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QoS Questions

November 26th, 2017 Go to comments

Question 1

Explanation

The following diagram illustrates the key difference between traffic policing and traffic shaping. Traffic policing propagates bursts. When the traffic rate reaches the configured maximum rate (or committed information rate), excess traffic is dropped (or remarked). The result is an output rate that appears as a saw-tooth with crests and troughs. In contrast to policing, traffic shaping retains excess packets in a queue and then schedules the excess for later transmission over increments of time. The result of traffic shaping is a smoothed packet output rate.

traffic_policing_vs_shaping.jpg

Note: Committed information rate (CIR): The minimum guaranteed data transfer rate agreed to by the routing device.

Question 2

Question 3

Explanation

The IP datagram header contains an 8-bit field called ToS (Type of Service). The field has been part of the IP header since the beginning, but it was rarely used until the recent introduction of Differentiated Services (Diff-Serv).

TOS.png

Note:
+ CoS does not exists in an IP header. It appears in the header of a 802.1Q frame only. CoS is used for QoS on a trunk link.
+ DSCP uses the first 6 bits of the TOS field.

Question 4

Explanation

The Resource Reservation Protocol (RSVP) protocol allows applications to reserve bandwidth for their data flows. It is used by a host, on the behalf of an application data flow, to request a specific amount of bandwidth from the network. RSVP is also used by the routers to forward bandwidth reservation requests.

Question 5

Explanation

The following diagram illustrates the key difference between traffic policing and traffic shaping. Traffic policing propagates bursts. When the traffic rate reaches the configured maximum rate (or committed information rate), excess traffic is dropped (or remarked). The result is an output rate that appears as a saw-tooth with crests and troughs. In contrast to policing, traffic shaping retains excess packets in a queue and then schedules the excess for later transmission over increments of time. The result of traffic shaping is a smoothed packet output rate.

traffic_policing_vs_shaping.jpg

Note: Committed information rate (CIR): The minimum guaranteed data transfer rate agreed to by the routing device.

Question 6

Explanation

Layer-3 marking is accomplished using the 8-bit Type of Service (ToS) field, part of the IP header. A mark in this field will remain unchanged as it travels from hop-to-hop, unless a Layer-3 device is explicitly configured to overwrite this field. There are two marking methods that use the ToS field:
+ IP Precedence: uses the first three bits of the ToS field.
+ Differentiated Service Code Point (DSCP): uses the first six bits of the ToS field. When using DSCP, the ToS field is often referred to as the Differentiated Services (DS) field.

TOS.png

Reference: http://www.routeralley.com/guides/qos_classification.pdf

Question 7

Explanation

The following diagram illustrates the key difference between traffic policing and traffic shaping. Traffic policing propagates bursts. When the traffic rate reaches the configured maximum rate (or committed information rate), excess traffic is dropped (or remarked). The result is an output rate that appears as a saw-tooth with crests and troughs. In contrast to policing, traffic shaping retains excess packets in a queue and then schedules the excess for later transmission over increments of time. The result of traffic shaping is a smoothed packet output rate.

traffic_policing_vs_shaping.jpg

Question 8

Question 9

Explanation

With Priority Queueing (PQ), traffic is classified into high, medium, normal, and low priority queues. The high priority traffic is serviced first, then medium priority traffic, followed by normal and low priority traffic.  -> Therefore we can assign higher priority for voice traffic.

Also with PQ, higher priority traffic can starve the lower priority queues of bandwidth. No bandwidth guarantees are possible -> It is still good because this network is mostly used for data traffic so voice traffic amount is small.

With First In First Out (FIFO) or Weighted Fair Queueing (WFQ), there is no priority servicing so they are not suitable here.

Reference: https://www.cisco.com/c/en/us/td/docs/ios/solutions_docs/qos_solutions/QoSVoIP/QoSVoIP.html

Weighted Random Early Detection (WRED) is just a congestion avoidance mechanism. WRED measures the size of the queues depending on the Precedence value and starts dropping packets when the queue is between the minimum threshold and the maximum threshold -> It does not have priority servicing either.

Comments (17) Comments
  1. ayesha
    June 11th, 2017

    hi

  2. Ayya
    July 12th, 2017

    Really useful info about traffic shapping and policing

  3. Crack
    July 24th, 2017

    Q2 – why not A?

    http://www.cisco.com/c/en/us/support/docs/switches/catalyst-6000-series-switches/23420-138.html

    Note: By default all ports are in the untrusted state when QoS is enabled.

  4. Anonymous
    August 23rd, 2017

    Question 2 needs more detail as every switch has different behavior regarding QoS. Site Admin please elaborate.

    Thanks

    Imran

  5. Anonymous
    August 24th, 2017

    Qos
    ===

    – The first thing to check is if the Qos is enable or not (show run | i Qos or show mls qos)
    *If it is enabled, then you have to trust the traffic comming into the port, msl qos trust dscp,
    *If you enable Qos and you do not trust the traffic comming in, it will strip all the tags or marking and forward the traffic on the best efforts basis, which implies you have lost your Qos.

    – By default , Qos is diabled.
    * so if it is diabled, then the switch will allow the traffic to pass through as it is.

    TEST-2960X#show mls qos
    QoS is enabled
    QoS ip packet dscp rewrite is enabled

  6. Imran Shahid
    August 24th, 2017

    So the default behavior of Qos will be

    A) IT is turned OFF
    B) Whatever traffic will be put through the switch, the switch will allow it on as it is basis whether it is video or voice or data.
    C) If you enable the Qos, you will need to trust the ports and apply Qos otherwise, it will strip off all the tags and forward traffic on best efforts basis.

    Please comment.

  7. Anonymous
    August 29th, 2017

    About Question 4 in the whole book there is no mention not once for the word RSVP it’s real question or what please help

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  9. ali
    September 28th, 2017

    in which byte of an ip packet can traffice be marked ?
    tos
    dscp

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  17. cthelite
    February 18th, 2018

    Boy, Q2 is just eating my lunch. 9TUT and every other website on the internet use the same word for word explanation supporting answer E, but I’m not getting it evidently. This same-ole explanation copied from site to site basically says the tag is removed prior to being delivered to the end station, then discusses native VLAN which deals with untagged data. How is E correct for this question?

    DEFAULT QoS behavior is DEFINITELY that the ports are untrusted! Answer should be A ?! By default, QoS is disabled on a switch, but that’s not the question. The question is about the default QoS behavior which implies QoS is at least enabled. Again, the answer should be A.

    Can someone please confirm or disagree? If E is actually correct, maybe someone else’s perspective will help me. I’ve been investigating this for hours and just don’t see it.

    Thanks!

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