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Cisco CCNA 3/3: Enterprise Networking, Security, and Automation v7 > Chapter 9 - QoS Concepts > Flashcards

Chapter 9 - QoS Concepts Flashcards

1
Q

how does congestion of packets occur?

A
  • multiple communicate lines aggregate onto a single device and data is placed on few/slow outbound interfaces
  • when large data packets prevent smaller packets from being transmitted in a timely manner
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2
Q

a technique for QoS

A

classifying data into multiple queues (high, medium, low priority)

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3
Q

code delay

A
  • source of delay
  • fixed amount of time it takes to compress data at the source before transmitting to the first internetworking device, usually a switch
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4
Q

packetization delay

A
  • source of delay

- fixed time it takes to encapsulate a packet with all the necessary header info

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5
Q

queuing delay

A
  • source of delay

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

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6
Q

serialization delay

A
  • source of delay

- fixed amount of time it takes to transmit a frame onto the wire

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7
Q

propagation delay

A
  • source of delay

- variable amount of time it takes for the frame to travel btwn src and dst

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8
Q

de-jitter delay

A
  • source of delay

- 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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9
Q

jitter

A

variance in time delays btwn packets

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10
Q

what is playout delay buffer

A

helps compensate for jitter by buffering packets and then plays them out in a steady stream on the outbound interface

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11
Q

voice traffic characteristics

A

smooth
benign
drop and delay sensitive
UDP priority

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12
Q

voice traffic minimum requirements

A

latency ≤ 150ms
jitter ≤ 30 ms
loss ≤ 1% bandwidth (30-128 Kbps)

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13
Q

video traffic characteristics

A 
bursty
greedy
drop and delay sensitive
UDP priority
(unpredictable, inconsistent and bursty)
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14
Q

video minimum requirements

A

latency ≤ 200-400 ms
jitter ≤ 30-50 ms
loss ≤ 0.1-1% bandwidth (384 Kbps - >20Mbps)

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15
Q

what port does Real-Time Streaming Protocol use?

A

UDP 554

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16
Q

what ports does Cisco products use for voice traffic

A

UDP 16384-32767

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17
Q

data traffic characteristics

A

smooth/bursty
benign/greedy
drop and delay insensitive
TCP retransmits

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18
Q

what is FIFO

A 
First-In, First-Out
Queuing Algorithm
buffers and forwards packets in the order of their arrival
all packets are treated equally
fastest method of queuing
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19
Q

what type of queuing algorithm is used by default by all interfaces except serial interfaces at E1 and below

A

FIFO

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20
Q

what is WFQ

A

Weighted Fair Queuing
Queuing Algorithm
applies priority to identified traffic and classifies it into flows then determines how much bandwidth each flow is allowed relative to other flows
- low bandwidth traffic flows receive preferential service while high-volume traffic flows share remaining capacity equally among themselves

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21
Q

what does WFQ use in the packet header to classify traffic

A
  • source and dst IP add
  • MAC add
  • port number
  • protocol
  • ToS
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22
Q

limitations of WFQ

A
  • not supported with tunneling and encryption

- no precise control over bandwidth allocation

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23
Q

what is CBWFQ

A

Class-Based Weighted Fair Queuing

- allows users to define traffic classes based on match criteria like protocols, ACLs and input interfaces

24
Q

characteristics you can assign after defining a class to its match criteria

A
  • bandwidth
  • weight
  • maximum packet limit
25
Q

when using CBWFQ, what happens when maximum packet limit is reached?

A

it causes packet drop or tail drop to take effect

26
Q

what is a tail drop

A

a router discards any packets that arrives at the tail end of a queue that has completely used up its packet-holding resources (default mode)

27
Q

what is LLQ

A

Low Latency Queuing
Combines CBWFQ and strict PQ (priority queuing)
Allows delay-sensitive packets like voice to be sent before packets in other queues

28
Q

what queuing algorithm simultaneously schedules interactive traffic to the front of a queue to reduce response times?

A

WFQ

29
Q

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

A

FIFO

30
Q

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

A

WFQ

31
Q

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

A

CBWFQ

32
Q

what queuing algorithm applies priority, or weight, to identify traffic and classify it?

A

WFQ

33
Q

benefits of the QoS Best-Effort Model

A

most scalable and only limited by available bandwidth
no special QoS mechanism required
easiest and quickest model to deploy

34
Q

which QoS model provides no guarantees

A

Best-Effort model

35
Q

which QoS model provides per-request policy admission control

A

IntServ

36
Q

which QoS model requires no special QoS mechanisms

A

Best Effort

37
Q

which QoS model provides many different levels of quality

A

DiffServ

38
Q

which QoS model uses explicit end-to-end resource admission control

A

IntServ

39
Q

Which QoS model is the most scalable?

A

DiffServ

40
Q

what is WRED

A

weighted random early detection, Cisco IOS QoS mechanism

  • used as part of congestion avoidance tools
  • buffer management and allows TCP traffic to decrease or throttle back before buffers are exhausted
  • can drop lower-priority packets before congestion occurs
41
Q

what are the 3 categories of QoS Tools

A
  • classification and marking tools
  • congestion avoidance tools
  • congestion management tools
42
Q

In Classification and Marking Tools, what method is used to classify packets at Layer 2 and 3?

A

Interfaces, ACLs and class maps

43
Q

In Classification and Marking Tools, what method is used to classify packets at Layer 4-7

A

NBAR - Network Based Application Recognition

44
Q

marking a packet should be done as close to the _____ device as possible

A

source

45
Q

In Layer 2 traffic marking, where is the CoS marking located?

A

In the first 3-bits of the TCI (Tag Control Information) field - called the Priority (PRI) field

46
Q

In Layer 3 traffic marking, where are marks specified?

A

IPv4 - ToS (Type of Service)

IPv6 - Traffic Class

47
Q

what is the DSCP field

A

Differentiated Services Code Point

6-bit field in ToS for IPv4 that allows packet marking

48
Q

what is ECN field

A

Explicit Congestion Notification

2-bits in the ToS field for IPv4 that informs downstream routers that there is a congestion in the packet flow

49
Q

DSCP values are organized into three categories

A
  • Best-Effort (BE)
  • Expedited Forwarding (EF)
  • Assured Forwarding (AF)
50
Q

which DSCP values category uses the AFxy formula

A

Assured Forwarding

  • first 3-bits are used to designate class
  • 4th-5th bits are used to designate drop preference
51
Q

which DSCP values category has a value of 46

A

Expedited Forwarding

52
Q

examples of trusted endpoints

A

IP phones, wireless APs, videoconferencing gateways and systems, IP conferencing stations and more

53
Q

where can trust boundaries be established?

A
  • at trusted endpoints
  • at Layer 2 switches
  • at Layer 3 switches/routers
54
Q

traffic shaping

A
  • Cisco IOS QoS software to prevent congestion
  • retains excess packets in a queue and then schedules excess for later transmission over increments of time
  • produces a smooth packet output rate
55
Q

which detects when traffic rates reach a configured maximum rate and drops excess traffic

A

traffic policing

Cisco CCNA 3/3: Enterprise Networking, Security, and Automation v7 (9 decks)

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