To get your feet wet, let’s start with a simple BRI configuration:
hostname R1
isdn switch-type basic-ni
!
interface BRI0
ip address 10.1.1.3 255.255.255.0
encapsulation ppp
isdn spid1 91955512120100 5551212
dialer map ip 10.1.1.1 name R2 555-1212
As you can see, the first statement defines the switch type. The BRI0 interface binds an IP address
and sets up PPP as the encapsulation type. The last two lines identify the SPID and the dialer map
command. These set the protocol with a next-hop address of 10./1.1.1, identify the remote host as
R2, and indicate that the dial string (telephone number) should be sent to the dialing device when
it the device recognizes packets that have specified addresses matching the configured access lists.
Under the old legacy way, you could have a main IP address under the physical interface,
along with several secondary addresses. This worked fine, but you ran into problems if you were
using a routing protocol, because the physical interface always uses its primary address when
sending out packets.
Here’s an example of a configuration using the old way. Notice the dialer map statements.
This enabled an administrator to tell the router which number to dial based on the destination
IP address in packets it received on one of the router’s incoming interfaces. See for yourself:
hostname R1
!
interface Serial 0/0:23
encapsulation ppp
ip address 192.168.250.1 255.255.255.0
ip address 192.168.251.1 255.255.255.0 secondary
dialer map ip 192.168.250.2 name R2 555-1212
dialer map ip 192.168.251.2 name R3 555-1234
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
!
end
hostname R2
!
interface BRI0
ip address 192.168.250.2 255.255.255.0
encapsulation ppp
isdn spid1 91955512120100 5551212
isdn spid2 91955512130100 5551213
dialer map ip 192.168.250.1 name R1 5551900
router ospf 100
network 192.168.250.2 0.0.0.0 area 0
!
end
hostname R3
!
interface BRI0
ip address 192.168.251.2 255.255.255.0
encapsulation ppp
isdn spid1 91955512230100 5551234
isdn spid2 91955512350100 5551235
dialer map ip 192.168.251.1 name R1 5551900
router ospf 100
network 192.168.251.2 0.0.0.0 area 0
!
end
You need to look at several points in this configuration. Host R1 is using a PRI ISDN interface
on Serial 0/0. The secondary command enables you to have a second route on the same
interface. We cover the Serial 0/0:23 command later in this chapter, but a quick explanation
is that for ISDN, the D channel time slot is equivalent to the :23 channel for channelized T-1.
And finally, we are using the dialer map to bind an IP address to an ISDN DN.
In these router configurations, both routers R2 and R3 will call into R1, but Open Shortest
Path First (OSPF) will work only between R1 and R2, because R2 uses the primary address on
R1, whereas R3 uses the secondary. This source IP address issue can be a real problem, but only
if you’re not aware of it. What is the solution to the primary IP address issue? Dialer interfaces.