This configuration uses a BRI circuit to backup a serial link. It also uses the Open Shortest Path First (OSPF) routing protocol between the two routers. Once the backup connection is activated, you must ensure that the routing table is updated to use the new backup link. Only one side (maui-soho-01) is configured to dial out. The other side (maui-nas-05) is configured to accept the call.

In this configuration we have a Cisco 1604 router connected to a Cisco 3640 router using a serial connection. Both routers are also equipped with BRI interfaces that will be used for the backup link. The Cisco 1604 is running Cisco IOS® Software Release 12.1(5)T and the Cisco 3640 is using Cisco IOS 12.1(2).

Note: Configure and verify that the DDR connection (dialer 1 with BRI0) is working properly before you configure the backup interface and backup delay commands. This will allow you to effectively manage troubleshooting dialer profiles, ISDN, Point-to-Point Protocol (PPP) and authentication issues before you configure the backup.
 
maui-soho-01 (1600) 
maui-soho-01#show running-config 
Building configuration...

Current configuration : 1687 bytes
!
version 12.1
no service single-slot-reload-enable
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname maui-soho-01
!
logging rate-limit console 10 except errors
aaa new-model
aaa authentication login default local
aaa authentication login NO_AUTHEN none
aaa authentication ppp default local
!--- basic AAA configuration for ppp calls
enable secret 5 <deleted>
!
username maui-nas-05 password 0 cisco
!--- username for remote router (maui-nas-05) and
shared secret
!---(used for CHAP authentication)
!--- shared secret must be the same on both sides
ip subnet-zero
no ip finger
!
isdn switch-type basic-ni
!
interface Loopback0
 ip address 172.17.1.1 255.255.255.0
!--- The loopback address will be used by OSPF for the
router ID.
!
interface Ethernet0
 ip address 172.16.1.1 255.255.255.0
!
interface Serial0
!--- Primary Interface
 backup delay 10 30
!--- Backup link will be activated 10 seconds after
primary link
 goes down.
!--- Backup link will be deactivated 30 seconds after primary
 link
 is restored.
 backup interface Dialer1
!--- Interface Dialer 1 will provide backup.
 ip address 192.168.10.2 255.255.255.252
 encapsulation ppp
 no fair-queue
 clockrate 64000
 ppp authentication chap
!
interface BRI0
 ip unnumbered Loopback0
!--- unnumbered to the Loopback 0 address
 encapsulation ppp
 dialer pool-member 10
!--- Assign BRI0 as member of dialer pool 10.
!--- Dialer pool 10 is specified in interface Dialer 1.
 isdn switch-type basic-ni
 isdn spid1 51255511110101 5551111
 isdn spid2 51255511120101 5551112
 ppp authentication chap
!--- Use ppp chap authentication.
 ppp multilink
!
interface Dialer1
!--- Dialer 1 provides backup for the serial link.
 ip address 172.20.10.2 255.255.255.0
!--- Address for the dialer interface. 
!--- The remote side dialer interface is in the same subnet.
 encapsulation ppp
 dialer pool 10
!--- defines Dialer pool 10
!--- BRI 0 is a member of this pool.
 dialer remote-name maui-nas-05
!--- specifies remote router name 
!--- This name must match that used by the remote router to
 authenticate itself.
 dialer idle-timeout 900
!--- Idle timeout is set to 900 seconds (15 minutes).
!--- The link will be disconnected if there is no interesting
 traffic for 900 secs.
!--- Since OSPF hellos are interesting traffic, this will reset
 the idle timeout and
!--- cause the link to stay up until the primary link is
!--- restored.
 dialer string 5552222
!--- defines the destination routers phone number
 dialer load-threshold 80 outbound
!--- This sets the outbound load level for traffic at
which 
!---additional connections will be added to the MP bundle load level.
!--- Values range from 1 (unloaded) to 255 (fully loaded). The threshold
!--- in this case is 80/255=32%.
 dialer-group 1
!--- apply interesting traffic definition from
dialer-list 1
 ppp authentication chap
!--- use ppp chap authentication
 ppp multilink
!
router ospf 5
 log-adjacency-changes
 network 172.16.1.0 0.0.0.255 area 0
 network 172.17.1.0 0.0.0.255 area 0
 network 172.20.10.0 0.0.0.255 area 0
 network 192.168.10.0 0.0.0.3 area 0
!
ip classless
no ip http server
!
dialer-list 1 protocol ip permit
!--- All IP traffic is designated as interesting.
!--- This is applied to Interface Dialer 1 using dialer-group 1.
!--- OSPF hello packets will trigger the dial.
!
line con 0
 exec-timeout 0 0
 login authentication NO_AUTHEN
 transport input none
line vty 0 4
!
end
maui-nas-05 (3640) 
maui-nas-05#show running-config 
Building configuration...

Current configuration:
!
version 12.1
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname maui-nas-05
!
aaa new-model
aaa authentication login default local
aaa authentication login NO_AUTHEN none
aaa authentication ppp default local
!--- basic AAA configuration for ppp calls
enable secret 5 <deleted>
!
username maui-soho-01 password 0 cisco
!--- username for remote router (maui-soho-01) and
shared secret
!---(used for CHAP authentication)
!--- shared secret must be the same on both sides
ip subnet-zero
!         
isdn switch-type basic-ni
!
interface Loopback0
 ip address 172.22.1.1 255.255.255.0
!--- The loopback address will be used by OSPF for the
router ID.
!
interface Ethernet0/0
 ip address 172.22.53.105 255.255.255.0
!
interface Ethernet0/1
 no ip address
 shutdown
!
interface BRI1/0
 ip unnumbered Loopback0
!--- unnumbered to the Loopback 0 address
 encapsulation ppp
 dialer pool-member 20
!--- Assign BRI0 as member of dialer pool 20.
!--- Dialer pool 20 is specified in interface Dialer 1.
 isdn switch-type basic-ni
 isdn spid1 51255522220101 5552222
 isdn spid2 51255522230101 5552223
 ppp authentication chap
!--- Use ppp chap authentication.
 ppp multilink
!
! <<-- Unused interface configurations have been
removed.
!
interface Serial2/0
!--- primary interface
 ip address 192.168.10.1 255.255.255.252
 encapsulation ppp
 no fair-queue
 ppp authentication chap
!
! <<--Unused interface configurations have been
removed.
!
interface Dialer1
!--- Dialer 1 provides backup for the serial link.
 ip address 172.20.10.1 255.255.255.0
!--- address for the dialer interface 
!--- The remote side dialer interface is in the same subnet.
 encapsulation ppp
 dialer remote-name maui-soho-01
!--- specifies remote router name 
!--- This name must match that used by the remote router to
 authenticate itself.
 dialer pool 20
!--- defines dialer pool 20
 dialer idle-timeout 900
!--- Idle timeout is set to 900 seconds (15 minutes).
!--- This is equal to the idle timeout set on maui-soho-01.
 Dialer max-call 4096
 dialer-group 1
!--- apply interesting traffic definition from
dialer-list 1
 ppp authentication chap
!--- Use ppp chap authentication.
 ppp multilink
!
router ospf 5
 network 172.20.10.0 0.0.0.255 area 0
 network 172.22.1.0 0.0.0.255 area 0
 network 172.22.53.0 0.0.0.255 area 0
 network 192.168.10.0 0.0.0.3 area 0
 default-information originate
!--- transmit ospf default information 
!--- This may be required for remote router to use the BRI DDR
 link.
!
ip classless
ip route 0.0.0.0 0.0.0.0 Ethernet0/0
no ip http server
!
dialer-list 1 protocol ip permit
!--- All IP traffic is designated as interesting.
!--- This is applied to Interface Dialer 1 using dialer-group
 1.
!
line con 0
 login authentication NO_AUTHEN
 transport input none
line 97 102
line aux 0
line vty 0 4
!
end

Debug and Show Commands

Before issuing debug commands, please see Important Information on Debug Commands.