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CCNP Core > Chap 14 - QOS (part 4) > Flashcards

Chap 14 - QOS (part 4) Flashcards

1
Q

When should the Single-rate Three-color Marker/Policer (srTCM) be used?

A
  • Makes sense only if the actions for each color differ.
  • If two actions are the same then use the Single-rate Two-color Marker/Policer
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2
Q

For the Two-rate Three-color Marker/Policer (trTCM):

  • How many buckets?
  • What are the two rates called?
  • What does it allow for?
  • How is traffic dropped?
A
  • two buckets
  • two rates - CIR and PIR
  • allows for different actions for exceeding the 2 different burst values
  • traffic can be dropped at a defined rate
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3
Q

What is the difference between the srTCM and the trTCM?

What does srTCM rely on?
What does that introduce?
What does trTCM introduce?

A
  • srTCM relies on excess tokens in Be bucket which introduces a certain level of variability and unpredictability in traffic flows
  • trTCM introduces two rates - CIR and PIR
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4
Q

What are the 2 rates used by the trTCM?

A

CIR and PIR

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5
Q

What is the difference in how srTCM uses its buckets vs how trTCM uses them?

A
  • srTCM uses overflow tokens from the Bc bucket
  • trTCM the Bc bucket token arrival rate is based on CIR and the Be bucket token arrival rate is based on PIR
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6
Q

How does the initial checking logic work on trTCM?

A
  • First it checks for a Violate condition, then an Exceed condition and lastly a conform condition.
  • This is the opposite of what srTCM does
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7
Q

In trTCM what does Be mean?

A

It represents the peak limit of traffic that can be sent during a subsecond interval

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8
Q

What are the 7 parameters used by the trTCM?

A
  • CIR
  • PIR - Peak Information Rate, max rate of traffic allowed, should be equal to or greater than CIR
  • Bc - committed burst size
  • Be - peak burst size
  • Tc - number of tokens in the Bc bucket
  • Tp - number of tokens in the PIR bucket
  • B - incoming packet length in bits
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9
Q

What is Congestion Management a combination of?

A

Congestion management involves a combination of queuing and scheduling.

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10
Q

What is Queuing, what is it also known as, when is it activated and when is it deactivated?

A
  • Temporary storage of excess packets
  • aka Buffering
  • Activated when an output interface is experiencing congestion and deactivated when congestion clears
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11
Q

How is congestion detected?

A

Detected by the queuing algorithm when a Layer 1 hardware queue present on physical interfaces, known as the transmit ring (Tx-ring or TxQ) , is full.

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12
Q

What are 2 causes of congestion?

A
  • The input interface is faster than the output interface.
  • The output interface is receiving packets from multiple input interfaces
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13
Q

What happens when congestion takes place?

What happens to the queues?
What happens to the packets?
What decides which packets to transmit next?

A
  • The queues fill up
  • Packets reordered by queuing algorithms so that higher-priority packets exit the output interface sooner than lower-priority ones
  • Scheduling algorithm decides which packets to transmit next
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14
Q

If there is no congestion is the scheduler still active?

A

Yes, it is always active

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15
Q

What are the 6 legacy queuing methods?

A
  • FIFO
  • Custom queuing
  • Priority queuing
  • Round robin
  • Weighted round robin (WRR)
  • Weighted Fair Queuing (WFQ)
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16
Q

What is weighted Round Robin queuing?

What was it developed to provide?
What is assigned to each queue?
Based on this algorithm what is provided to each queue?

A
  • Developed to provide prioritization to Round Robin queuing
  • A weight is assigned to each queue
  • Interface bandwidth proportional to the weight assigned to each queue.
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17
Q

What is Custom Queuing?

What is it similar to?
What type of scheduler?
How many queues?
How are the queues customized?
What happens to unused bandwidth?
Algorithm within each queue?
What is its 2 downsides?

A
  • Cisco implementation of WRR
  • Round robin scheduler
  • 16 queues
  • Customized with a portion of interface bandwidth based on traffic type
  • If a traffic type is not using the bandwidth reserved for it other traffic types may use it
  • FIFO within each queue
  • Causes long delays
  • Suffers the same problems as FIFO in each queue
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18
Q

What is Priority Queuing?

How many queues?
Algorithm within each queue?
In what order are the queues serviced?
What is the downside?

A
  • 4 queues - high, medium, normal, low
  • FIFO within each queue
  • High priority queue always serviced first
  • Low priority queue not serviced until higher ones are empty
  • Low priority queues could be starved
19
Q

What is Weighted Fair Queueing?

How does it divide bandwidth and how is it weighted?
How does it allocate bandwidth?
What is the upside to this?
What does it not provide?

A
  • Automatically divides bandwidth by number of flows (weighted by IP Precedence)
  • Allocates bandwidth fairly among all flows
  • It provides better service for high-priority real time flows
  • Does not provide a fixed-bandwidth guarantee for any particular flow.
20
Q

Which 2 queueing algorithms are recommended for rich media networks?

A
  • Class-based weighted fair queuing (CBWFQ)
  • Low Latency Queuing
21
Q

What is Class-based Weighted Fair Queuing (CBWFQ)

How many queues?
How many traffic classes?
What 2 ways is each queue customized?
What does it support?

A
  • Enables 64 queues
  • 64 traffic classes
  • Each queue serviced based on:
    • Bandwidth assigned to that class
    • Type of policing - tail drop or WRED
  • Supports user-defined traffic classes
22
Q

In CBWFQ what 4 factors is classification based on?

A
  • QoS markings
  • Protocols
  • ACLs
  • Input interfaces
23
Q

In CBWFQ what 4 parameters can be assigned to each class?

A
  • Bandwidth
  • Weight
  • Queue limit
  • Maximum packet limit
24
Q

In CBWFQ what happens when a queue gets full?

A

Excess packets are dropped

25
Q

In CBWFQ what is meant by assigning bandwidth to the class?

A

The minimum bandwidth delivered to the class during congestion.

26
Q

In CBWFQ what is meant by the queue limit?

A

It is the maximum number of packets allowed to be buffered in the class queue.

27
Q

In CBWFQ is there a guarantee of latency?

A

No, it does not provide a latency guarantee and is only suitable for non-real-time data traffic.

28
Q

What is Low Latency Queuing (LLQ)?

What is it a combination of?
Why was it developed?
How many queues?
What type of traffic is assigned to this?
Is all traffic assigned to this queue treated the same?

A
  • LLQ is CBWFQ combined with priority queueing (PQ)
  • Was developed for real-time traffic, such as voice
  • 2 classes can be defined but are both are serviced by a single internal priority queue
  • All real time traffic is assigned to this queue?
  • Traffic assigned to this queue can be classified and given seperate bandwidth guarantees
29
Q

In LLQ what happens in times of congestion?

A

Real-time traffic that goes beyond the assigned bandwidth guarantee is policed by a congestion-aware policer to ensure that the non-priority traffic is not starved.

30
Q

In LLQ what happens if a traffic class is not using the bandwidth assigned to it?

A

it is shared among the other classes.

31
Q

In LLQ what 2 guarantees are provided?

A
  • Latency guarantee
  • Bandwidth guarantee
32
Q

In LLQ the policing rate is based on bandwidth. What 2 ways can bandwidth be defined as?

A
  • A fixed amount of bandwidth or
  • A percentage of the interface bandwidth.
33
Q

What does the CBWFQ scheduler do?

A

Guarantees bandwidth to each class.

34
Q

In LLQ how many traffic classes are there and why?

Also, if there are multiple traffic classes which class gets priority?
In what order is traffic sent out from the LLQ?
A
  • LLQ allows for two different traffic classes to be assigned to it so that different policing rates can be applied to different types of high-priority traffic like voice and video
  • No LLQ class gets priority because all the LLQ classes go into the same internal low latency queue.
  • Traffic in the LLQ is still sent out by FIFO
35
Q

How does LLQ provide low latency?

A

LLQ creates a high-priority queue that is always serviced first. During times of congestion, LLQ priority classes are policed to prevent the PQ from starving the CBWFQ non-priority classes

36
Q

What do congestion avoidance tools do?

A

Congestion-avoidance techniques monitor network traffic loads to anticipate and avoid congestion by dropping packets.

37
Q

For congestion avoidance what is the default method of dropping packets?

A

Tail drop

38
Q

What is Tail Drop?

A
  • Treats all traffic equally
  • When output queue buffers are full all packets trying to enter the queue are dropped
39
Q

Why should tail drop be avoided?

What does tail drop cause?
What is the result of tail drop?

A

It should be avoided with TCP traffic because it can cause TCP global synchronization, which results in significant link underutilization.

40
Q

What does Random Early Detection (RED) do?

A

It provides congestion avoidance by randomly dropping packets before the queue buffers are full.

41
Q

What is Weighted RED?

Who developed it?
How is randomness controlled?
What can WRED also do?

A
  • Cisco implementation of RED is known as Weighted RED (WRED)
  • Can use either IPP or DSCP to control randomness
  • Also can set the ECN bit indicating the packet was handled during a time of congestion
42
Q

What is the difference between RED and Weighted RED?

A

The difference between RED and WRED is that the randomness of packet drops can be manipulated by traffic weights denoted by either IP Precedence (IPP) or DSCP.

43
Q

What does WRED do to manpulate the randomness of packet drops?

A

Packets with lower IPP value dropped more aggressively than higher IPP values; IPP 3 would be dropped more aggressively than IPP 5 or DSCP, AFx3 would be dropped more aggressively than AFx2, and AFx2 would be dropped more aggressively than AFx1.

CCNP Core (46 decks)

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