How to configure HDLC and PPP step by step guide and example

In this article I will demonstrate how can you configure wan encapsulation protocols. HDLC is the default encapsulation for synchronous serial links on Cisco routers. You should only use the encapsulation hdlc command to return the link to its default state.

For practical example of HDLC PPP create a simple topology as shown in figure or download this pre configured topology and load it in packet tracer.

Pre configured topology for PPP and HDLC

Double click on R1 and check the default encapsulation

Router>

Router#show interfaces serial 0/0/0

Serial0/0/0 is up, line protocol is up (connected)

Hardware is HD64570

Internet address is 20.0.0.1/8

MTU 1500 bytes, BW 64 Kbit, DLY 20000 usec, rely 255/255, load 1/255

Encapsulation HDLC, loopback not set, keepalive set (10 sec)

    [output is omited]

As you can verify that default encapsulation on router is HDLC. A wan link work only when it detects same protocols on same sides. To check it change the default encapsulation to PPP.

Router#configure terminal

Router(config)#interface serial 0/0/0

Router(config-if)#encapsulation ppp

Router(config)#exit

Router#show interfaces serial 0/0/0

Serial0/0/0 is up, line protocol is down (disabled)

Hardware is HD64570

Internet address is 20.0.0.1/8

MTU 1500 bytes, BW 64 Kbit, DLY 20000 usec, rely 255/255, load 1/255

Encapsulation PPP, loopback not set, keepalive set (10 sec)

 [output is omited]

as you can see that line protocols is disable. To enable it set the encapsulation back to HDLC and restart the port with shut down command

Router#configure terminal

Router(config)#interface serial 0/0/0

Router(config-if)#encapsulation hdlc

Router(config-if)#shutdown

Router(config-if)#no shutdown

Router(config-if)#exit

Router(config)#exit

Router#show interfaces serial 0/0/0

Serial0/0/0 is up, line protocol is up (connected)

Hardware is HD64570

Internet address is 20.0.0.1/8

MTU 1500 bytes, BW 64 Kbit, DLY 20000 usec, rely 255/255, load 1/255

Encapsulation HDLC, loopback not set, keepalive set (10 sec)

    [output is omited]

Configuration of PPP

Now we will configure PPP encapsulations on both router. We will also authenticate it with CHAP. Hostname of Router are R1 and R2 and password is vinita.

Double Click on R1 and configure it

Router>enable

Router#configure terminal

Router(config)#hostname R1

R1(config)#username R2 password vinita

R1(config)#interface serial 0/0/0

R1(config-if)#encapsulation ppp

R1(config-if)#ppp authentication chap

R1(config-if)#exit

R1(config)#

Now configure R2 for PPP

Router>enable

Router#configure terminal

Router(config)#hostname R2

R2(config)#username R1 password vinita

R2(config)#interface serial 0/0/0

R2(config-if)#encapsulation ppp

R2(config-if)#ppp authentication chap

R2(config-if)#exit

%LINEPROTO-5-UPDOWN: Line protocol on Interface Serial0/0/0,

changed state to up

R2(config)#

HDLC PPP command reference sheet

Router(config)#interface serial 0/0/0	Moves to interface configuration mode
Router(config-if)#encapsulation hdlc	Sets the encapsulation mode for this interface to HDLC
Router(config)#interface serial 0/0/0	Moves to interface configuration mode
Router(config-if)#encapsulation ppp	Changes encapsulation from default HDLC to PPP
Router(config)#username R1 password vinita	Sets a username of R1 and a password of vinita for authentication from the other side of the PPP serial link. This is used by the local router to authenticate the PPP peer
Router(config)#interface serial 0/0/0	Moves to interface configuration mode.
Router(config-if)#ppp authentication pap	Turns on Password Authentication Protocol (PAP) authentication only
Router(config-if)#ppp authentication chap	Turns on Challenge Handshake Authentication Protocol (CHAP) authentication only.
Router(config-if)#ppp authentication pap chap	Defines that the link will use PAP authentication, but will try CHAP if PAP fails or is rejected by other side.
Router(config-if)#ppp authentication chap pap	Defines that the link will use CHAP authentication, but will try PAP if CHAP fails or is rejected by other side.
Router(config-if)#ppp pap sentusername R1 password vinita	This command must be set if using PAP in Cisco IOS Software Release 11.1 or later
Router#show interfaces serial x	Lists information for serial interface x
Router#show controllers serial x	Tells you what type of cable (DCE/DTE) is plugged into your interface and whether a clock rate has been set
Router#debug serial interface	Displays whether serial keepalive counters are incrementing
Router#debug ppp	Displays any traffic related to PPP
Router#debug ppp packet	Displays PPP packets that are being sent and received
Router#debug ppp negotiation	Displays PPP packets related to the negotiation of the PPP link