Configuration Example and Explanation

This next section illustrates how to configure a router for DDR. We've broken each applicable section of a router configuration into manageable chunks with explanations. We start with Listing 8.2 with static routes and authentication.

Listing 8.2 Sample configuration

 dialer-list 1 protocol ip permit ip route 20.1.1.0 255.255.255.0 10.1.2.1 ip route 20 1.2.0 255.255.255.0 10.1.3.1 ip route 21.1.2.0 255.255.255.0 11.1.2.1 ip route 21.1.3.0 255.255.255.0 11.1.3.1 ip route 22.1.2.0 255.255.255.0 12.1.2.1 ip route 22.1.3.0 255.255.255.0 12.1.3.1 username router1 password cisco username router2 password cisco username router3 password cisco username router4 password cisco username router5 password cisco username router6 password cisco 

The dialer-list command tells the router what traffic is considered interesting. In this case, all IP traffic is interesting and will bring up a link pointing to dialer list 1. You can also specify an access list to get more granular.

The ip route statements tell the router that to get to the specified network, it needs to send packets to the specified IP address. This address is a remote address.

The username and password statements indicate that when the router calls a remote device with a hostname equal to the username, it needs to use the associated password.

The following code adds dynamic routing to the router and includes the static routes in our routing updates through the redistribute static command:

 router rip        network 10.0.0.0        network 11.0.0.0        network 12.0.0.0        redistribute static 

This setup allows other RIP devices to know where to send packets to us, if they need to get packets to the remote networks.

Next, we configure the physical interfaces:

 interface bri 0         encapsulation ppp         dialer-pool member 1 priority 200         ppp multilink 

You must configure encapsulation, pool membership, and multilink here, whereas you configure everything else on the dialer interfaces. The dialer-pool statement tells the router that this interface is a member of dialer pool 1, and because it has a higher priority than the other interface in pool 1, this interface is used first for dial out.

The next code performs the same task as earlier on this interface, but this interface is a member of two pools:

 interface bri 1         encapsulation ppp         dialer-pool member 1 priority 150         dialer-pool member 2 priority 200         ppp multilink 

It has a high priority on one pool and a medium on the other. The way we are configuring the pools means that this interface is the primary for pool 2 and the secondary for pool 1.

We need to configure a couple more interfaces and set them into the appropriate pools. Then, the dialer interfaces must be able to use the pools to decide which physical interface to make the call with.

Next, the dialer interfaces get most of the configuration information. We need to configure IP addressing on this interface, and we can either configure the interface so that the network overlaps with both destinations or configure two IP addresses and specify one as a secondary address:

 interface bri 2         encapsulation ppp         dialer-pool member 2 priority 200         dialer-pool member 3 priority 150         ppp multilink interface bri 3         encapsulation ppp         dialer-pool member 3 priority 200         ppp multilink interface dialer 1         ip address 10.1.1.1 255.255.0.0         ppp authentication chap         dialer map ip 10.1.2.1 name router1 dial-string 5551212         dialer map ip 10.1.3.1 name router2 dial-string 5551313         dialer-group 1         dialer pool 1 

We set up Challenge Handshake Authentication Protocol (CHAP) authentication, specified that this interface will use physical interfaces that are members of dialer pool 1 to make our calls, and specified dialer group 1 to determine interesting traffic. (See dialer list 1 earlier.)

The dialer map statements allow us to call multiple sites from one interface by specifying a remote IP address; the remote router name, which is used for authentication; and the phone number we need to call to get there.

The following sample configuration shows how multiple dialer interfaces use dialer pools. When a dialer interface needs to forward a packet, it checks what interfaces are available by referring to the dialer pool that has been configured. Dialer interface 2 will only use physical interfaces that are members of dialer pool 2, whereas dialer interface 3 will only use physical interfaces that are members of pool 3:

A single interface may be a member of more than one pool.

 interface dialer 2         ip address 11.1.1.1 255.255.0.0         ppp authentication chap         dialer map ip 11.1.2.1 name router3 dial-string 5551414         dialer map ip 11.1.3.1 name router4 dial-string 5551515         dialer-group 1         dialer pool 2 interface dialer 3         ip address 12.1.1.1 255.255.0.0         ppp authentication chap         dialer map ip 12.1.2.1 name router5 dial-string 5551616         dialer map ip 12.1.3.1 name router6 dial-string 5551717         dialer-group 1         dialer pool 3