CISCO IOS in a Nutshell

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; 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 ...