Bypassing the Software Queue

Bypassing the Software Queue

• When a packet is being forwarded, the router will bypass the software queue if:
• The software queue is empty, and
• The hardware queue is not full

• Software Queue
• Empty?

• Hardware Queue
• Full?

• Hardware
• Queue
• (TxQ)

• Yes

• No

• Software
• Queuing
• System

• Yes

• No

Hardware Queue (TxQ) Size

• Routers determine the length of the hardware queue based on the configured bandwidth of the interface.
• Long TxQ may result in poor performance of the software queue.
• Short TxQ may result in a large number of interrupts which causes high CPU use and low link use.

Queuing Components

• Software Queuing System

• Hardware
• Queuing System

• Each queuing mechanism has three main components that define it:
• Classification (selecting the class)
• Insertion policy (determining whether a packet can be enqueued)
• Service policy (scheduling packets to be put into the hardware queue)

• Class 1?

• Class 2?

• Class n?

• Queue 1

• Queue 2

• Queue n

• Scheduler

• Interface

• Forwarded Packets

• Hardware Q

• Add/Drop

• Add/Drop

• Add/Drop

Summary

• Upon completing this lesson, you should be able to:
• Understand how queuing works on Cisco routers
• List the most used queuing mechanisms