Multilink PPP

Multilink PPP (MLP) allows connections over multiple links to have the same remote network address, therefore allowing packet fragmentation and dynamic load balancing across the links. MLP can be configured for async, bri, and pri interfaces. In this example, we have two ports (modems) dedicated to one connection. The traffic is distributed across both links, resulting in a higher bandwidth connection. We use the dialer load-threshold command to tell the router to bring up the second link whenever the first link is at 100% capacity. In this case, it says to bring up the other link whenever either one of the interfaces reaches 100% capacity:

 interface async 1
 no ip address
 encapsulation ppp
 dialer rotary-group 1
 !
 interface async 2
 no ip address
 encapsulation ppp
 dialer rotary-group 1
 !
 interface dialer 1
 ip unnumber ethernet 0
 encapsulation ppp
 dialer in-band
 dialer load-threshold 100 either
 ppp authentication chap
 ppp multilink

Multilink PPP is commonly used to tie the two channels of a BRI ISDN line into a single 128 Kbps connection. Here's what that configuration looks like. We define two ISDN interfaces (one for each B channel), we place those interfaces in the same rotary group, and then we configure a dialer interface for the rotary group using PPP encapsulation. The dialer interface has the multilink command, which allows you to use both BRI interfaces simultaneously:

 interface BRI0
 no ip address
 encapsulation ppp
 dialer idle-timeout 300
 dialer rotary-group 1
 dialer load-threshold 30 either
 !
 interface BRI1
 no ip address
 encapsulation ppp
 dialer idle-timeout 300
 dialer rotary-group 1
 dialer load-threshold 30 either
 !
 ! We configure Dialer1 because BRI0 and BRI1 have been
 ! configured to use rotary group 1 (dialer1)
 !
 interface Dialer1
 ip address 10.1.1.2 255.255.255.0
 encapsulation ppp
 dialer in-band
 dialer idle-timeout 500
 dialer map ip 10.1.1.1 name baltimore broadcast 55512340101
 dialer load-threshold 30 either
 dialer-group 1
 ppp authentication chap
 ppp multilink