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Cisco CCNP ENCOR > QoS > Flashcards

QoS Flashcards

1
Q

What is QoS?

A

Quality of Service - a collection of technologies and methods that ensures critical application communication is delivered reliably across the network by prioritizing certain types of traffic.

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2
Q

What are the three QoS implementation models?

A
  • Best Effort: no QoS
  • Integrated Services (IntServ) - old
  • Differentiated Services (DiffServ) - modern
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3
Q

What is the key component of how the Integrated Services Model (IntServ) functions?

A

End to End bandwidth reservations using RSVP (Resource Reservation Protocol)

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4
Q

What are the key component to how the Differentiated Services Model (DiffServ) functions?

A
  • Classification and Marking of Traffic
  • IP Precedence (service levels)
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5
Q

Given the following Layers of the OSI Model, what are the ways that traffic can be marked for classification at each layer? (Layer 1, 2, 3, 4, and 7)

A
  • Layer 1: Interface/Port
  • Layer 2: MAC Address, 802.1Q Class of Service
  • Layer 3: DSCP, IP Precedence, Source/Destination IP address
  • Layer 4: TCP or UDP ports
  • Layer 7: NBAR (Network Based Application Recognition)
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6
Q

In practice, what are the most common methods to classify (identify) QoS traffic on a Cisco device?

A
  • Access Lists (ACLs)
  • Class-Maps: class maps can match traffic on an array of criteria such as interface, protocol, dscp, etc.
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7
Q

At Layer 2, what 802.1Q header field and sub-fields are responsible for marking traffic?

A

Tag Control Information (TCI) field which is composed of three sub-fields:

  • Priority Code Point (PCP)
  • Drop Eligible Indicator (DEI)
  • VLAN Identifier (VLAN ID)
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8
Q

What are the possible Class of Service (CoS) marking values found in the PCP field of an 802.1Q header and which ones are more preferred?

A
  • CS0 Background Traffic (Lowest)
  • CS1 Best Effort (default)
  • CS2 Excellent Effort
  • CS3 Critical Applications
  • CS4 Video
  • CS5 Video with low jitter/latency
  • CS6 Internetwork control
  • CS7 Network Control (Highest)
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9
Q

What is the ToS field and what is the relationship to IP Precedence?

A

The ToS field is is an 8-bit field that is part of a packet header (Layer 3). Only the first 3 bits of the field are used and they are known as IP Precedence.

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10
Q

What is the ToS field and what is the relationship to the DiffServ QoS Model?

A

DiffServ redefines the ToS field to use 6 bits for marking traffic at Layer 3 (instead of 3) known as Differentiated Services Code Point (DSCP).

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11
Q

What are the four Per Hop Behavior classes that are defined in DiffServ’s DSCP marking?

A
  • Class Selector (CS)
  • Default Forwarding (DF)
  • Assured Forwarding (AF)
  • Expedited Forwarding (EF)
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12
Q

What are the DSCP marking value equivalents compared to Class of Service values?

A
  • CS7 (No DSCP equivalent)
  • CS6 (No DSCP equivalent)
  • CS5 (EF)
  • CS4 (AF4)
  • CS3 (AF3)
  • CS2 (AF2)
  • CS1 (AF1)
  • CS0 (BE)
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13
Q

When looking at DSCP values such as AF41 and AF 43 which one is more likely to be dropped?

A

AF43

*the second number is the drop probability - higher is more likely to be dropped.

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14
Q

What is special about traffic marked with CS6 and CS7

A

This is reserved for network control and routing protocol types of packets only.

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15
Q

When marking packets, what is best practice for where in the network the traffic should be marked?

A

As close to the source as possible.

In some cases endpoints, such as phones, can mark their own traffic. When this is not possible, it should be marked on the switch as it enters the switchport.

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16
Q

What is a Trust Boundary as it relates to DSCP and QoS?

A

The trust boundary is the concept of where a DSCP value will be trusted and forwarded on.

By default a PC connected to the PC port on a phone will have QoS markings “zeroed out” meaning that the phone will not trust the DSCP markings coming from the PC. In this scenario, the phone is the trust boundary.

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17
Q

In a wireless network, where is the Trust Boundary located by default?

A

At the wireless LAN controller, before the traffic is placed on the wired network (on the other end of the CAPWAP tunnel).

18
Q

What is Scavenger Traffic?

A

Traffic that is classified as being lower than Best Effort. This traffic is marked with CS1.

19
Q

After traffic has been identified on a network device. What can be done to “treat” or take action on the traffic?

A
  • Marking
  • Shaping
  • Policing
20
Q

What is the difference between a Shaper and a Policer?

A
  • Policers will drop or re-mark traffic
  • Shapers will attempt to delay, buffer, or queue traffic
21
Q

What is Markdown?

A

When a Policer re-marks a packet’s DSCP value to a lower priority due to exceeding a defined bit-rate.

22
Q

What are Token Bucket Algorithms used by Policers and Shapers?

A

Different types of algorithms that use a concept of “token” to determine if packets should be buffered, dropped, or marked down when congestion occurs on a network link.

23
Q

In Token Bucket Algorithms, what is Token Bucket?

A

A bucket size defines how many tokens it has. Tokens are removed from the bucket when a packet arrives and is checked. New tokens are added into the bucket at a rate that is equal to the Committed Information Rate. If the bucket gets empty, the enforcement mechanism will take affect.

24
Q

In Token Bucket Algorithms, what is CIR?

A

Committed Information Rate - The defined rate, in bits per second, at which traffic will be policed.

25
Q

In Token Bucket Algorithms, what is Tc?

A

Committed Time Interval - the defined time range, in milliseconds, over which the Committed Burst Size is sent.

26
Q

In Token Bucket Algorithms, what is Bc?

A

Committed Burst Size - the maximum size (defined in bits) of the Committed Information Rate token bucket that is allowed during the defined Committed Time Interval.

To state this more plainly - this is the size of the token bucket.

27
Q

In Token Bucket Algorithms, what is a Token?

A

A token represents 8 bits of a packet’s size. If a packet is 1500 bytes in size then it counts as 1500 tokens from the token bucket to process.

28
Q

What are the three types of QoS Policers?

A
  • Single Rate two-color marker: One Bucket, CIR Conforming or Exceeding
  • Single Rate three-color marker: Two Buckets, CIR Conforming, Exceeding, or Violating
  • Two Rate three-color marker: Two Buckets, CIR Conforming, Exceeding, or Violating
29
Q

When referring to three-color markers, what is the difference between exceeding and violating?

A

Three-color markers introduce the Excess Burst Size (Be) parameter and Be Bucket Token Count (Te). If the Bc bucket is empty but there are tokens in the Be, the traffic is considered “exceeding” and can be handled as desired (dropped or marked-down).

30
Q

What additional parameter is introduced in the Two-Rate Three Color Policer?

A

In addition to CIR is PIR (Peak Information Rate) which is a rate greater than the CIR which allows policed traffic to flow more evenly instead of dropping way off when traffic is policed.

31
Q

What are the six legacy, Pre-MQC queueing algorithms?

A
  • First In First Out (FIFO)
  • Round Robin
  • Weighted Round Robin
  • Custom Queueing (CQ)
  • Priority Queueing (PQ)
  • Weighted Fair Queueing (WFQ)
32
Q

What are the two modern MQC-supported algorithms best for rich media networks?

A
  • Class-based Weighted Fair Queueing (CBWFQ)
  • Low-latency queueing (LLQ)
33
Q

What are the characteristics of CBWFQ?

A
  • 256 possible queues/256 traffic classes
  • Packet classification using DSCP markings, protocols, ACLs, or interfaces
  • Classes can be assigned minimum bandwidth, weight, queue limit, and packet limits
34
Q

What are the characteristics of LLQ?

A
  • CBWFQ combined with PQ
  • Adds a strict-priority queue to the CBWFQ queues
  • Strict Priority Queue should be used for real-time voice and video
  • Multiple classes for real time traffic can be assigned to Strict Priority Queue
  • Unused bandwidth by a given class can be shared to other queues
35
Q

What are the two types of Congestion Avoidance?

A
  • Random Early Detection (RED)
  • Weighted Random Early Detection (WRED) - DSCP used to manipulate “randomness” of packet selection
36
Q

What are the commands to create a named ACL used for QoS purposes to identify all voice/RTP traffic? (minimum 2 commands)

A

SW1(config)# ip access-list extended VOICE_ACL
SW1(config-ext-nacl)# permit udp any eq 16384 32767 any eq 16384 32767

37
Q

What are the commands to create a class that identifies Voice traffic using an ACL named VOICE_ACL? (2 commands)

A

SW1(config)# class-map match-any VOICE_CLASS
SW1(config-cmap)# match access-group name VOICE_ACL

38
Q

What are the commands to mark voice traffic with the appropriate DSCP value using a class map named VOICE_CLASS? (3 commands)

A

SW1(config)# policy-map MARKING_POLICY
SW1(config-pmap)# class VOICE_CLASS
SW1(config-pmap-c)# set dscp ef

39
Q

What are the commands to use a single-rate two-color policer at a rate of 128Mbps with a class map named HTTP_CLASS? (4+ commands)

A

SW1(config)# policy-map QOS_POLICY
SW1(config-pmap)# class HTTP_CLASS
SW1(config-pmap-c)# police 128000000
SW1(config-pmap-c-police)# conform-action transmit
SW1(config-pmap-c-police)# violate-action drop

40
Q

What is the command to apply a policy map outbound on an interface?

A

SW1(config-if)# service-policy output POLICY_NAME

41
Q

What is the command to verify a policy-map applied to an interface?

A

SW1# show policy-map interface {name} [output | input]

Cisco CCNP ENCOR (22 decks)

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