What is queue? How software and hardware queue works? Congestion management using Queuing.

Routers used a queue to store traffic until it can be processed or serialized. switches and routers interfaces have ingress (inbound) queues and egress (outbound) queues. An ingress (inbound) queue stores packets until the switch or router CPU can forward the data to the appropriate interface.




Cisco routers can be configured to perform well queuing for packets that are waiting to exit an interface. Queues are buffers in devices that hold data to be processed. Queues provide bandwidth reservation and prioritization of traffic as it enters or leaves a network device. If the queues are not emptied, they overflow and drop traffic.

We have two types of queues and fancy queueing QoS tools like CBWFQ  & LLQ.

Hardware queue 

Software queue 

Hardware queues provide the following features:

  • When an interface finishes sending a packet, the next packet from the hardware queue can be encoded and send out the interface, without ensuring a software interrupt to the CPU—ensuring full use interface bandwidth.
  • Always use FIFO logic first in first out.
  • Cannot be affected by IOS queuing tools.
  • IOS automatically shrinks the length of the hardware queue to a smaller length than the default when a queuing tool is present.
  • Short hardware queue length means packets are more likely to be in the controllable software queues, giving the software queuing more control of the traffic leaving the interface.
  • The only function of a hardware queue that can be manipulated is the length of the queue.

Congestion management using Queuing

Congestion can occur at any point in the network where there are points of speed mismatches or aggregation, Speed mismatches are the most typical cause of congestion. when the packets traveling from LAN to WAN. Generally, a full hardware queue indicates interface congestion, and software queuing is used to manage it. The software queuing system can be selected and configured depending on the platform and cisco IOS version. Queuing manages congestion to provide bandwidth and delay guarantees.



Creation of queues. Assignment of packets to those queues based on the classification of the packets, and scheduling of the packets in a queue for transmission. Allow us to control congestion by determining the order in which packets are sent out an interface based on priorities assigned to those packets. 

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