Here is the quick and dirty configuration for bonding two Serial (T1) PPP links on a Cisco 1841 router.

interface Mulitlink1
ip address xxx.xxx.xxx.xxx xxx.xxx.xxx.xxx
ppp multilink
ppp multilink group 1

interface Serial 0/0
no ip address
encapsulation ppp
ppp multilink
ppp multilink group 1

interface Serial 0/1
no ip address
encapsulation ppp
ppp multilink
ppp multilink group 1

The two serial interfaces then look like one – the Multilink1 interface.  The PPP ip address for this end will be the ip address of the Multilink1 interface. You can still check the status of each serial interface using “show interface serial 0/x” .  This will also show you what Multilink group the serial interface is a member of

Link is a member of Multilink bundle Multilink1

You can also check the status of the Multilink interface by using “show interface Multilink1″.  The command “show ppp multilink interface multilink1″ will display what interfaces belong to the multilink group (useful if you are on a larger router with multiple multilink ppp groups).

I have set up these multilink interfaces in the past, but I can say I am not sure what happens if just one of the T1s fail.  I would guess that the PPP would not like this and bring the whole interface down.

Does losing one T1 bring the whole Multilink interface down or will it the protocol stay up? Is there a better way to do this that will keep the interface up if one of the T1s fail?

Going to have to test this when I get a chance.