dial-on demand routing (ddr)是用公共电话网提供了网络连接。通常的，广域网大多数使用专线连接的，路由器连接到类似MODEM OR ISDNTAS 的数据终端DCE设备上，他们支持同步V.25BITS协议，你可以用SCRIPTS AND DIALER命令设定拨号串。
DDR比较适用于用户对数率要求不高，偶尔有数据传输或只是在特定时候传输数据，比如银行每晚传送报表等等情况下。
当一个感兴趣的包到达路由器时，产生一个DDR请求，路由器发送呼叫建立信息给指定的串口的DCE设备，这个呼叫就把本地的和远程的设备连接起来，一旦没有数据传输，空闲时间开始记时，超过设置的记时时间，连接终止。DDR现在都用静态路由来传输数据，避免路由交换引起的DDR拨号。和XNS可以通过DDR路由寻址，同步串口，异步串口和ISDN端口可以配置成到一个或多个目的地的DDR连接。
下面是一个典型的DDR连接：
在配置DDR过程中，我们可以把一个或几个物理端口配置成一个逻辑拨号接口，他可以是同步V.25方式，同步DYR启动拨号或异步CHATSCRIPT方式。
具体配置：
！
interface serial 0
ip address 131.108.126.1 255.255.255.0
dialer in-band
dialer-group 1
!
dialer map ip 131.108.126.2 55551234
!
dialer idle-timeout 300
dialer backup example
A) 同步V.25bits方式
configuration for routerA:
interface serial 10:0
backup delay 0 10
backup interface serial 110
ip address 16.217.30.2 255.255.255.252
!
interface serial 110
ip address 16.30.16.81 255.255.255.25.0
encapsulation ppp
dialer in-band
dialer string 8292
dialer-group 1
pulse-time 1
!
dilar-list 1 protocol ip permit
B) 辅助口作拨号备份
configuration for routerA:
chat-script mydial “””atdt 8292” timeout 60 “connect”
!
interface serial 0
backup delay 0 0
backup interface async1
ip address 16.3.1.1 255.255.255.0
encapsulation ppp
!
interface async1
ip address 16.3.2.1 255.255.255.0
encapsulation ppp
keepalive 9
async default routing
async dynamic address
async dynamic routing
async mode dedicated
dialer string 8292
dialer-group 1
!
dialer-list 1 protocol ip permit
!
line aux 0
script dialer mydial
modeinout
transport output none
stopbits 1
flowcontrol hardware
speed 9600
subinterface example (frame realay)
configuration for routerA:
interface serial 0
encapsulation frame-relay
interface serial 0.1 multipoint
ip address 11.10.11.1 255.255.255.0
frame-relay interface-dlci 41
frame-relay interface-dlci42
configuration for routerC:
interface serial 0
encapsulation frame-relay
interface serial 0.1 point-point
ip address 11.10.16.2 255.255.255.0
frame-realy interface-dlci 46
configuration for routerB:
interface serial 0
encapsulation frame-relay
interface serial 0.1 multipoint
ip address 11.10.11.3 255.255.255.0
frame-relay interface-dlci 43
frame-relay interface-dlci 44
!
interface serial 0.2 point-point
ip address 11.10.13.1 255.255.255.0
frame-realy interface-dlci 48
frame relay switching example
configuration for routerA:
frame-relay switching
no ip address
frame-relay encapsulation
frame-relay route 163 tun0 43
frame-relay intf-type dce
!
interface serial0
ip address 131.108.100.1 255.255.255.0
!
interface tu 0
tunnel source serial 1
tunnel destintion 131.108.13.2
configuration for routerB:
frame-relay switching
!
interface serial 0
no Ip address
frame-relay encapsulation
frame-relay router 9 tun0 43
frame-relay interface-type dce
!
int tu 0
tunnel source serial 1
tunnel destination 131.108.100.1
channelized E1interface example
假设是7500系列路由器，E1接口（MIP板）在插槽4上面。
一个CHANNEL-GROUP 可对应多个时间槽，本例中SERIAL4/0：1有5*64的速率
configuration for router :
controller e1 0
framing no-crc4
chanel-group 0 timeslots1
channel-group 1 timeslots 2,7-9,20 speed 64
!
interface serial4/0:0
Ipaddress 16.217.30.2 255.255.255.252
Encapsulation ppp
!
interface serial 4/0:1
ip address 16.205030.5 255.255.255.252
X.25 EXAMPLE
在配置X.25 时，为减少路由交换引起的呼叫，通常用静态路由，而当一对多情况下，不在一个子网中用subinface 配置
configuration for router:
interface serial 0
ip address 131.108.100.1 255.255.255.0
encapsulation X.25
X.25 address 041673226839
x.25 htc 16
x.25 map ip 131.108.100.2 041675222222
int s 0.1
ip address 131.108.101.1 255.255.255.0
x.25 mapip 131.108.101.2 041674222222
!
ip router 131.108.100.0 255.255.255.0 131.108.100.2
ip router 131.108.101.0 255.255.255.0 131.108.101.2
