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Troubleshooting Serial Lines

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Serial Line Summary

Several encapsulations and protocols may be used over serial lines. Because
of this variety, many different problems can occur. It is also important
to realize that the output of many show commands may differ, depending on
the interface configuration.

To aid you in diagnosing and resolving serial line problems, this section

includes two quick reference tables for your convenience.

Symptoms and Problems

Table 7.1 lists several common serial line conditions and their related
possible problems.

T A B L E 7 . 1

Serial Line Symtoms and Problems

Symptom or
Condition

Associated Problem(s)

Interface is
administratively
down; line
protocol is down

(a) The interface has been placed in shutdown via a
configuration command
(b) Duplicate IP addresses are not allowed and one of
the two interfaces with the same IP address will be
shut down

Interface is down;
line protocol
is down

(a) Improper cabling
(b) No carrier signal from local provider.
(c) Hardware failure (interface or CSU/DSU, cabling)

Interface is up;
line protocol
is down

(a) Misconfigured interface, local or remote
(b) Local provider problem
(c) Keepalive sequencing not incrementing
(d) Hardware failures (local or remote interfaces and
CSU/DSU)
(e) Noisy line
(f) Timing mismatches

Interface is up;
line protocol is up
(looped)

(a) The circuit is in loopback

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