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How to prevent the input queue drops and output queue drops in Catalyst switches running Cisco IOS system software


Core issue

When a packet enters the device, the device attempts to forward it at interrupt level. If a match cannot be found in an appropriate cache table, the packet is queued in the input queue of the incoming interface to be processed. Some packets are always processed. With the appropriate configuration and in stableHowever, if the input queue is full, an input queue drop results. 

Output queue drops are caused by a congested interface. For example, the traffic rate on the outgoing interface cannot accept all packets that must be sent out.

This is a sample output of the show interfaces command:

Switch#show interfaces gigabitEthernet 1/0/1

GigabitEthernet1/0/1 is up, line protocol is up


Input queue: 0/75/0/0 (size/max/drops/flushes);

Total output drops: 0

Output queue: 0/40 (size/max) ...

Also, both the ingress and egress queues are serviced by SRR, which controls the rate at which packets are sent. On the ingress queues, SRR sends packets to the stack ring. On the egress queues, SRR sends packets to the egress port.

Scheduling services the queues based on their configured shaped round robin (SRR) weights. One of the ingress queues is the priority queue, and SRR services it for its configured share before the other queue is serviced.


In order to avoid drops, you can configure the SRR, which controls the rate at which packets are sent.

Refer to the SRR Shaping and Sharing section of Configuring QoS for more information.

Refer to Troubleshooting Input Queue Drops and Output Queue Drops for more information on input queue drops and output queue drops.