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QoS Questions

March 16th, 2020 in ENCOR 350-401 Go to comments

Question 1

Question 2


Weighted Random Early Detection (WRED) is just a congestion avoidance mechanism. WRED drops packets selectively based on IP precedence. Edge routers assign IP precedences to packets as they enter the network. When a packet arrives, the following events occur:

1. The average queue size is calculated.
2. If the average is less than the minimum queue threshold, the arriving packet is queued.
3. If the average is between the minimum queue threshold for that type of traffic and the maximum threshold for the interface, the packet is either dropped or queued, depending on the packet drop probability for that type of traffic.
4. If the average queue size is greater than the maximum threshold, the packet is dropped.

WRED reduces the chances of tail drop (when the queue is full, the packet is dropped) by selectively dropping packets when the output interface begins to show signs of congestion (thus it can mitigate congestion by preventing the queue from filling up). By dropping some packets early rather than waiting until the queue is full, WRED avoids dropping large numbers of packets at once and minimizes the chances of global synchronization. Thus, WRED allows the transmission line to be used fully at all times.

WRED generally drops packets selectively based on IP precedence. Packets with a higher IP precedence are less likely to be dropped than packets with a lower precedence. Thus, the higher the priority of a packet, the higher the probability that the packet will be delivered.

Reference: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/qos_conavd/configuration/15-mt/qos-conavd-15-mt-book/qos-conavd-cfg-wred.html

WRED is only useful when the bulk of the traffic is TCP/IP traffic. With TCP, dropped packets indicate congestion, so the packet source will reduce its transmission rate. With other protocols, packet sources may not respond or may resend dropped packets at the same rate. Thus, dropping packets does not decrease congestion.

Reference: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/qos_conavd/configuration/xe-16/qos-conavd-xe-16-book/qos-conavd-oview.html

Note: Global synchronization occurs when multiple TCP hosts reduce their transmission rates in response to congestion. But when congestion is reduced, TCP hosts try to increase their transmission rates again simultaneously (known as slow-start algorithm), which causes another congestion. Global synchronization produces this graph:


Question 3


QoS Packet Marking refers to changing a field within a packet either at Layer 2 (802.1Q/p CoS, MPLS EXP) or Layer 3 (IP Precedence, DSCP and/or IP ECN).

Reference: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/qos_mqc/configuration/xe-16/qos-mqc-xe-16-book/qos-mrkg.html

Question 4


Cisco routers allow you to mark two internal values (qos-group and discard-class) that travel with the packet within the router but do not modify the packet’s contents.

Reference: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/qos_mqc/configuration/xe-16-6/qos-mqc-xe-16-6-book/qos-mrkg.html

  1. Tosha
    May 24th, 2020

    Question 1

    answer is not C

    The CoS marking allows a Layer 2 Ethernet frame to be marked with eight different levels of priority values, 0 to 7, where 0 is the lowest priority and 7 is the highest. 0 (lowest)-Background, 1 (default)-Best effort.

    so, the answer “D. The Cos value of each tagged packet is modified” ?

  2. Anonymous
    May 26th, 2020

    Anyone get an answer on Question 1? If the default CoS (PCP) port value is 0 then its priority is 1 which is the default value.


  3. guy
    May 27th, 2020

    Use the Default CoS page to configure the default CoS values for incoming packets through each LAN interface. The possible field values are 0 to 7. The default value is 0.

    June 9th, 2020

    Q1 Here the explanation:

    This is how the switch treats frames by default after the QoS is enabled:

    – A frame enters the switch port and it does not have the frame tagged (it means the port is access port and the frame enters the switch does not have ISL or dot1q encapsulation).
    – The switch encapsulates the frame with the dot1q (ignore ISL because dot1q is the default on all the new switches).
    – Inside the dot1q frame tag, there are three bits called 802.1p priority bits available which are also called CoS. These bits are set to 0.
    – Then, the switch calculates DSCP value based on the CoS-DSCP map table. As per the table, the switch sets the DSCP value to 0. DSCP value is located at the IP header of the packet.

    In summary, the CoS and DSCP values of the frame enter the switch set to 0 by default if the QoS is enabled on the switch. So C is correct.

    Reference: https://www.cisco.com/c/en/us/support/docs/switches/catalyst-3750-series-switches/91862-cat3750-qos-config.html

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