ppp'ing...

globally set the username as the hostname of the other router, password the same on each router, and encap ppp on each interface and you're ppp'ing...

to wit:

r2620_01#sh int s0/0
Serial0/0 is up, line protocol is up
Hardware is PowerQUICC Serial
Internet address is 30.0.0.1/24
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation PPP, LCP Open
Open: IPCP, CDPCP, loopback not set
Keepalive set (10 sec)


r2620_01#sh int s0/1
Serial0/1 is up, line protocol is up
Hardware is PowerQUICC Serial
Internet address is 20.0.0.1/24
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation PPP, LCP Open
Open: IPCP, CDPCP, loopback not set
Keepalive set (10 sec)

and:

r2620_03#ping 2.2.2.2
(ping from router 3 to router 2 via router 1)
Sending 5, 100-byte ICMP Echos to 2.2.2.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 60/67/80 ms
r2620_03#

finally,
sh ip int will not show you the new encapsulation, sh int does...