Queuing Components

Forwarded Packets

Forwarded Packets

• The hardware queuing system always uses FIFO queuing.

• The software queuing system can be selected and configured depending on the platform and Cisco IOS version.

Queuing on routers is necessary to accommodate bursts when the arrival rate of packets is greater than the departure rate, usually because of one of these two reasons:

■ Input interface is faster than the output interface

■ Output interface is receiving packets coming in from multiple other interfaces

Initial implementations of queuing used a single FIFO strategy. More complex scheduling mechanisms were introduced when special requirements needed routers to differentiate between packets of different levels of importance.

Queuing was split into these two parts:

■ Hardware queue: Uses FIFO strategy, which is necessary for the interface drivers to transmit packets one by one. The hardware queue is sometimes referred to as the transmit queue (TxQ).

■ Software queue: Schedules packets into the hardware queue based on the QoS requirements.

Forwarded Packets

Forwarded Packets

• The hardware queuing system always uses FIFO queuing.

• The software queuing system can be selected and configured depending on the platform and Cisco IOS version.

The figure illustrates these actions that have to be taken before a packet can be transmitted:

■ Most queuing mechanisms include classification of packets.

■ After a packet is classified, a router has to determine whether it can put the packet into the queue or drop the packet. Most queuing mechanisms will drop a packet only if the corresponding queue is full (tail drop). Some mechanisms use a more intelligent dropping scheme, such as weighted fair queuing (WFQ), or a random dropping scheme, such as weighted random early detection (WRED).

■ If the packet is allowed to be enqueued, it will be put into the FIFO queue for that particular class.

■ Packets are then taken from the individual per-class queues and put into the hardware queue.

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