System information

Troubleshooting Serial Line Problems 15-319
Serial Lines: Increasing Input Drops on Serial Link
Serial Lines: Increasing Input Drops on Serial Link
Input drops appear in the output of the show interfaces serial exec command (refer to Figure 15-1)
when too many packets from that interface are still being processed in the system.
Symptom: Increasing number of input drops on serial link.
Table 15-3 outlines the possible problem that might cause this symptom and describes solutions to
that problem.
Table 15-3 Serial Lines: Increasing Input Drops on Serial Link
Serial Lines: Increasing Input Errors in Excess of 1% of Total Interface Traffic
If input errors appear in the show interfaces serial output (refer to Figure 15-1), there are several
possible sources of those errors. The most likely sources are summarized in Table 15-4.
Note Any input error value for cyclic redundancy check (CRC) errors, framing errors, or aborts
above 1 percent of the total interface traffic suggests some kind of link problem that should be
isolated and repaired.
Symptom: Increasing number of input errors in excess of 1 percent of total interface traffic.
Table 15-4 Serial Lines: Increasing Input Errors in Excess of 1% of Total Interface Traffic
Possible Problem Solution
Input rate exceeds the capacity of the router or
input queues exceed the size of output queues
Note: Input drop problems are typically seen when traffic is being
routed between faster interfaces (such as Ethernet, Token Ring, and
FDDI
1
) and serial interfaces. When traffic is light, there is no
problem. As traffic rates increase, backups start occurring. Routers
drop packets during these congested periods.
Step 1 Increase the output queue size on common destination
interfaces for the interface that is dropping packets. Use the
hold-queue out interface configuration command. Increase
these queues by small increments (for instance, 25%) until
you no longer see drops in the show interfaces output. The
default output hold queue limit is 100 packets.
Step 2 Reduce the input queue size, using the hold-queue in
interface configuration command, to force input drops to
become output drops. Output drops have less impact on the
performance of the router than do input drops. The default
input hold queue is 75 packets.
1 FDDI = Fiber Distributed Data Interface