This document describes the output queue overflow on an
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This document is not restricted to specific software and hardware
Each interface owns an output queue onto which the Routing Processor
(RP) places outgoing packets to be sent on the interface. Sometimes the rate of
outgoing packets placed on the output queue by the RP exceeds the rate at which
the interface can send the packets.
Each output queue has a size that indicates the maximum number of
packets that may be held on the queue. Once the output queue becomes full (the
max number of packets is on queue), the RP drops additional outgoing packets.
The output queue overflow scenario occurs most often when the RP tries to send
many packets at once.
Assume a remote source-route bridging / Transmission Control Protocol
(RSRB/TCP) local-ack configuration:
The RP is responsible for flow control of the Logical Link Control,
type 2 (LLC2) sessions.
If the RP is local-acking 50 LLC2 sessions and the TCP pipe is
suddenly closed, the RP sends disconnect requests (DISCs) for each LLC2
50 DISCs are placed on the output queue of the output interface, but
some may be dropped if the output queue
The following show interface
<interface-identifier> output shows
the current output queue levels and the number of outgoing packets dropped:
dspu-7k#show interface channel 4/2
Channel4/2 is up, line protocol is up
Hardware is cxBus IBM Channel
MTU 4472 bytes, BW 98304 Kbit, DLY 100 usec, rely 255/255, load 1/255
Encapsulation CHANNEL, loopback not set, keepalive not set
Last input 1:09:19, output 1:10:29, output hang never
Last clearing of "show interface" counters never
Output queue 35/40, 67 drops; input queue 0/75, 0 drops
5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
7668 packets input, 252270 bytes, 0 no buffer
Received 0 broadcasts, 0 runts, 0 giants
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
7661 packets output, 258070 bytes, 0 underruns
0 output errors, 0 collisions, 0 interface resets, 0 restarts
0 output buffer failures, 0 output buffers swapped out
The show interface
provides the following information:
The output queue x/y counter shows the current
number of packets on the output queue "x" and the current size of the output
The drops counter indicates the number of outgoing
If the current number of packets on the output queue is consistently
at or greater than 80 percent of the current size of the output queue, the size
of the output queue may require tuning to accommodate the outgoing packet rate.
Even if the current number of packets on the output queue never seems
to approach the size of the output queue, bursts of packets may still overflow
If the drops counter increments at a high rate, the
size of the output queue may require tuning to accommodate the
Note: The size of the output queue may be tuned using the
hold-queue interface configuration command, as shown
in the example below.
interface channel 4/2
hold-queue 125 output