Implementation – Low Latency Queueing

CBT Nuggets Lab Example



Under interface turn on NBAR (Internet facing Serial interface)

ip nbar protocol-discovery

load-interval 60 (Default 5 min average… lower average to 60 seconds instead)

1. Class Map to match protocol HTTP (NBAR)

2. Class Map to match protocol FTP (NBAR)

3. Single Policy Map to mark traffic. (MARK_TRAFFIC)

4. For HTTP Class Map: set dscp AF21 for HTTP

5. For FTP Class Map: set dcsp AF11 for FTP

6. show policy-map to review

7. service-policy input MARK_TRAFFIC (Marking is typically done on the INBOUND interface from the source.)

8. show policy-map interface eth0 (applied interface)


On R2

1. Class Map to match DSCP AF11 and AF21 for HTTP and FTP

2. Policy Map named ‘LLQ’ class match AF11 (FTP)

Guarantee FTP bandwidth

bandwidth bitrate/percent (10 percent for FTP) (CBWFQ)

bandwidth remaining percent 10 (Look at 75 percent of interface, assign 10 percent of remaining bandwidth) You will never breach the 75 percent best practice)

class class-default fair-queue (Enable WFQ, this is now a CBWFQ setup)


For HTTP – AF21

Not just bandwidth, but priority bandwidth!

priority percent 70

On the outbound/local eth0/0 direction interface on R2:

service-policy output LLQ (Policy Map name)

Only an output feature!

class mark and police only on an inbound interface

75 percent rule applies to Serial/WAN

Eth is 65 percent (Legacy?)

max-reserved-bandwidth – change default reserved bandwidth value! be careful!

Remember that this only applies when there is congestion! Eth interface of 100Mbps, may not actually be congested. Issue is across the cloud/internet…

On the WAN interface of the exit router on R3 we need to apply QoS….

R3 Serial 0/0

policy-map LLQ


priority percent 100

int Serial 0/0

service-policy output LLQ

max-reserved-bandwidth 100 (Lab!!!)

FTP will eventually die due to this configuration!

This is true end to end QoS.

642-642 QoS Video Training: Congestion Management: Concepts & Design

Congestion Management: Concepts & Design

Legacy Queueing

First In First Out (FIFO)

  • Number of Queues: 1
  • Method: FIFO
  • Delay Guarantee: No
  • Bandwidth Guarantee: No
  • Recommended for Voice: No

Priority Queueing (PQ)

  • Number of Queues: 4 (High, Medium, Normal, Low) (High queue is priority, other 3 queues will be ‘starved’)
  • Method: Strict Priority
  • Delay Guarantee: Yes, but only for High Priority
  • Bandwidth Guarantee: No
  • Recommended for Voice: Somewhat

Custom Queueing (CQ)

  • Number of Queues: 16 (Each queue has a fixed amount of bytes set)
  • Method: Round Robin (Can be bad….Voice can be delayed as the queues are processed in turn)
  • Delay Guarantee: No
  • Bandwidth Guarantee: Yes
  • Recommended for Voice: No
  • (*Uses Round Robin to send traffic into queues, when queue is hit the set amount of bandwidth is assigned)
  • (*Not recommended for time sensitive applications)

Weighted Fair Queueing (WFQ)

  • Number of Queues: Per Flow
  • Method: Weighted Fair
  • Delay Guarantee: No
  • Bandwidth Guarantee: No
  • Recommended for Voice: No
  • Low talkers are given the fair opportunity to speak and are not choked out by continual high talkers.

Modern Queueing

Class Based Weighted Fair Queueing (CBWFQ)

  • Number of Queues: Up to 256
  • Method: N/A
  • Delay Guarantee: No
  • Bandwidth Guarantee: Yes
  • Recommended for Voice: No
  • Percentage can be set on queue as apposed to number of bytes
  • Class-Default = Anything that you havent specified.
  • ‘Leftover traffic’ Queueing with WFQ. CQ is used for everything else.

Low Latency Queueing (LLQ)

  • Number of Queues: 1 PQ and CBWFQ (PQ can have a bandwidth limit to avoid choking out all bandwidth just for voice)
  • Method: N/A
  • Delay Guarantee: Yes for PQ only
  • Bandwidth Guarantee: Yes
  • Recommended for Voice: Yes

QoS Design: User Expectations

AVVID Strategy from Cisco – http://www.cisco.com/web/offer/powernow/docs/ipcomm/Cisco_AVVID_IPComm.pdf

Goal: Voice and Video are not to impact existing traffic types.

LLQ Bandwidth Provisioning

General rule of thumb from Cisco..

  • Priority 33% (Voice and Video – Policed Priority/Strict Priority)
  • Sum of all guarantees = 75%
  • 75-33 = 42% left for everything else
  • Total Link Capacity = 100%