ISDN and Multilink with load-threshold

Configuration Steps:

1. Configure the BRI interfaces

Router(config)# interface bri 0/0/0
Router(config-if)# ip address 10.0.0.1 255.255.255.0
Router(config-if)# encapsulation ppp
Router(config-if)# isdn spid1 0835866101 8358661
Router(config-if)# isdn spid2 0835866301 8358663
Router(config-if)# dialer string 8358662
Router(config-if)# dialer-group 1
Router(config-if)# ppp authentication chap
Router(config-if)# ppp chap hostname RouterA
Router(config-if)# ppp chap password cisco
Router(config-if)# exit

2. Configure the second BRI interface

Router(config)# interface bri 0/0/1
Router(config-if)# ip address 10.0.1.1 255.255.255.0
Router(config-if)# encapsulation ppp
Router(config-if)# isdn spid1 0835866201 8358662
Router(config-if)# isdn spid2 0835866401 8358664
Router(config-if)# dialer string 8358663
Router(config-if)# dialer-group 1
Router(config-if)# ppp authentication chap
Router(config-if)# ppp chap hostname RouterA
Router(config-if)# ppp chap password cisco
Router(config-if)# exit

3. Create the multilink interface

Router(config)# interface multilink 1
Router(config-if)# ip address 192.168.1.1 255.255.255.0
Router(config-if)# encapsulation ppp
Router(config-if)# dialer string 8358662
Router(config-if)# dialer-group 1
Router(config-if)# dialer load-threshold 128 either
Router(config-if)# ppp authentication chap
Router(config-if)# ppp chap hostname RouterA
Router(config-if)# ppp chap password cisco
Router(config-if)# ppp multilink
Router(config-if)# exit

4. Assign physical interfaces to the multilink group

Router(config)# interface bri 0/0/0
Router(config-if)# ppp multilink
Router(config-if)# ppp multilink group 1
Router(config-if)# exit

Router(config)# interface bri 0/0/1
Router(config-if)# ppp multilink
Router(config-if)# ppp multilink group 1
Router(config-if)# exit

5. Configure dialer list and routing

Router(config)# dialer-list 1 protocol ip permit
Router(config)# ip route 0.0.0.0 0.0.0.0 192.168.1.2

Key Configuration Points:

• The dialer load-threshold 128 either command sets the threshold at 50% (128 out of 255) utilization on either inbound or outbound traffic
• When traffic exceeds this threshold, the second BRI interface will be brought up automatically
• Both BRI interfaces must be configured with PPP multilink and assigned to the same multilink group
• The multilink interface handles the IP addressing and dialing logic
• CHAP authentication must match on both ends of the connection

Verification Commands:

Router# show interfaces multilink 1
Router# show dialer
Router# show ppp multilink
Router# show isdn status
Router# show isdn active

This configuration allows for automatic load balancing across multiple ISDN BRI connections, with the second line being brought up when traffic load exceeds the configured threshold.