Using a dialer interface solves the primary IP address/secondary IP address problem because
each interface can be assigned its own primary address. The dialer map command does not
have to be used because each interface has its own IP address and dial number configured by
using the dialer string command.
A virtual interface must be associated with a dialer pool. The dialer pool is a group of one
or more physical interfaces in charge of placing calls. Here’s an example of a configuration
using dialer interfaces:
hostname R1
!
isdn switch-type basic-5ess
!
interface Ethernet0
ip address 192.168.1.1 255.255.255.0
!
interface Serial0/0:23
no ip address
encapsulation ppp
dialer pool-member 1 priority 100
!
interface Dialer1
ip address 192.168.250.1 255.255.255.0
encapsulation ppp
dialer remote-name R2
dialer idle-timeout 300
dialer string 5551212
dialer load-threshold 50 either
dialer pool 1
dialer-group 1
!
interface Dialer2
ip address 192.168.251.1 255.255.255.0
encapsulation ppp
dialer remote-name R3
dialer string 5551234
dialer load-threshold 150 either
dialer pool 1
dialer-group 1
!
router ospf 100
network 192.168.250.1 0.0.0.0 area 0
network 192.168.251.1 0.0.0.0 area 0
!
dialer-list 1 protocol ip permit
!
end
Notice how the interface Dialer1 creates a virtual interface with the correct configuration—
ip address, encapsulation, and dialer string—the same items that were bound to the
physical interface BRI0 in the first example. The interface Dialer carries a 1 as its index. The virtual
Dialer interface is then bound to a dialer string that references the ISDN DN. The dialer
pool 1 is then bound to the virtual Dialer to point to the physical interface that will be carrying
out the dialing. The index of the dialer pool 1 maps to the dialer pool-member 1—the physical
interface of Serial 0/0. The last configuration command worthy of noting is the dialerlist
1 protocol ip permit, which tells the router what traffic is interesting and to bring the
connection up when interesting traffic is identified.
OSPF will work properly because the source address on both sides of the link matches the network
statement. The source address of a packet originating at a router is the primary address on
the outgoing interface. Dialer interfaces are easy to configure. Consider the following example:
router#config t
router(config)#interface dialer 2
router(config-if)#
Now the network administrator can create the configuration as you would under a physical
interface. The physical PRI interface—Serial0/0:23—is designated as a member of a dialer pool
using the dialer pool-member command.