notes: Congestion Management

 

- Prioritization, protection and isolation of traffic based on markings.

Software Queues and Hardware Queues

The queues created on an interface by the popularly known queuing tools are called software queues, as these queues are implemented in software. However, when the queuing scheduler picks the next packet to take from the software queues, the packet does not move directly out the interface. Instead, the router moves the packet from the interface software queue to a small hardware FIFO (first-in, first-out) queue on each interface. Cisco calls this separate, final queue either the transmit queue (Tx queue) or transmit ring (Tx ring), depending on the model of the router; generically, these queues are called hardware queues.

Hardware queues provide the following features:

■ When an interface finishes sending a packet, the next packet from the hardware queue can be encoded and sent out the interface, without requiring a software interrupt to the CPU—ensuring full use of interface bandwidth.
■ Always use FIFO logic.
■ 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 lengths mean packets are more likely to be in the controllable software queues, giving the software queuing more control of the traffic leaving the interface.

Note:  The only function of a hardware queue that can be manipulated is the length of the queue.

Queuing tools decide how packets are emptied from an interface’s output queue. Several queuing tools are available in the Cisco IOS Software:

1.  First-In, First-Out (FIFO): The default queuing mechanism on high-speed interfaces (that is, greater than 2.048 Mbps), which does not reorder packets.

2.  Weighted Fair Queuing (WFQ): The default queuing mechanism on low-speed interfaces, which makes forwarding decisions based on a packet’s size and Layer 3 priority marking.

3.  Low latency queuing (LLQ): The preferred queuing method for voice and video traffic, in which traffic can be classified in up to 64 different classes, with different amounts of bandwidth given to each class; includes the capability to give priority treatment to one or more classes.

4.  Priority queuing: A legacy queuing approach with four queues, in which higher-priority queues must be emptied before forwarding traffic from any lower-priority queues.

5.  Custom queuing: A legacy queuing approach that services up to 16 queues in a round-robin fashion, emptying a specified number of bytes from each queue during each round-robin cycle.

6.   Class-based weighted fair queuing (CBWFQ): Similar to LLQ, with the exception of having no priority queuing mechanism

7.  IP RTP priority: A legacy queuing approach for voice traffic that placed a range of UDP ports in a priority queue, with all other packets treated with WFQ