Your CCNA & CCENT exam success depends on knowing how to configure & troubleshoot HDLC & PPP on Cisco routers, as well as
knowing the differences & similarities between the two. Today we will take a look at HDLC & some basic router configurations, & in future CCNA & CCENT
certification exam tutorials, we will examine PPP in detail.
With a point-to-point WAN link such as the following, we have got two options for encapsulation - HDLC
and PPP. (HDLC, PPP, & Frame Relay do not run on LANs - you would not configure them on Ethernet interfaces.)
We’re not going to discuss HDLC for
long, since there’s not very 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 & IP connectivity will be lost. To illustrate, I will 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, & 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!
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage, home of free
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