QoS

Question 1

What is the command to configure a voice VLAN on a switchport

Answer 1

switchport voice vlan vlan

Question 2

Jitter is defined as:

Answer 2

The variation in one-way delay between packets sent by the same application

Question 3

Explain Random Early Detection (RED)

Answer 3

To prevent TCP global synchronization, once the amount of traffic in queue reaches a certain threshold, the device will start randomly dropping packets from select TCP flows.

Those TCP flows that dropped packets will reduce the rate at which they are sending traffic, but TCP global synchronization is avoided.

In standard RED, all kinds of traffic are treated the same, but with WRED (Weighted RED), you can control which packets are dropped depending on the traffic class

Question 4

Which of the following are recommended standards for acceptable interactive video quality?

a) Delay: 30 ms or less
b) Delay: 150 ms or less
c) Jitter: 30 ms or less
d) Jitter: 50 ms or less
e) Loss: 1% or less
f) Loss: 2% or less

Answer 4

B, C, E

Question 5

Which of the following is a negative effect of tail drop?

a) TCP sliding window
b) RED
c) WRED
d) TCP global synchronization

Answer 5

D

Question 6

Which of the following is the default manner of forwarding queued packets?

a) FIFO
b) LIFO
c) LILO
d) FILO

Answer 6

A) FIFO

Question 7

Which of the following is a congestion avoidance method that drops lower-priority packets if network congestion is detected?

a) WFQ
b) RED
c) FIFO
d) WRED

Answer 7

d) WRED

Question 8

Define classification

Answer 8

Organizing network traffic into traffic classes, in order to be able to prioritize certain types of traffic

Question 9

Name an L2 header field and an L3 header field that can be used to identify high/low priority traffic

Answer 9

L2: PCP (Priority Code Point) field of dot1q tag header (so only applicable when there is VLAN tagging)

L3: DSCP (Differentiated Services Code Point) field of IP header

Question 10

IEEE 802.1p specifies that traffic with a PCP == 0 is:

Answer 10

Best-effort traffic. There is no guarantee that data is delivered or that it meets any QoS standard. This is regular traffic

Question 11

IEEE 802.1p specifies that traffic with PCP == 3 is:

Answer 11

Critical applications traffic.

IP phones mark call signaling traffic (used to establish calls) as PCP-3. They mark the actual voice traffic as PCP-5

Question 12

IEEE 802.1p specifies that traffic with PCP == 5 is:

Answer 12

Voice traffic. Specifically, the actual voice traffic, excluding the call signaling traffic.

Question 13

IEEE 802.1p specifies that call signaling traffic is marked as PCP-?

Answer 13

PCP-3, critical applications traffic

Question 14

IEEE 802.1p specifies that voice traffic is marked as PCP-?

Answer 14

PCP-5, voice traffic

Question 15

IEEE 802.1p specifies that by default traffic is marked as PCP-?

Answer 15

PCP-0, best effort

Question 16

T/F: Because the PCP field is only found in the dot1q header, it can only be used in trunk links and access links with a voice VLAN

Answer 16

T

The PCP header field is only found in the dot1q (VLAN) tag.

In addition, traffic going between two routers has no dot1q tagging, so it can’t be marked with PCP fields

Question 17

T/F: Because the PCP field is found in the L3 IP header, it is more versatile than the DSCP field of the L2 MAC header.

Answer 17

F

The PCP field is found at L2 header, and the DSCP field is found at the L3 header

Question 18

The IP ToS (Type of Service) byte is composed of these two fields:

Answer 18

DSCP and ECN

(Differentiated Code Services Point) (Explicit Congestion Notification)

Question 19

DSCP: DF is used for:

Answer 19

Best-effort traffic

DSCP == 000000 , or 0

Question 20

DSCP: EF is used for:

Answer 20

Traffic that requires low loss/latency/jitter

DSCP == 101110 , or 46

Question 21

Describe the AF DSCP tag

Answer 21

AF defines four traffic classes. All packets in a class have the same priority. Within each class, there are 3 levels of drop precedence. The higher the drop precedence the more likely the packet will be dropped during connection

Question 22

How is an AF priority tag written:

Answer 22

AFXY

Where X is the decimal value of the first 3 bits (which class), and Y is the decimal value of the drop precedence (bits 4 and 5)