Cisco ASR 1000 Configuration Manual page 330

Performing Basic Interface Troubleshooting
Table 20-1
Status Line 
Condition
Serial x is up, line
protocol is up
(looped)
Cisco ASR 1000 Series Aggregation Services Routers SIP and SPA Software Configuration Guide
20-6
Serial Lines: show interfaces serial Status Line Conditions (continued)
Possible Problem
A loop exists in the circuit.
The sequence number in the
keepalive packet changes to a
random number when a loop
is initially detected. If the
same random number is
returned over the link, a loop
exists.
Chapter 20 Troubleshooting the Serial SPAs
Solution
If the line protocol does not come up in local
5.
loopback mode, and if the output of the debug
serial interface EXEC command shows that
the keepalive counter is not incrementing, a
router hardware problem is likely. Swap router
interface hardware.
If the line protocol comes up and the keepalive
6.
counter increments, the problem is not in the
local router.
7. If you suspect faulty router hardware, change
the serial line to an unused port. If the
connection comes up, the previously connected
interface has a problem.
Use the show running-config privileged EXEC
1.
command to look for any loopback interface
configuration command entries.
If you find a loopback interface configuration
2.
command entry, use the no loopback interface
configuration command to remove the loop.
If you do not find the loopback interface
3.
configuration command, examine the
CSU/DSU to determine whether they are
configured in manual loopback mode. If they
are, disable manual loopback.
Reset the CSU or DSU, and inspect the line
4.
status. If the line protocol comes up, no other
action is needed.
If the CSU or DSU is not configured in manual
5.
loopback mode, contact the leased-line or other
carrier service for line troubleshooting
assistance.
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