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This chapter is from the book

Weighted Random Early Detection (WRED)

The previous section addressed queuing, which is a congestion-management QoS mechanism. However, this section focuses on congestion avoidance. Specifically, you do not want a queue to fill to capacity, because all arriving traffic flows would be dropped and enter Transport Control Protocol (TCP) slow start. Some negative consequences arise from having multiple flows simultaneously enter TCP slow start. To prevent such an occurrence, you can configure Weighted Random Early Detection (WRED).

These flash cards review the need for WRED. You also must be familiar with the inner workings of the industry-standard Random Early Detection (RED) approach, in addition to the configuration of WRED. Even though WRED can be configured from interface-configuration mode or as part of a Modular Quality of Service Command-Line Interface (MQC) configuration, these flash cards focus specifically on WRED’s MQC implementation. Finally, you need to recall the characteristics and configuration of the Explicit Congestion Notification (ECN) mechanism.

Question 1

What is TCP slow start?

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Question 2

 Which QoS tool does Cisco use to prevent a queue from filling to capacity?

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Question 3

Describe the following WRED parameters: minimum threshold, maximum threshold, and Mark Probability Denominator (MPD).

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Question 4

When configuring WRED, you configure an MPD of 4 for traffic that is marked with a Differentiated Services Code Point (DSCP) value of 46. When the queue depth is at the maximum threshold for DSCP 46 traffic, what is the probability (in percent) that a packet marked with a DSCP value of 46 will be discarded?

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Question 5

Identify the QoS marking that WRED references if the following command is issued:

Router(config-if)#random-detect

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Question 6

Define global synchronization.

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Question 7

Describe the three drop modes of RED: no drop, random drop, and full drop.

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Question 8

Besides policy-map-class configuration mode, list two other configuration modes from which WRED can be configured.

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Question 9

Describe the following WRED parameters: minimum threshold, maximum threshold, and Mark Probability Denominator (MPD).

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Question 10

What policy-map-class configuration-mode command enables WRED and specifies that the WRED profiles should be based on DSCP values, as opposed to IP Precedence values?

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Question 11

Which WRED command specifies a minimum threshold of 24, a maximum threshold of 45, and an MPD of 10 for an IP Precedence value of 2?

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Question 12

Which command can you use to see the WRED parameters that are associated with a particular interface using the MQC process?

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Question 13

Define Explicit Congestion Notification (ECN).

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Question 14

What are the names of the ECN bits that are located in the 2 right-most bit positions of the ToS byte?

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Question 15

What happens to packets that are sent between two ECN-capable routers when the queue depth for those packets is below the minimum threshold?

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Question 16

What is being indicated when a packet is received with both of its ECN bits set to 1?

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Question 17

What happens to packets that are sent between two ECN-capable routers when the queue depth is above the packets’ maximum threshold?

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Question 18

What happens when an ECN-marked packet is received by a router that is not ECN capable?

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Question 19

Under what configuration mode (or modes) can you configure ECN?

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Question 20

Which command enables ECN?

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