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.

show isdn status - Ensure that the router is properly communicating with the ISDN switch. In the output, verify that Layer 1 Status is ACTIVE, and that the Layer 2 Status state = MULTIPLE_FRAME_ESTABLISHED appears. This command also displays the number of active calls.

show ip interface brief - This will show dialer 1(on maui-soho-01) in standby mode. Using dialer 1 as the backup interface means the physical interface (BRI0) is not dedicated to the backup. Therefor, BRI 0 can be used for normal DDR while the primary link is up

debug dialer - This is used to display DDR information about the packets received on the dialer interface.

debug isdn events - This is used to see ISDN activity occurring on the user side of the ISDN interface.

debug isdn q931 - Shows call setup and tear down of the ISDN network connection (Layer 3) between the router and the ISDN switch.

debug ppp negotiation - Displays information on PPP traffic and exchanges while negotiating the PPP components including Link Control Protocol (LCP), Authentication, and NCP. A successful PPP negotiation will first open the LCP state, then Authenticate, and finally negotiate NCP.

debug ppp authentication - Displays the PPP authentication protocol messages, including Challenge Authentication Protocol (CHAP) packet exchanges and Password Authentication Protocol (PAP) exchanges. If you observe a failure verify that the chap username and password are configured correctly.

debug ppp error - This is used to display protocol errors and error statistics associated with PPP connection negotiation and operation.