imagine someone comes to job and ask you to build a network using for primary access an isdn connection with a modem dialer as backup..... nice and worthless because of the 2 mins of re-establishing connection yet even if cisco says it isn't supposed to be like that... for them a isdn is already a backup so is modem dial up... so why have a backup of a backup???
well it is possible as usual and here is the config enjoy ....
the major problem was to get the router to switch routes while having the same destinations ... can't use the 2 ip routes with differents metrics since it will get stuck to isdn and never swich unless a manual shut in BRI0 is done ...
as for the route with equal metric ... fun but not practical... half the packet one way the others the rest yeahhhh!!!!





!
version 12.2
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
!
hostname central
!
aaa new-model
!
!
aaa authentication login default local
aaa authentication login NO_AUTHEN none
aaa authentication ppp default local
aaa session-id common
enable secret 5 $1$SSMd$RzXwmXaFE8.kYpijR0x1T0
!
username office password 0 cisco
username admin password 0 cisco
memory-size iomem 25
ip subnet-zero
no ip domain-lookup
ip host office 192.168.2.1
!
isdn switch-type basic-net3
!
!
!
interface Loopback1
ip address 2.2.2.2 255.255.255.0
!
interface BRI0
description Ligne principal d'acces MARNIS
no ip address
encapsulation ppp
no ip route-cache
dialer pool-member 1
isdn switch-type basic-net3
no cdp enable
!
interface FastEthernet0
ip address 192.168.1.1 255.255.255.0
no ip route-cache
no keepalive
speed auto
half-duplex
no cdp enable
!
interface Serial0
no ip address
shutdown
no fair-queue
!
interface Serial1
physical-layer async
description Backup du resaux MARNIS,via modem externe
no ip address
encapsulation ppp
dialer in-band
dialer pool-member 1
async mode interactive
no peer default ip address
ppp authentication chap callin
!
interface Dialer1
ip address 10.10.10.21 255.255.255.0
encapsulation ppp
dialer pool 1
dialer remote-name office
dialer fast-idle 1
dialer string 555555555
dialer string 555553333
dialer-group 1
pulse-time 0
no cdp enable
ppp authentication chap
ppp multilink
!
ip classless
ip route 192.168.2.0 255.255.255.0 10.10.10.20
no ip http server
!
dialer-list 1 protocol ip permit
dialer-list 2 protocol ip permit
no cdp run
!
line con 0
line 2
modem InOut
modem autoconfigure type usr_sportster
transport input all
autoselect ppp
speed 115200
flowcontrol hardware
line aux 0
line vty 0 4
password cisco
!
end