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152 Chapter 6: Using ISDN and DDR Technologies
Note that the example does not reference BRI 0 as the outbound interface. Although it is a valid
command configuration to place BRI 0 at the end of the default route command (that is, the
router allows it), this configuration will not function because the routing table entry is the origin
of the next hop address information that triggers the call to the other side of the network. In
addition, the dialer map association ties a phone number to that next hop address. If there's no
routing table entry, the device has no way of knowing the next hop address. Therefore, it does
not know which dialer map to utilize for the call and the call fails.
Rate Adaptation
Earlier in the chapter, a solution was discussed that involved the use of non-native ISDN routers
being placed into service in an ISDN network. A short discussion described how this is possible
using TA.
In this type of implementation, the dialer in-band command is a necessary part of the
configuration which effectively takes 8 kbps from each B channel for use by the signaling
entity. In other words, the 16 kbps that would normally be out-of-band in the D channel now
has to be taken from the B channels. Effectively, the throughput is now 56 kbps for each B
channel.
Should a native solution dial into a non-native solution with out-of-band signaling, the native
solution would need to step down its speed to 56 kbps. This is done with rate adaptation. The
implementation of rate adaptation is simply an extension of the dialer map command. The
dialer-map command tells the router that to reach a specific next hop address, a specific phone
number must be called. The dialer-map command simply associates the destination protocol
address with the appropriate phone number to dial to get there. Example 6-15 illustrates the
configuration of the router dialing into the non-native 56-kbps installation.
Example 6-15
Rate Adaptation
RouterA(config)#interface BRI0
RouterA(config-if)#ip address 10.12.1.1 255.255.255.0
RouterA(config-if)#encapsulation ppp
RouterA(config-if)#dialer idle-timeout 180
RouterA(config-if)#dialer map ip 10.12.1.2 speed 56 2145552222
RouterA(config-if)#dialer map ip 10.12.1.2 speed 56 2145552223
RouterA(config-if)#dialer-group 1
!
RouterA(config)#dialer-list 1 protocol ip permit
!
RouterA(config)#ip route 10.13.1.0 255.255.255.0 10.12.1.2