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CCNP Encor > QoS > Flashcards

QoS Flashcards

1
Q

When does congestion occur ?

A

When a router or switch receives more traffic than it can forward.

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2
Q

What is FIFO queuing?

A

When a packet is stored and forwarded in the same order it was received.

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3
Q

What happens when the buffer fills up?

A

All subsequent traffic is dropped.

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4
Q

What is DiffServ?

A

Differentiated services - QoS architecture which populates TOS field with a DSCP value. This value is then used to match traffic to a PHB group.

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5
Q

What is a PHB group?

A

An arbitrary collection of values or factors which impact forwarding behaviour. These can include buffer size and bandwidth.

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6
Q

What is a class selector?

A

8 tier class model which maps to the DSCP field

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7
Q

What are the Class Selector code points?

A 
CS0
LE
CS1
CS2
CS3
CS4
CS5
CS6
CS7
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8
Q

Which of the 3 Code Selectors has the highest priority

CS0
CS1
LE

A

CS0 - considered best effort. The other 2 are considered undesirable.

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9
Q

What is the DSCP binary value of the CS0 code point?

A

000000

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10
Q

What is the DSCP binary value of the LE class selector?

A

000001

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11
Q

What is the DSCP binary value of the CS1 class selector?

A

001000

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12
Q

What is the DSCP binary value of the CS2 class selector?

A

010000

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13
Q

What is the DSCP binary value of the CS3 class selector?

A

011000

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14
Q

What is the DSCP binary value of the CS4 class selector?

A

100000

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15
Q

What is the DSCP binary value of the CS5 class selector?

A

101000

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16
Q

What is the DSCP binary value of the CS6 class selector?

A

110000

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17
Q

What is the DSCP binary value of the CS7 class selector?

A

111000

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18
Q

What is AF (assured forwarding)?

A

PHB group which allows you to prioritise traffic and drop probability during congestion.

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19
Q

How many classes does assured forwarding have?

A

12

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20
Q

What are the 12 AF classes?

A

AF11 AF12 AF13
AF21 AF22 AF23
AF31 AF32 AF33
AF41 AF42 AF43

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21
Q

Which AF class is of highest priority and most likely to be dropped?

A

AF43

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22
Q

Which AF class is lowest priority and least likely to be dropped?

A

AF11

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23
Q

Which AF class is lowest priority and most likely to be dropped?

A

AF13

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24
Q

Which AF class is lowest priority and most likely to be dropped?

A

AF13

25
Q

When is Assured Forwarding most beneficial?

A

For TCP applications which can tolerate packet loss.

26
Q

What is expedited forwarding (EF)?

A

PHB group with code point with value 101110

Best used for important traffic with low bandwidth I.e voice.

27
Q

What are the 4 PHB groups used by diffserv?

A

DF (direct forwarding) - best effort

EF (Expedited forwarding) - important, low bandwidth

AF (assured forwarding) - assurance of delivery under specified conditions

Class Selectors - 0,LE,1-7

28
Q

What is Layer 2 marking?

A

3 bit Class of service (CoS) used to mark Layer 2 traffic.

Sometimes called PCP field.

29
Q

What values can the layer 2 Cos field hold?

A

Priority 0-7

0 - best effort
1- low pri
2-7 - increasing in priority.

30
Q

Where in the network should QoS marking be applied?

A

As close to source as possible.

Sometimes the host itself

31
Q

What is a Class Map?

A

A mechanism for classifying packets via defined attributes.

I.e IP address, source mac, port and protocol numbers, DSCP and CoS tagging.

32
Q

What is ‘Next-Generation NBAR’?

A

A mechanism of deep packet inspection used for classifying hard to define traffic.

33
Q

What is the difference between a Class Map and a Policy Map

A

Both will mark traffic for QoS, but only Policy maps will police, queue and shape traffic.

34
Q

Where are policy maps applied?

A

To interfaces, matching both ingress and egress traffic.

35
Q

What are the 4 wireless layer 3 QoS profiles?

A

Platinum - EF
Gold - AF41
Silver - CS0
Bronze - AF11

36
Q

Where are wireless layer 3 QoS applied?

A

To the ssid

37
Q

What is the Committed Information Rate (CIR)?

A

The maximum amount of packets which can be sent before policing starts dropping packets

38
Q

What 3 policing actions can be applied to conforming or exceeding packets

A

Transmitted
Dropped
Marked with DSCP priority

39
Q

What is single rate, 2 colour policing?

A

Conforming traffic is marked green
Exceeding traffic is marked red.

Green traffic is forwarded and may be tagged

Red traffic is dropped or tagged low priority.

40
Q

What is a policing token bucket?

A

Counter applied to an interface based on the CIR.

As packets egress the port, counters are removed. Tokens replenish over time.

41
Q

What is single rate 3 colour policing?

A

3 tier system soft policy system

Green traffic is confirming and is sent (with optional priority tagging)

Yellow traffic is exceeding but still sent. It is marked as low pri DSCP

Red traffic is exceeding and dropped

42
Q

What are the 2 token buckets used by 3 colour policing?

A

BC - committed burst

BE - excess burst (manually defined)

43
Q

How does two-rate policing work?

A

Two-rate policing has a CIR (BC) and PIR (peak information rate) (BE) bucket.

Both buckets fill simultaneously. Traffic is deducted from BC bucket first (conforming) and from BE bucket of BC depletes.

If BC and BE buckets deplete; traffic is said to be violating.

44
Q

What is class based weighted fair queuing (CBWFQ)?

A

A queuing mechanism.

45
Q

How many queues and traffic classes does CBWFQ have?

A

256

46
Q

What type of traffic is CBWFQ best suited to?

A

TCP Traffic which can adapt to bandwidth changes and packet drops.

47
Q

What 2 queuing methods does CBWFQ support?

A

Tail drop and WRED (weighted random early detection)

48
Q

How does Tail Drop queuing work?

A

Size of queue is set in packets. When the queue exceeds its packet limit, all end (tail) traffic is dropped.

49
Q

Why must you Segment TCP and UDP traffic when using Tail Drop?

A

When TCP traffic experiences packet loss, the sender slows its transfer rate.

When UDP traffic experiences packet loss, the sender has no mechanism to detect and continues forwarding.

This creates a TCP starvation/UDP dominance scenario.

50
Q

What is the benefit of WRED?

A

Mitigation of ‘TCP global synchronisation’ phenomenon.

If TCP traffic is Tail dropped, all senders begin adjusting their transmission window in syncro.

This leads to inefficient bandwidth usage.

51
Q

How does WRED work?

A

Your queue size can be much larger, but WRED will randomly drop packets from the queue to balance the TCP traffic and prevent Global Synchronisation.

52
Q

Which PHB DSCP group does WRED use to drop packets?

A

Assured forwarding

53
Q

What is LLQ?

A

Low latency Queuing

54
Q

How does LLQ work?

A

LLQ is considered high pri and is always serviced first. It has dedicated bandwidth and can burst (default 20%) over this value.

LLQ still has a tail drop or WRED for handling congestion.

55
Q

What is Explicit Congestion Notification?

A

Extension of WRED configuration that prevents global synchronisation without dropping TCP traffic.

56
Q

How does Explicit Congestion Notification work?

A

ECN slows TCP traffic by reducing the senders congestion window.

TCP Sender flags traffic with CE tag, which when received by Receiver will be tagged with ECE tag halving the congestion window.

57
Q

What is Traffic shaping?

A

Queuing/buffering traffic when you know the receiving end is policing traffic.

58
Q

How does traffic shaping work?

A

CIR is set, with any traffic sent below this sent immediately.

If traffic exceeds the CIR, it’s buffered and sent gradually to not exceed the line rate.

CCNP Encor (9 decks)

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