Queuing Mechanisms Objectives

CQ Design Guideline

• Configure the amount to remove from a queue in each round to configure the proportional “weight” of the queue.
• The amounts to remove should approximate a small multiple of the interface’s MTU.
• The ratio between largest and smallest queue should be a small positive integer, not more than 10:1.

Delay vs. Bandwidth Allocation

• BW (Queue 1) = bc1/(bc1+bc2+bc3) = 4500/9000 = 50%
• Delay (Queue 1) = (bc2+bc3)/Bandwidth = 562ms

• 4500

• 3000

• 1500

• 5999

• 4499

• 2999

• Worst-case Delay (Queue 1) = [(bc2+1499) +(bc3+1499)]/Bandwidth = 937ms

• Round-
• Robin
• Scheduler

• 64 kbps

• Queue 1

• Queue 2

• Queue 3

• MTU=1500

Worst-Case Delay

• MTU=1500, byte count (4500, 3000, 1500) Max(delay)=(5999+4499)*8/64000=1312 ms
• MTU=1000, byte count (4500, 3000, 1500) Max(delay)=(5499+3999)*8/64000=1187 ms
• MTU=250, byte count (450, 300, 150) Max(delay)=(699+549)*8/64000=156 ms Expected delay=(500+500)*8/64000=125 ms
• Custom queuing is not appropriate for a low-delay environment. Changing the MTU and byte counts might be a workaround.

Benefits and Drawbacks of Custom Queuing

• Benefits
• Guarantees throughput to traffic classes (prevents starvation between traffic classes)
• Supported on most platforms
• Supported in all IOS versions (above 10.0)
• Drawbacks
• All drawbacks of FIFO queuing within a single class
• Manual configuration of classification on every hop
• Inaccurate bandwidth allocation
• High jitter due to implementation of scheduling