When companies who have a T1 connections to the internet run out of bandwidth they have a few choices. One is to order the latest service called metro ethernet. The second is to order a second T1 from your provider. When you order the T1 line let them know you want to line bonded, or in cisco router terms MPPP, or multilink PPP. This allows you to aggregate T1 bandwidth in 3M, 4.5M and so on. This looks like one big pipe when packets go out to the internet.

Multilink PPP allows you to bond multiple T1 lines into one larger line. This MPPP line is assigned one set of ip addresses, while the individual lines that make up the MPPP line do not. The intersting thing is if either of your T1 connections where to drop, the MPPP connection stays up. Once the cisco router is setup for MPPP its very easy to add or subtract T1 lines.

The first thing to do in your router is to create the MPPP interface:
config t
interface Multilink2
description Multilink PPP connection
ip address 192.168.0.2 255.255.255.252
ip verify unicast reverse-path
no ip redirects
no ip proxy-arp
no ip route-cache
no cdp enable
ppp multilink
multilink-group 1
no clns route-cache

The important part of this is the “multilink-group 1″ part. This says any T1 that is assigned this group id, is part of the multilink interface.

Now we go configure our normal serial port like this:
conf t
interface Serial2/0/9:0
description first multilink PPP T1 line
no ip address
ip verify unicast reverse-path
no ip redirects
no ip proxy-arp
encapsulation ppp
no ip route-cache
load-interval 30
fair-queue 64 16 0
down-when-looped
no cdp enable
ppp multilink
multilink-group 1
no clns route-cache

notice how we dont assign an ip address to the serial port. Also we assign the “multilink-group1″ so it knows its part of the multilink PPP connection.

We would do the same for any other serial connection that we want to be part of the parent multilink line.

The one single catch to all of this is you need to have cisco IOS version 12.2(13)T or greater. This is when Multilink PPP was introduced.

In addition you cant just do this on your side alone. The opposite side also has to be configured as MPPP for the whole thing to work.

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