, network layer addressing, and the routing strategy. The configuration for each central-site access router is the same with the following exceptions:
  • The IP address specified for loopback interface 0
  • The IP address specified for Ethernet interface 0
  • The name of the router as specified by the hostname global configuration command

Username Configuration for the Remote Sites
The configuration of each central-site access router includes the username global configuration commands . Each remote router can dial in to any of the three central-site access routers, so there is a username global configuration command for each remote router. When a remote router logs in, it specifies a name (for example, Router2) and a password (for example, outthere) that must match the values specified by a username command. Each remote site uses a chat script to log in and specify its host name (which must match a value specified by the username command) and password.

Dial-Up Configuration for the Remote Sites
The configuration of each central-site access router includes the chat-script global configuration commands . The three chat-script global configuration commands establish three scripts named CALL1020, REM, and usrv32bis. CALL1020 and REM are invoked by the dialer map commands to dial and log in to the remote sites, respectively. The script reset command specifies that the USRV32BIS script is to be run whenever an asynchronous line is reset in order to ensure that the central-site modems are always configured correctly.

Loopback Interface Configuration
The configuration of each central-site access router includes the commands for configuring loopback interfaces. The IP address for loopback interface 0 is unique for each access router and, to satisfy the rules by which OSPF selects the router ID, must be the highest loopback IP address on the router. The IP address for loopback interface 1 is the same for each central-site access router.

The goal is for all three access routers to appear to have the same IP address during IPCP negotiation with the remote sites. (IPCP is the part of PPP that brings up and configures IP support.) This goal is accomplished by creating a loopback interface, assigning to it the same IP address on each central-site access router, and running the ip unnumbered interface configuration command using the loopback interface address. The problem with this strategy is that OSPF takes its router ID from the IP address of a loopback interface, if one is configured, which would mean that all three access routers would have the same OSPF router ID.

The solution is to create loopback interface 0 and assign to it a unique IP address (which results in a unique OSPF router ID for each router). The configuration then creates loopback interface 1 and assigns to it the same IP address on each router. Loopback interface 1 allows the ip unnumbered command to be applied to dialer rotary group 20 later in the configuration.

Asynchronous Line Configuration
For each of the 16 asynchronous interfaces provided by the access router, the configuration uses the
ip unnumbered interface configuration command to specify that the asynchronous interface is to use the IP address of loopback interface 1 as the source address for any IP packets that the asynchronous interface generates. The IP address of loopback interface 1 is also used to determine which routing processes are sending updates over the asynchronous interface.

The async dynamic address interface configuration command enables dynamic addressing on the asynchronous interface. This command is required to allow each remote router to specify its IP address when it logs in. The async dynamic routing interface configuration command allows the interface to run a routing protocol, in this case RIP.

The async mode interactive interface configuration command allows a remote router to dial in and access the EXEC command interface, which allows the remote router to start PPP and specify its IP address.

The dialer in-band interface configuration command allows chat scripts to be used on the asynchronous interface. The chat scripts allow the access router to dial the remote sites. The dialer rotary-group interface configuration command assigns each asynchronous interface to dialer rotary group 20.

Dialer Interface Configuration
The configuration of each central-site access router includes the commands for configuring dialer rotary group 20. The
interface dialer global configuration command defines dialer rotary group 20. Any interface configuration commands that are applied to a dialer rotary group apply to the physical interfaces that are its members. When the router's configuration includes multiple destinations, any of the interfaces in the dialer rotary group can be used to place outgoing calls.

The ip unnumbered interface configuration command specifies that the IP address of loopback interface 1 is to be used as the source address for any IP packets that dialer rotary group 20 might generate. The dialer idle-timeout interface configuration command will cause a disconnection if 60 seconds elapses without any interesting traffic.

The configuration includes a dialer map interface configuration command for each remote router that the central-site access router might dial. The IP keyword specifies that the dialer map is to be used for IP packets, the IP address is the next-hop address of the destination that is to be called, and the name keyword specifies the host name of the remote router that is to be called. The modem-script keyword specifies that the CALL1020 chat script is to be used, and the system-script keyword specifies that the REM chat script is to be used. The last value specified by the dialer map command is the telephone number for the remote router. The dialer map commands do not specify the broadcast keyword, so RIP updates are not sent to the remote sites.

For the Dialer20 interface, the dialer-group interface configuration command defines interesting packets to be those packets defined by the corresponding dial-list command. Interesting packets cause a call to be made or cause a call to be maintained. In this case, access list 101 defines RIP as uninteresting. (RIP uses User Datagram Protocol [UDP] port 520.) All other packets are defined as interesting.