了解排队拥塞管理队列实现组件硬件系统始终使用fifo.pdfVIP

Queuing Components • The hardware queuing system always uses FIFO queuing. • The software queuing system can be selected and configured depending on the tform and Cisco IOS version. The Software Queue • Generally, a full hardware queue indicates interface congestion, and software queuing is used to manage it. • When a packet is being forwarded, the router will bypass the software queue if the hardware queue has space in it (no congestion). Hardware Queue (TxQ) Size • Routers determ he length of the hardware queue based on the configured bandwidth of the interface. • The length of the hardware queue can be adjusted with the tx-ring-limit command. • Reducing the size of the transmit ring has two benefits: – It reduces the um amount of time that packets wait in the FIFO queue before being transmitted. – It accelerates the use of QoS in the Cisco IOS software. • Improper tuning of the hardware queue may produce undesirable results: – Long TxQ may result in poor performance of the software queue. – Short TxQ may result in a large number of interrupts, which cause high CPU utilization and low link utilization. Congestion on Software Interfaces • Subinterfaces and software interfaces do not have their own separate Tx ring, therefore no congestion can occur. – Dialers, tunnels, Frame Relay subinterfaces – They congest when their main hardware interface Tx ring congests • The Tx-ring state (full, not full) is, therefore, an indication of congestion for software interfaces. • Only hardware interfaces have a Tx ring. Queuing Implementations isco IOS • Priority queuing – Implementation of priority queuing – Four queues (high, medium, normal, low) • Custom queuing – Implementation of weighted round robin – Up to 16 queues – Threshold based on number of bytes