Troubleshooting Serial Lines
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unit/digital service unit (CSU/DSU), through a serial cable that is connected
to another CSU/DSU, and then connected to Router B.
F I G U R E 7 . 1
Serial line setup
HDLC Encapsulation
High-level Data Link Control (HDLC) is an encapsulation method used by
serial links. You learned in Chapter 2 that HDLC provides a 32-bit
checksum and three different transfer modes: normal, asynchronous
response, and asynchronous balanced.
HDLC is used by default on Cisco serial interfaces. The first important
point of troubleshooting serial line problems is to verify that both sides of
the link are using the same encapsulation types. Here is a look at a serial
interface from a Cisco 2501. Notice that the encapsulation type is HDLC:
Router_A>show interface serial0
Serial0 is administratively down, line protocol is down
Hardware is HD64570
Internet address is 172.16.20.6/30
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely 255/
255, load 1/255
Encapsulation HDLC, loopback not set, keepalive set (10
sec)
Last input never, output never, output hang never
Last clearing of "show interface" counters never
Input queue: 0/75/0 (size/max/drops); Total output
drops: 0
Queueing strategy: weighted fair
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