The configuration of PPP on Cisco routers isn’t difficult, but can vary depending upon which options you choose to implement. For example, configuring PPP can be as simple as specifying that the appropriate serial interface on each router should use PPP encapsulation. This is accomplished by issuing the encapsulation ppp command from interface configuration mode.
RouterA#config t
Enter configuration commands, one per line. End with CNTL/Z.
RouterA(config)#int s0
RouterA(config-if)#encapsulation ?
atm-dxi ATM-DXI encapsulation
frame-relay Frame Relay networks
hdlc Serial HDLC synchronous
lapb LAPB (X.25 Level 2)
ppp Point-to-Point protocol
smds Switched Megabit Data Service (SMDS)
x25 X.25
RouterA(config-if)#encapsulation ppp
After configuring interface S0 on RouterA to use PPP, notice the truncated output of the show int s0 command shown below. While the encapsulation is set to PPP, the status of the port shows that Serial 0 is up, but the line protocol is down. This is because the other end of the link on my network is still configured to use HDLC encapsulation. Recall from Chapter 7 that frame type mismatches will result in this message. Notice also that the LCP status shows a REQsent message (if properly connected, this would be open), and that the NCP used to configure IP (IPCP) is also closed.
RouterA#show int s0
Serial0 is up, line protocol is down
Hardware is HD64570
Internet address is 192.168.2.200/28
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely 255/255, load 1/255
Encapsulation PPP, loopback not set, keepalive set (10 sec)
LCP REQsent
Closed: IPCP
After changing the encapsulation type on the other end of the link to PPP, the output of the show int s0 command on RouterA displays the following:
RouterA#show int s0
Serial0 is up, line protocol is up
Hardware is HD64570
Internet address is 192.168.2.200/24
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely 255/255, load 1/255
Encapsulation PPP, loopback not set, keepalive set (10 sec)
LCP Open
Open: IPCP
Now that both ends of the link are configured to use PPP encapsulation, the line protocol has changed to up; LCP is open, as is the NCP for IP (IPCP). IP traffic should now be able to move across the link without a problem. If additional protocols like IPX or AppleTalk were also configured on both interfaces, their associated NCPs would also be listed.
