Cisco Multilink PPP used to bundle two or more Serial links or BRI/PRI channels. When we upgraded an existing Serial/E1/T1 link with one more parallel Serial/E1/T1 link, it is easy to bundle Serial links by using Multilink PPP. If the first E1/ T1 circuit is a point to point circuit, first you must make sure that encapsulation is Point to Point protocol, as apposed to HDLC. This we can do by using the simple command encapsulation ppp on the serial interface of each router. Once this is done, a second circuit is easy to add by creating a Multilink interface. The following configuration example will connect Router1 and Router2 by using a multilink interface to bundle the 2 serial links (serial1 1/5:0 and serial 4/0).