电话网络(PSTN)是目前普及程度最高、成本最低的公用通讯网络，它在网络互连中也有广泛的应用。电话网络的应用一般可分为两种类型，一种是同等级别机构之间以按需拨号(DDR)的方式实现互连，一种是ISP为拨号上网为用户提供的远程访问服务的功能。
1. 远程访问
1.1.Access Server基本设置：
选用Cisco2511作为访问服务器,采用IP地址池动态分配地址.远程工作站使用WIN95拨号网络实现连接。
全局设置：
基本接口设置命令：
line拨号线设置：
访问服务器设置如下：
Router:
hostname Router
enable secret 5 $1$EFqU$tYLJLrynNUKzE4bx6fmH//
!
interface Ethernet0
ip address 10.111.4.20 255.255.255.0
!
interface Async1
ip unnumbered Ethernet0
encapsulation ppp
keepalive 10
async mode interactive
peer default ip address pool Cisco2511-Group-142
!
ip local pool Cisco2511-Group-142 10.111.4.21 10.111.4.36
!
line con 0
exec-timeout 0 0
passWord cisco
!
line 1 16
modem InOut
modem autoconfigure discovery
flowcontrol hardware
!
line aux 0
transport input all
line vty 0 4
password cisco
!
end
相关调试命令：
show interface
show line
1.2. Access Server通过Tacacs服务器实现安全认证：
使用一台WINDOWS NT服务器作为Tacacs服务器，地址为10.111.4.2,运行Cisco2511随机带的Easy ACS 1.0软件实现用户认证功能.
相关设置：
访问服务器设置如下：
hostname router
!
aaa new-model
aaa authentication login default tacacs+
aaa authentication login no_tacacs enable
aaa authentication ppp default tacacs+
aaa authorization exec tacacs+
aaa authorization network tacacs+
aaa accounting exec start-stop tacacs+
aaa accounting network start-stop tacacs+
enable secret 5 $1$kN4g$CvS4d2.rJzWntCnn/0hvE0
!
interface Ethernet0
ip address 10.111.4.20 255.255.255.0
!
interface Serial0
no ip address
shutdown
interface Serial1
no ip address
shutdown
!
interface Group-Async1
ip unnumbered Ethernet0
encapsulation ppp
async mode interactive
peer default ip address pool Cisco2511-Group-142
no cdp enable
group-range 1 16
!
ip local pool Cisco2511-Group-142 10.111.4.21 10.111.4.36
tacacs-server host 10.111.4.2
tacacs-server key tac
!
line con 0
exec-timeout 0 0
password cisco
login authentication no_tacacs
line 1 16
login authentication tacacs
modem InOut
modem autoconfigure type usr_courier
autocommand ppp
transport input all
stopbits 1
rxspeed 115200
txspeed 115200
flowcontrol hardware
line aux 0
transport input all
line vty 0 4
password cisco
!
end
2. DDR（dial-on-demand routing）实例
此例通过Cisco 2500系列路由器的aux端口实现异步拨号DDR连接。Router1拨号连接到Router2。其中采用PPP/CHAP做安全认证，在Router1中应建立一个用户，以对端路由器主机名作为用户名，即用户名应为Router2。同时在Router2中应建立一个用户，以对端路由器主机名作为用户名，即用户名应为Router1。所建的这两用户的password必须相同。
相关命令如下：
hostname Router1
!
enable secret 5 $1$QKI7$wXjpFqC74vDAyKBUMallw/
!
username Router2 password cisco
chat-script cisco-default "" "AT" TIMEOUT 30 OK "ATDT T" TIMEOUT 30 CONNECT c
!
interface Ethernet0
ip address 10.0.0.1 255.255.255.0
!
interface Async1
ip address 192.200.10.1 255.255.255.0
encapsulation ppp
async default routing
async mode dedicated
dialer in-band
dialer idle-timeout 60
dialer map ip 192.200.10.2 name Router2 modem-script cisco-default 573
dialer-group 1
ppp authentication chap
!
ip route 10.0.1.0 255.255.255.0 192.200.10.2
dialer-list 1 protocol ip permit
!
line con 0
line aux 0
modem InOut
modem autoconfigure discovery
flowcontrol hardware
Router2:
hostname Router2
!
enable secret 5 $1$F6EV$5U8puzNt2/o9g.t56PXHo.
!
username Router1 password cisco
!
interface Ethernet0
ip address 10.0.1.1 255.255.255.0
!
interface Async1
ip address 192.200.10.2 255.255.255.0
encapsulation ppp
async default routing
async mode dedicated
dialer in-band
dialer idle-timeout 60
dialer map ip 192.200.10.1 name Router1
dialer-group 1
ppp authentication chap
!
ip route 10.0.0.0 255.255.255.0 192.200.10.1
dialer-list 1 protocol ip permit
!
line con 0
line aux 0
modem InOut
modem autoconfigure discovery
flowcontrol hardware
!
相关调试命令：
debug dialer
debug ppp authentication
debug ppp error
debug ppp negotiation
debug ppp packet
show dialer
3. 异步拨号备份DDN专线：
此例主连接采用DDN专线，备份线路为电话拨号。当DDN专线连接正常时，主端口S0状态为up，line protocol亦为up,则备份线路状态为standby，line protocol为down，此时所有通信均通过主接口进行。当主接口连接发生故障时，端口状态为down，则激活备份接口，完成数据通信。此方法不适合为X.25做备份。因为，配置封装为X.25的接口只要和X.25交换机之间的连接正常其接口及line protocol的状态亦为 up,它并不考虑其它地方需与之通信的路由器的状态如何，所以若本地路由器状态正常，而对方路由器连接即使发生故障，本地也不会激活备份线路。例4将会描述如何为X.25做拨号备份。
以下是相关命令：
hostname c2522rb
!
enable secret 5 $1$J5vn$ceYDe2FwPhrZi6qsIIz6g0
enable password cisco
!
username c4700 password 0 cisco
ip subnet-zero
chat-script cisco-default "" "AT" TIMEOUT 30 OK "ATDT T" TIMEOUT 30 CONNECT c
chat-script reset atz
!
interface Ethernet0
ip address 16.122.51.254 255.255.255.0
no ip mroute-cache
!
interface Serial0
backup delay 10 10
backup interface Serial2
ip address 16.250.123.18 255.255.255.252
no ip mroute-cache
no fair-queue
!
interface Serial1
no ip address
no ip mroute-cache
shutdown
!
interface Serial2
physical-layer async
ip address 16.249.123.18 255.255.255.252
encapsulation ppp
async mode dedicated
dialer in-band
dialer idle-timeout 60
dialer map ip 16.249.123.17 name c4700 6825179
dialer-group 1
ppp authentication chap
!
interface Serial3
no ip address
shutdown
no cdp enable
!
interface Serial4
no ip address
shutdown
no cdp enable
!
interface Serial5
no ip address
no ip mroute-cache
shutdown
!
interface Serial6
no ip address
no ip mroute-cache
shutdown
!
interface Serial7
no ip address
no ip mroute-cache
shutdown
!
interface Serial8
no ip address
no ip mroute-cache
shutdown
!
interface Serial9
no ip address
no ip mroute-cache
shutdown
!
interface BRI0
no ip address
no ip mroute-cache
shutdown
!
router eigrp 200
network 16.0.0.0
!
ip classless
!
dialer-list 1 protocol ip permit
!
line con 0
line 2
script dialer cisco-default
script reset reset
modem InOut
modem autoconfigure discovery
rxspeed 38400
txspeed 38400
flowcontrol hardware
line aux 0
line vty 0 4
password cisco
login
!
end
c2522rb#
4. 异步拨号备份X.25:
设置X.25的拨号备份,首先X.25连接的端口必须运行动态路由协议,异步拨号口必须使用静态路由.本例选择EIGRP作为路由选择协议,将静态路由的Metric的值设置为200,由于EIGRP的默认Metric为90,所以当同时有两条路径通往同一网段时,其中Metric值小的路径生效,而当X.25连接出现问题时,路由器无法通过路由协议学习到路由表，则此时静态路由生效，访问通过拨号端口实现。当X.25连接恢复正常时，路由器又可以学习到路由表，则由于 Metric值的不同，静态路由自动被动态路由所代替，这样就实现了备份的功能。
路由器Router1配置如下:
hostname router1
!
enable secret 5 $1$UTvD$99YiY2XsRMxHudcYeHn.Y.
enable password cisco
!
username router2 password cisco
ip subnet-zero
chat-script cisco-default "" "AT" TIMEOUT 30 OK "ATDT T" TIMEOUT 30 CONNECT c
chat-script reset atz
interface Ethernet0
ip address 202.96.38.100 255.255.255.0
!
interface Serial0
ip address 202.96.0.1 255.255.255.0
encapsulation x25
x25 address 10112227
x25 htc 16
x25 map ip 202.96.0.2 10112225 broadcast
!
interface Serial1
no ip address
shutdown
!
!
interface Async 1
ip address 202.96.1.1 255.255.255.252
encapsulation ppp
dialer in-band
dialer idle-timeout 60
dialer map ip 202.96.1.2 name router2 modem-script cisco-default 2113470
dialer-group 1
ppp authentication chap
!
router eigrp 200
redistribute connected
network 202.96.0.0
!
ip route 202.96.37.0 255.255.255.0 202.96.1.2 200
dialer-list 1 protocol ip permit
line con 0
line aux 0
script dialer cisco-default
script reset reset
modem InOut
modem autoconfigure discovery
transport input all
rxspeed 38400
txspeed 38400
flowcontrol hardware
line vty 0 4
password cisco
login
!
end
路由器Router2配置如下:
hostname router2
!
enable secret 5 $1$T4IU$2cIqak8f/E4Ug6dLT0k.J0
enable password cisco
!
username router1 password cisco
ip subnet-zero
chat-script cisco-default "" "AT" TIMEOUT 30 OK "ATDT T" TIMEOUT 30 CONNECT c
chat-script reset atz
!
interface Ethernet0
ip address 202.96.37.100 255.255.255.0
!
interface Serial0
ip address 202.96.0.2 255.255.255.0
no ip mroute-cache
encapsulation x25
x25 address 10112225
x25 htc 16
x25 map ip 202.96.0.1 10112227 broadcast
!
interface Serial1
no ip address
shutdown
!
interface Async1
ip address 202.96.1.2 255.255.255.252
encapsulation ppp
keepalive 30
async default routing
async mode dedicated
dialer in-band
dialer idle-timeout 60
dialer wait-for-carrier-time 120
dialer map ip 202.96.1.1 name router1 modem-script cisco-default 2113469
dialer-group 1
ppp authentication chap
!
router eigrp 200
redistribute static
network 202.96.0.0
!
no ip classless
ip route 202.96.38.0 255.255.255.0 202.96.1.1 200
dialer-list 1 protocol ip permit
!
line con 0
exec-timeout 0 0
line aux 0
script reset reset
modem InOut
modem autoconfigure discovery
transport input all
rxspeed 38400
txspeed 38400
flowcontrol hardware
line vty 0 4
password cisco
login
!
end
