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CCNP Core > Chap 14 - QOS (part 2) > Flashcards

Chap 14 - QOS (part 2) Flashcards

1
Q

What does NBAR2’s Protocol Discovery do?

A

It enables NBAR2 to discover and get real-time statistics on applications currently running in the network.

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2
Q

What can be done with the statistics from the NBAR2 Protocol Discovery mode?

A

They can be used to define QoS classes and policies using MQC configuration.

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3
Q

What is Modular QOS CLI used for?

A

Matches protocols to classes and then applies QOS policies based on those classes.

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4
Q

What is Packet Marking?

A

QoS mechanism that colors a packet by changing a field within a packet or a frame header with a traffic descriptor so it is distinguished from other packets during the application of other QoS mechanisms

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5
Q

What 4 Traffic Descriptors are used when marking traffic?

A
  • Internal: QOS groups
  • Layer 2: 802.1Q/p Class of Service (CoS) bits
  • Layer 2.5: MPLS Experimental (EXP) bits
  • Layer 3: Differentiated Services Code Points (DSCP) and IP Precedence (IPP)
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6
Q

What is the Traffic Descriptor ‘Internal: QOS Groups’ used for and when are they removed?

A

QoS groups are used to mark packets as they are received and processed internally within the router and are automatically removed when packets egress the router.

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7
Q

When are QOS Groups used?

A

They are used only in special cases in which traffic descriptors marked or received on an ingress interface would not be visible for packet classification on egress interfaces due to encapsulation or de-encapsulation.

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8
Q

When does congestion take place?

A

When the maximum capacity of the lowest-bandwidth link is surpassed.

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9
Q

What is 802.1q?

A

is an IEEE specification for implementing VLANs in Layer 2 switched networks.

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10
Q

How many bits is the TPID Field, what does TPID stand for, and what is its value on an 802.1q frame?

A
  • 16 bits
  • Tag Protocol Identifier
  • Set to 0x8100 identifying it as an 802.1q frame
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11
Q

How many bits are in the TCI Field in an 802.1q frame and what does TCI stand for?

A
  • 16 bits
  • Tag Control Information
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12
Q

What are 3 components of the TCI Field and how many bits are in each?

A
  • Priority Code Point (PCP) 3 bits
  • Drop Eligible Indicator (DEI) 1 bit
  • VLAN Identifier (VLAN ID) 12 bits
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13
Q

What does 802.1p standard describe?

A

The PCP field

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14
Q

What is the PCP field used for?

A

This field is used to mark packets as belonging to a specific CoS.

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15
Q

What are the 8 COS settings?

A
  • 0 - Background
  • 1 - Best Effort (default)
  • 2 - Excellent Effort
  • 3 - Critical Applications
  • 4 - Video (< 100 ms latency and jitter
  • 5 - Voice (< 10 ms latency and jitter)
  • 6 - Internetwork Control
  • 7 - Network Control
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16
Q

What are the 8 COS acronyms?

A
  • 0 - BK
  • 1 - BE
  • 2 - EE
  • 3 - CA
  • 4 - VI
  • 5 - VO
  • 6 - IC
  • 7 - NC
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17
Q

What is a drawback to using COS?

A

Frames lose their CoS markings when traversing a non-802.1Q link or a Layer 3 network.

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18
Q

What is the solution to the problem of COS markings being lost on non-802.1q links?

A

Mark the frames at a higher level by mapping the COS markings into TOS values at the router

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19
Q

How many bits is the DEI (Drop Eligible Indicator), what can be done with it, and what is its default value?

A
  • 1 bit
  • Can be used independently or in conjunction with PCP to indicate frames that are eligible to be dropped during times of congestion.
  • Default is 0
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20
Q

In the DEI bit what is the default setting and what does it mean?

A

0 indicating the frame cannot be dropped

21
Q

What is the VLAN ID field?

A

It is a 12-bit field that defines the VLAN used by 802.1Q.

22
Q

How many bits are in the TOS field, where is it located in the IP header, what fields make up the TOS field when used for IPP?

A
  • 8 bit field
  • Sits btw the Header Length and Total Length fields of the IP Header
  • Only the most significant 3 bits are used as IPP (IP Precedense) with the rest of the bits unused
23
Q

What are the possible values of the 3 IPP bits?

A

Range in values between 0 to 7 however 6 and 7 are reserved for internal network use

24
Q

What is the TOS byte called in IPV6?

A

Traffic Class byte

25
Q

How have newer standards redefined the TOS Byte?

A

Redefined it to the DiffServ byte

26
Q

What is the DiffServ byte composed of?

A
  • 6 bits are DSCP allowing values 0 to 63
  • 2 bits used for Explicit Congestion Notification (ECN)
27
Q

What is the DiffServ byte used for?

A

Used to mark packets according to their classification into DiffServ Behavior Aggregates (BAs).

28
Q

What is a Behavior Aggregate?

A

It is a collection of packets or applications with the same DiffServ value crossing a link in a particular direction.

29
Q

What is Per Hop Behavior?

A

It is the externally observable forwarding behavior (forwarding treatment) applied at a DiffServ-compliant node to a collection of packets with the same DiffServ value crossing a link in a particular direction (DiffServ BA).

30
Q

What does a Per Hop Behavior do?

A

PHB is expediting, delaying, or dropping a collection of packets by one or multiple QoS mechanisms on a per-hop basis, based on the DSCP value.

31
Q

Where should only simple PHB, based on DiffServ BAs be done?

A

At the Core of the network.

32
Q

What should be done at the network edge?

A

The edge devices should perform classification, marking, policing, and shaping operations.

33
Q

What are the 4 PHBs have been defined and characterized for general use?

A
  • Class Selector (CS) PHB
  • Default Forwarding (DF) PHB
  • Assured Forwarding (AF)
  • Express Forwarding (EF) PHB
34
Q

What is the Class Selector (CS) PHB used for?

A

The first 3 bits of the DSCP field are used as CS bits. The CS bits make DSCP backward compatible with IP Precedence because IP Precedence uses the same 3 bits to determine class.

35
Q

What is Default Forwarding (DF) PHB used for?

A

Best-effort service

36
Q

What is Assured Forwarding (AF) PHB used for?

A

Used for guaranteed bandwidth service

37
Q

What is Express Forwarding (EF) PHB used for?

A

Used for low delay service

38
Q

Why was the Class Selector (CS) PHB defined?

A

To provide backward compatibility for DSCP with IP Precedence (IPP).

39
Q

How does DSCP provide backward compatibility with IPP?

A

To achieve backward compatibility set the first 3 bits of the DSCP field to 0. Then use the last 3 bits to get the same values (0 thru 7) that IPP would have had.

40
Q

What are 4 times when Best Effort forwarding is done?

A
  • When the DSCP value is all 0’s
  • When packets cannot be classified by a QoS mechanism such as queueing, shaping, or policing
  • When a QoS policy on the node is incomplete
  • When DSCP values are outside the ones that have been defined for the CS, AF, and EF PHBs.
41
Q

What 2 things does Assured Forwarding (AF) do?

A
  • It guarantees a certain amount of bandwidth to an AF class
  • Allows access to extra bandwidth, if available.
42
Q

What are the Assured Forwarding settings?

A
  • Queues - 1 (worst) to 4 (best)
  • Drop Possibility - 1 (most likely) to 3 (least likely)
43
Q

What is the formula for calculating the binary number of an AF value?

A

8x + 2y where x=IP Precedence and y=Drop Probability so for AF23 it would be 8(2) + 2(3) = 22 binary

44
Q

What is the AF name composed of?

A

Afxy where x=IP Precedence and y=Drop Probability

45
Q

What is Weighted RED?

A
  • Weighted Random Early Detection
  • A congestion avoidance algorithm
  • Can use DSCP or IPP to drop more aggressively
46
Q

For Assured Forwarding and Expedited Forwarding which of the two does policing and which does Congestion avoidance?

A

Assured Forwarding does Congestion Avoidance and Expedited Forwarding does Policing

47
Q

In Assured Forwarding what is the first bit of the DSCP field always set to?

A

First DSCP bit is always set to 0

48
Q

In Assured Forwarding what is the format of the DSCP field?

A

aaadd0 where aaa is the binary value of the AF class and dd is the binary value of Drop Probability, and the last bit is always set to 0

49
Q

In the 2 COS settings for voice and video what are the maximum latency/jitter values for each?

A
  • Voice > 10
  • Video > 100

CCNP Core (46 decks)

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