Your CCNA and CCENT exam success depends on knowing how to configure and troubleshoot HDLC and PPP on Cisco routers, as well as knowing the differences and similarities between the two. Today we'll take a look at HDLC and some basic router configurations, and in future CCNA and CCENT certification exam tutorials, we'll examine PPP in detail.

With a point-to-point WAN link such as the following, we've got two options for encapsulation - HDLC and PPP. (HDLC, PPP, and Frame Relay do not run on LANs - you wouldn't configure them on Ethernet interfaces.)

We're not going to discuss HDLC for long, since there's not much to say about it - but what there is to say is very important! The version of HDLC used by Cisco routers is the default encapsulation type on Serial interfaces, verifiable with the show interface serial command. (Only the portion of this command's output dealing with the PTP WAN link will be shown in this section.)

R1#show interface serial 1

Serial1 is up, line protocol is up

Hardware is HD64570

Internet address is 172.12.13.1/24

MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,

reliability 255/255, txload 1/255, rxload 1/255

Encapsulation HDLC, loopback not set

R3#show int serial1

Serial1 is up, line protocol is up

Hardware is HD64570

Internet address is 172.12.13.3/24

MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,

reliability 255/255, txload 1/255, rxload 1/255

Encapsulation HDLC, loopback not set

At this point, each partner in the PTP link can ping the other.

R1#ping 172.12.13.3

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 172.12.13.3, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 36/36/36 ms

R3#ping 172.12.13.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 172.12.13.1, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 32/35/36 ms

This particular version of the High Data-Link Control encapsulation is Cisco-proprietary, a fancy way of saying "only Cisco routers understand this encapsulation type". If a router at the end of a PTP link is running HDLC encapsulation, the remote partner in the link must do so as well.

If one of the routers is running another encapsulation type, the physical interfaces will still be up, but the line protocol will go down and IP connectivity will be lost. To illustrate, I'll change the encapsulation type on R3's Serial1 interface to the Point-To-Point Protocol (PPP).

R3(config-if)#exit

R3(config)#int serial 1

R3(config-if)#encapsulation ppp

A few seconds later, the line protocol goes down on R3.

2d04h: %SYS-5-CONFIG_I: Configured from console by console

2d04h: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1, changed state to down

show interface serial 1 on both routers verifies that the physical interface is up, but the line protocol is down. IP connectivity is lost.

R3#show interface serial 1

Serial1 is up, line protocol is down

R3#ping 172.12.13.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 172.12.13.1, timeout is 2 seconds:

.....

Success rate is 0 percent (0/5)

R1#show interface serial 1

Serial1 is up, line protocol is down

R1#ping 172.12.13.3

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 172.12.13.3, timeout is 2 seconds:

.....

Success rate is 0 percent (0/5)

The encapsulation mismatch has brought the line protocol down, and to bring it back up, we simply need to make the encapsulation types match again. We'll do just that in the next installment of my exclusive Cisco CCENT / CCNA certification exam tutorial series!