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CCNA: Enterprise Networking, Security, and Automation > Module 9: QoS Concepts > Flashcards

Module 9: QoS Concepts Flashcards

1
Q

Name three types of congestion points for network traffic.

A
  1. Aggregation - Where multiple traffic streams combine into a single path.
  2. Speed mismatch - When traffic moves between links with differing bandwidths.
  3. LAN to WAN - The transition from a local area network to a typically slower wide area network like the Internet.
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2
Q

What is Quality of Service (QoS)?

A

A technique for prioritising the flow of traffic in congested networks.

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3
Q

What is Delay in terms of network traffic?

A

Delay, or latency, is the time it takes for a packet to travel from the source to a destination.

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4
Q

What is fixed delay?

A

The specific amount of time a process takes, such as the process of placing a bit on the transmission media.

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5
Q

What is variable delay?

A

The unspecified and variable amount of time a process takes, such as how long a packet is queued before being transmitted on a link.

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6
Q

What is Code Delay? Is it a fixed or variable delay?

A

The fixed amount of time it takes to compress data at the source before transmitting to the first internetworking device, typically a switch.

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7
Q

What is Packetisation Delay? Is it a fixed or variable delay?

A

The fixed time it takes to encapsulate a packet with all the necessary header information.

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8
Q

What is Queing Delay? Is it a fixed or variable delay?

A

The variable amount of time a frame or packet waits to be transmitted on the link.

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9
Q

What is Serialisation Delay? Is it a fixed or variable delay?

A

The fixed amount of time it takes to transmite a frame onto the wire.

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10
Q

What is Propagation Delay? Is it a fixed or variable delay?

A

The variable amount of time it takes for the frame to travel between the source and the destination.

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11
Q

What is De-jitter Delay? Is it a fixed or variable delay?

A

The fixed amount of time it takes to buffer a flow of packets and then send them out in evenly spaced intervals.

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12
Q

What is Jitter?

A

The variation in the delay of packets received.

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13
Q

True or false?
At the sending side packets are sent at a variable rate.

A

False. They are sent in a continuous stream with the packets evenly spaced apart.

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14
Q

True or false?
Removing the delay in sending traffic also removes the jitter.

A

False. Both delay and jitter need to be controlled and minimised to support real-time and interactive traffic.

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15
Q

What is Packet Loss?

A

The loss of packets during transmission.

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16
Q

What can cause packet loss in an otherwise connected transmission?

A

Congestion in network devices such as switches and routers may fill up their memory, causing them to drop packets.

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17
Q

Why might QoS mechanisms be important on frequently congested devices?

A

In order to ensure that time-sensitive packets, like real-time voice and video, will not be dropped. Instead the network devices will drop non time-sensitive packets, such as email and web browsing, which can be resent without as much impact.

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18
Q

Which type of traffic tends to consume a large portion of network capacity?

A

Data

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19
Q

Which type of traffic requires at least 384 Kbs of bandwidth?

A

Video

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20
Q

Which type of traffic is unpredictable, inconsistent, and bursty?

A

Video

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21
Q

Which type of traffic can be predictable and smooth?

A

Voice

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22
Q

Which type of traffic cannot be retransmitted if lost?

A

Voice

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23
Q

Which type of traffic must receive a higher UDP priority?

A

Voice

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24
Q

What does FIFO queuing stand for, and how does it work?

A

First In First Out (FIFO) works on a first-come first-served bases. It is effectively ‘no Quality of Service’ (QoS)

25
Q

What does WFQ queuing stand for, and how does it work?

A

Weighted Fair Queuing (WFQ) provides bandwidth to traffic such as voice first, then distributes remaining bandwidth among other transfers.

26
Q

True or false?
WFQ queuing cannot support tunneling or encryption.

A

True, these features modify packet information for classification of the packets.

27
Q

What does CBWFQ queuing stand for, and how does it work?

A

Class-Based Weighted Fair Queuing (CBWFQ) allows for user-defined traffic classes, based on criteria for certain protocols, ACLs and/or interfaces. A FIFO queue is created for each class, and packets matching the criteria of that class get placed within that FIFO queue.

28
Q

What does LLQ queuing stand for, and how does it work?

A

LLQ (Low Latency Queuing) integrates strict priority queuing (PQ) into CBWFQ to ensure delay-sensitive traffic like voice is sent first, reducing jitter and improving real-time communication.

29
Q

Which queuing algorithm simultaneously schedules interactive traffic to the front of a queue to reduce response time?

A

WFQ (Weighted Fair Queing)

30
Q

Which queuing algorithm provides support for user-defined traffic classes?

A

Class-Based Weighted Fair Queuing (CBWFQ)

31
Q

Which queuing algorithm is effective for large links that have little delay and minimal congestion?

A

WFQ (Weighted Fair Queing)

32
Q

Which queuing algorithm classifies traffic into different flows based on packet header addressing?

A

WFQ (Weighted Fair Queing)

33
Q

Which queuing algorithm allows delay-sensitive data such as voice to be sent before packets in other queues?

A

LLQ (Low Latency Queuing)

34
Q

Which queuing algorithm applies priority, or weights, to identify traffic and classify it?

A

WFQ (Weighted Fair Queing)

35
Q

What is the QoS ‘Best-effort model’?

A

This is not really an implementation of QoS as it is not having QoS configured. It should be used when QoS is not required.

36
Q

What is the QoS ‘Integrated Services (IntServ)’ model?

A
  • Provides very high QoS to IP packets with guaranteed delivery.
  • Defines a signaling process for applications to state they require special QoS for a period and bandwidth should be reserved for them.
  • Can severally limit the scalability of a network.
37
Q

What is the QoS ‘Differentiated Services (DiffServ)’ model?

A
  • Network devices recognise traffic classes and provide different levels of QoS to them.
  • Provides high scalability and flexibility.
38
Q

Which QoS model provides per-request policy admission control?

A

Integrated Services (IntServ)

39
Q

Which QoS model requires no special QoS mechanisms?

A

Best-effort

40
Q

Which QoS model provides many different levels of quality?

A

Differential Services (DiffServ)

41
Q

Which QoS model uses explicit end-to-end resource admission control?

A

Integrated Services (DiffServ)

42
Q

Which QoS model is the most scalable?

A

Best-effort

43
Q

What does the queing mechanism WRED stand for, and how does it work?

A

Weighted Random Early Detection (WRED) allows for congestion avoidance on network interfaces by providing buffer management. It allows for TCP traffic to be throttled back before buffers are exhuasted.

44
Q

What approaches can be taken to prevent packet loss and drops of packets in sensitive applications?

A
  • Increaes link capacity
  • Guarantee enough bandwidth and increase buffer space for the drop-sensitive applications via WFQ, CBWFQ, and LLQ
  • Drop lower-priority packets before congestion occurs, via mechanisms like WRED
45
Q

What three parts are there of the QoS Sequence?

A
  1. Classification and marking
  2. Congestion avoidance
  3. Congestion management
46
Q

What is classification and marking in QoS?

A

It is the first part of the QoS sequence, whereby the process of inspecting packets and classifyting them into different classes takes place. They are then marked so that they can be identified by these classes.

47
Q

What is congestion avoidance in QoS?

A

It is the second part of the QoS sequence, whereby marked traffic is policed and selectively dropped where necessary to avoid congestion.

48
Q

What is congestion management in QoS?

A

It is the third part of the QoS sequence, whereby congestion has already occured despite congestion avoidance techniques, so it now must be placed into queues based on its class, and then scheduled and shaped so that resources are allocated to them based on their class policies.

49
Q

How are packets classified and marked in QoS? At what layers does this take place?

A

It varies by implementation, but they all rely on a Marking Field applied to the data. This takes place at either Layer 2 or Layer 3.

50
Q

What IEEE standard supports VLAN tagging at Layer 2 on Ethernet networks, and adds two fields to the frame, one of which is used for QoS marking?

A

The 802.1Q standard

51
Q

The 802.1Q standard adds what field to the Ethernet Frame for QoS marking? How many bits is it?

A

The User Priority (PRI) field, used for Class of Service (CoS) value markings.

52
Q

The CoS value markings allow for how many levels of priority? Think about how many bits the PRI field contains

A

8 levels from 0-7. The PRI field contains 3-bits so values 000 to 111 in binary.

53
Q

Marking at Layer 3 takes place within the IPv4 and IPv6 Packet Headers via an 8-bit field. What is the name for the IPv4 field and the IPv6 field (they are not the same)

A

IPv4 = Type of Service (ToS) field
IPv6 = Traffic Class (TC) field

54
Q

What RFC standard specifies the IPv4 and IPv6 field used for QoS classification?

A

RFC 2472, which replaced RFC 791

55
Q

In IPv4 and IPv6 packet classification, what is the DSCP field and what does it do?

A

It is part of the ToS/TC field that allows for assigning packets to one of three categories
- Best-Effort (BE)
- Expedited Forwarding (EF)
- Assured Forwarding (AF)

56
Q

What is the DSCP category ‘Best-Effort’ and what DSCP value does it have?

A

It is essentially ‘No QoS’ and applied by default to all packets. It has a DSCP value of 0.

57
Q

What is the DSCP category ‘Expedited Forwarding (EF)’ and what DSCP value does it have? Why does it have this DSCP value?

A

It is used for voice packets. It has the DSCP value of 46. It has this value because in binary the first three bits are 101, which directly maps to the Layer 2 CoS value 5 used for voice traffic, because 46 = 101110.

58
Q

What is the DSCP category ‘Assured Forwarding (AF)’ and what DSCP values does it have?

A

Used to indicate class (1-4, with 4 being the best) and drop preference (Low, Medium, High). It has many DSCP values as shown in the image.

59
Q

What is the name given to the 3 most significant bits in the DSCP field? What do they indicate?

A

They are called the Class Selector (CS) bits and indicate the class given to the packet. They directly map to the CoS field to maintain compatibility CoS, as demonstrated in the image.

CCNA: Enterprise Networking, Security, and Automation (9 decks)

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