QoS

Question 1

Traditional vs. Converged Networks

Answer 1

On old traditional networks, data, voice, & video had their own separate network infrastructure and did not impact each other

On modern networks, data/voice/video run over the same shared infrastructure

This enables cost savings & advanced features for voice/video

Data/voice/video are all fighting for the same shared bandwidth

Question 2

Voice/Video Quality Requirements

Answer 2

Voice & traditional standard definition video packets must meet the recommended requirements to be an acceptable quality call:

• –Latency (delay): < 150 ms
• –Jitter (variation in delay): < 30 ms
• –Loss < 1%

These are one way requirements, meaning a packet sent from a phone in HQ has 150ms to reach the phone in the branch, & vice versa

HD video has stricter requirements

Question 3

FIFO

Answer 3

First In First Out:
Whenever congestion is experienced on a router/switch, packets are sent out in a FIFO manner by default

Congestion can be experience whenever it is possible for packets to come in quicker than they can be sent out

Question 4

Effects of Congestion

Answer 4

Congestion causes delay to packets as they wait in the queue

As the size of the queue changes it causes jitter

There is a limit to the size of the queue. If a packet arrives when the queue is full the router will drop it

Voice & video calls (and applications) will be unacceptable quality of they do not meet their delay/jitter/loss requirements

Question 5

Mitigating Congestion

Answer 5

Add more bandwidth (obviously costs more money)

Use QoS to give better service to the traffic that needs it
—The router recognizes voice packets & moves them to the front of the queue to minimize delay

Question 6

Effects of QoS Queuing

Answer 6

Reduce latency/jitter/loss for particular traffic

VoIP was the original QoS driver
—Can also be used to give better service to data apps

If you give better service to one type of traffic, others get worse service

Designed to mitigate temporary periods of congestion
—If a link is permanently congested, get more bandwidth

Question 7

Classification & Marking

Answer 7

For a router/switch to give a particular level of service to a type of traffic…
It must recognize the traffic

Question 8

CoS

Answer 8

Class of Service:

Layer 2 Marking

There is a 3-bit field in the Layer 2 802.1q frame header which is used to carry the CoS QoS marking

Values of 0-7 can be set.
—Default value is 0 = best effort traffic

CoS 6 & 7 are reserved for network use

IP phones mark their call signaling traffic as CoS 3
—And voice payload as CoS 5

Question 9

DSCP

Answer 9

Differentiated Service Code Point:

The preferred classification & marking method
—Router can very quickly gather the info from a single byte in the IP header

The ToS (Type of Service) byte in the Layer 3 IP header is used to carry the DSCP QoS marking

6 bits are used = 64 possible values (0-63); each value has a nickname
—Default value is 0 = best effort

IP phones mark call signaling traffic as 24 (CS3) & voice payload as 46 (EF)

There are standard markings for other traffic types

• –26 = AF31 = Mission critical data
• –34 = AF41 = SD video

Question 10

The Trust Boundary

Answer 10

The switch should be configured to trust markings from the IP phone & pass them on unchanged

But mark traffic from the PC down to CoS 0 & DSCP 0

Question 11

QoS: ACLs

Answer 11

Can be used to recognize traffic based on Layer 3 & 4 info

Example:
SSH traffic going to & from the router 10.10.100.10 on TCP port 22

Should be done as close to the source as possible with a DSCP value added

Question 12

NBAR

Answer 12

Network Based Application Recognition:

Can be used to recognize traffic based on Layer 3 to 7 info

Signatures can be downloaded from Cisco & loaded on your router which will recognize well-known apps

Should be done as close to the source as possible with a DSCP value added

Question 13

CBWFQ

Answer 13

Class Based Weighted Fair Queuing:

Gives bandwidth guarantees to specified traffic types

Question 14

LLQ

Answer 14

Low Latency Queuing:

Is CBWFQ with a priority queue

Example:

• –You can give a priority value to voice & video
• –And use bandwidth guarantees on data applications

Question 15

MQC (Modular QoS CLI)

Answer 15

Cisco QoS configuration uses the MQC Modular QoS CLI

3 Sections:
Class Maps - define traffic to take action on
Policy Maps - take the action on that traffic
Service Policies - apply the policy to an interface

Question 16

Policing & Shaping

Answer 16

Policing & shaping can be used to control traffic rate
They both measure the rate of traffic through an interface & take an action if the rate is above a configured limit

Shaping:
—Buffers any excess traffic so the overall traffic stays within the desired rate limit

Policing:
—Drops or re-marks excess traffic to enforce the specified rate limit

Classification can be used to allow different rates for different traffic types

Question 17

Policing within Enterprises

Answer 17

Another use case for policing is worm/junk traffic mitigation

An enterprise can configure classification & marking to recognize worm/junk traffic like peer to peer file sharing applications

• –“Scavenger” traffic
• –Recommended DSCP value is DSCP 8 (CS1)

Policing can be used to rate limit junk traffic down to prevent it from taking bandwidth from business apps