Symptom: Increasing output drops on serial link
The possible problem that might cause this symptom could be that input rate to serial interface exceeds
bandwidth available on serial link. The solution to this problem is as follows:
1
Minimize periodic broadcast traffic, such as routing and Service Advertising Protocol (SAP) updates, by
using access lists or by other means. For example, to increase the delay between SAP updates, use the ipx
sap-interval interface configuration command.
2
Increase the output hold queue size in small increments (for instance, 25 percent), using the hold-queue
out interface configuration command.
3
Implement priority queuing on slower serial links by configuring priority lists. For information on
configuring priority lists, see the Cisco IOS configuration guides and command references for your Cisco
IOS software release.
Output drops are acceptable under certain conditions. For instance, if a link is known to be overused (with
no way to remedy the situation), it is often considered more preferable to drop packets than to hold them.
This is true for protocols that support flow control and can retransmit data (such as TCP/IP and Novell
Internetwork Packet Exchange [IPX]). However, some protocols, such as DECnet and local-area transport,
are sensitive to dropped packets and accommodate retransmission poorly, if at all.
Note
Serial Lines: Increasing Input Drops on Serial Link
Input drops appear in the output of the show interfaces serial EXEC command when too many packets from
that interface are still being processed in the system.
Symptom: Increasing number of input drops on serial link
The possible problem that might cause this symptom could be input rate exceeds the capacity of the router,
or input queues exceed the size of output queues. The solution to this problem is as follows:
1
Increase the output queue size on common destination interfaces for the interface that is dropping packets.
Use the hold-queue
number
out interface configuration command. Increase these queues by small increments
(for instance, 25 percent) until you no longer see drops in the show interfaces command output. The default
output hold queue limit is 40 packets.
2
Reduce the input queue size, using the hold-queue
number
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.
Serial Lines: Increasing Input Errors in Excess of 1 Per Cent of Total Interface
Traffic
If input errors appear in the show interfaces serial command output, there are several possible sources of those
errors. The most likely sources, along with possible solutions, are summarized in the following table.
Cisco ASR 1000 Series Aggregation Services Routers SIP and SPA Software Configuration Guide, Cisco IOS
XE Everest 16.5
332
OL-14127-17
Troubleshooting the Serial SPAs
Serial Lines: Increasing Input Drops on Serial Link