Queuing Mechanisms Objectives

Lesson Review

• Which queuing mechanisms do Cisco routers support?
• When are software queuing mechanisms not used?
• How does TxQ length affect the software queuing system?

FIFO Queuing

• © 2001, Cisco Systems, Inc.

• QOS v1.0—3-

Objectives

• Upon completing this lesson, you will be able to:
• Describe FIFO queuing
• Describe the drawbacks of FIFO queuing
• Configure FIFO queuing on Cisco routers
• Monitor and troubleshoot FIFO queuing

FIFO Queuing

• The software FIFO queue is basically an extension of the hardware FIFO queue.

• FIFO Queuing System

• Hardware
• Queuing System

• All in one
• queue

• Queue 1

• FIFO
• Scheduler

• Interface

• Forwarded Packets

• Hardware Q

• Tail-drop

• All packets are classified into one class.

• Newly arriving packets are dropped if the queue is full.

• FIFO uses one single queue.

• Routers serve packets in the first-come, first-serve fashion.

Benefits and Drawbacks of FIFO Queuing

• Benefits
• Simple and fast (one single queue with a simple scheduling mechanism)
• Supported on all platforms
• Supported in all switching paths
• Supported in all IOS versions
• Drawbacks
• Unfair allocation of bandwidth among multiple flows
• Causes starvation (aggressive flows can monopolize links)
• Causes jitter (bursts or packet trains temporarily fill the queue)