Quality of Service

Question 1

First-in First-out (FIFO Queuing)

Answer 1

Buffers begin to fill and the packets are queued up in the order in which they’re received

Question 2

Classification, Marking and Queuing

Answer 2

Prioritizing some packets over others according to how they tolerate latency and loss

Question 3

Policing and Shaping

Answer 3

Policing: To avoid queue delays, policing drops packets

Shaping: Stores in a queue until bandwidth is available. Does not drop packets.

Question 4

Differentiated Services (DiffServ)

Answer 4

A QoS architecture that uses the TOS byte of the IP field to classify a packet

Question 5

Per-Hop Behavior Groups

Answer 5

A description of the externally observable forwarding behavior. Classifying packets by priority.

Question 6

Class Selector (CS)

Answer 6

Offers 8 different classes ranging from CS0 to CS7.

CS0 is the highest priority, default class, and is best-effort traffic. CS1 and LE are intended for lower effort traffic

Question 7

Assured Forwarding

Answer 7

Allows you to specify not only the priority of a packet, but also the likelihood that it will be dropped while it’s waiting in a queue.

Use formula 8x +2y to find the DSCP value of any AF

example AF11 would be (8x1)+(2x1)=10

Question 8

Expedited Forwarding

Answer 8

Single code point with the value 46. It’s intended for high priority traffic that should be bandwidth limited. An example would be VoIP.

Question 9

Class Map

Answer 9

Can match packets according to a variety of criteria, such as IP address, protocol, port number, source mac address, packet length, DSCP value, or 801.1Q CoS value

Default class-map class-default matches all traffic

Question 10

Policy Maps

Answer 10

Mark packets and perform policing, shaping, and queuing. They are applied to interfaces and applied either inbound or outbound traffic

Question 11

Wireless QOS

Answer 11

WLC has 4 preset QOS profiles that define max DSCP values for wireless client traffic

Profile Purpose DSCP

Platinum Voice EF
Gold Video AD41
Silver Best Effort CS0
Bronze Background AF11

Question 12

WiFi Multimedia (WMM)

Answer 12

Clients that support WMM can mark their WLAN frames with one of 4 WMM values

Access class WMM 802.11e Value Trans Cos

Voice 6 5
Video 5 4
Best Effort 0 0
Background 1 1

Question 13

Single-Rate, Two-color Policing

Answer 13

Exceeding traffic is red and conforming traffic is green

Question 14

Single-Rate, Three-color Policing

Answer 14

Adds yellow packets that are considered exceeding but still allowed. Uses 2 tokens instead of one:

The committed burst bucket (bc) and the excess burst bucket (be)

Question 15

Two-Rate Policing

Answer 15

Define a CIR and a peak information rate (PIR). The CIR has a bc bucket and the PIR has a be bucket

Question 16

Class-based Weighted Fair Queuing (CBWFQ)

Answer 16

It can create up to 256 queues for 256 different classes. Not good for delay-sensitive traffic such as voice. For the size of the queue, you have 2 choices: tail drop and WRED