Configuring And Troubleshooting HDLC On Point-To-Point Links

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



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


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.