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    CCIE R&S 350-001 Q&As – QoS Voice (6-10)

    By admin | June 18, 2014

    Tagged with:

    Section 9 – Implement Quality of  Service (QoS)

    QUESTION 6
    An expanding company is deploying leased lines between its main site and two remote sites. The bandwidth of the leased lines is 128kb/s each, terminated on different serial interfaces on the main router. These links are used for combined VOIP and data traffic. The network administrator has implemented a VOIP solution to reduce costs, and has therefore reserved sufficient bandwidth in a low latency queue on each interface for the VOIP traffic. Users now complain about bad voice quality although no drops are observed in the low latency queue.
    What action will likely fix this problem?
    A.    mark VOIP traffic with IP precedence 6 and configure only ‘fair-queue’ on the links
    B.    configure the scheduler allocate 3000 1000 command to allow the QoS code to have enough CPU cycles
    C.    enable class-based traffic shaping on the VoIP traffic class
    D.    enable Layer 2 fragmentation and interleaving on the links
    E.    enable Frame Relay on the links and send voice and data on different Frame Relay PVCs
    Answer: D

    Explanation:
    Link fragmentation and interleaving (LFI) is a Layer 2 technique in which all Layer 2 frames are broken into small, equal-size fragments, and transmitted over the link in an interleaved fashion. When fragmentation and interleaving are in effect, the network device fragments all frames waiting in the queuing system where it prioritizes smaller frames. Then, the network device sends the fragments over the link. Small frames may be scheduled behind larger frames in the WFQ system. LFI fragments all frames, which reduces the queuing delay of small frames because they are sent almost immediately. Link fragmentation reduces delay and jitter by normalizing packet sizes of larger packets in order to offer more regular transmission opportunities to the voice packets.
    The following LFI mechanisms are implemented in Cisco IOS:
    Multilink PPP with interleaving is by far the most common and widely used form of LFI.
    FRF.11 Annex C LFI is used with Voice over Frame Relay (VoFR).
    FRF.12 Frame Relay LFI is used with Frame Relay data connections.

    QUESTION 7
    You are the network administrator of an enterprise with a main site and multiple remote sites. Your network carries both VOIP and data traffic. You agree with your service provider to classify VOIP and data traffic according to the different service RFCs. How can your data and VOIP traffic be marked?
    A.    data marked with DSCP AF21, VOIP marked with DSCP EF
    B.    data marked with DSCP AF51, VOIP marked with DSCP EF
    C.    data marked with the DE-bit, VOIP marked with the CLP-bit
    D.    data marked with DSCP EF, VOIP marked with DSCP AF31
    E.    data marked with IP precedence 5, VOIP marked with DSCP EF
    Answer: A
    Explanation:
    Expedited Forwarding
    RFC 2598 defines the Expedited Forwarding (EF) PHB: “The EF PHB can be used to build a low loss, low latency, low jitter, assured bandwidth, end-to-end service through DS (Diffserv) domains. Such a service appears to the endpoints like a point-to- point connection or a “virtual leased line.” This service has also been described as Premium service.” Codepoint 101110 is recommended for the EF PHB, which corresponds to a DSCP value of 46.
    Again, vendor-specific mechanisms need to be configured to implement these PHBs. Refer to RFC 2598 for more information about EF PHB.
    DSCP was designed to be more granular and more scalable than IP precedence BUT with backward compatibility.
    The priority field (or type of service, ToS) was originally 3 bits, giving it the IP prec.values 0-7 (0 being the lowest priority, 7 the highest). DSCP has an 8 bit field, of which 6 bits are used for markings (the 6th bit is always 0). This gives it a larger number of values (both for per hop behavior, or PHB, and drop precedence. More on this later)
    The last 2 bits are used for ECN or explicit congestion notification. This is a brand new feature (as of Windows Vista) and is purportedly going to revolutionize internetwork traffic flow. More on this later.
    With the 6 bits allotted to DSCP, the first three (left to right) are used for Major Class, or Per Hop Behavior. These match up with the old IPprec values of 0-7.
    The second 3 bits identify the drop precedence. Higher = more likely to be dropped. This means that a DSCP marking of AF21 (major class 2, drop precedence of 1) will be preferred over AF22 or AF23. An AF3x will beat any AF1x or AF2x–the major class of 3 is higher than the major class, or PHB, of 2.
    It is important to note the drop precedence is only used on classes 1-4. (here is a table from wikipedia) A marking of 0 indicates ‘best effort’
    Class 1 Class 2 Class 3 Class 4
    Low Drop AF11 (DSCP 10) AF21 (DSCP 18) AF31 (DSCP 26) AF41 (DSCP 34)
    Med Drop AF12 (DSCP 12) AF22 (DSCP 20) AF32 (DSCP 28) AF42 (DSCP 36)
    High Drop AF13 (DSCP 14) AF23 (DSCP 22) AF33 (DSCP 30) AF43 (DSCP 38)
    The notation DSCP xx is the bit notation. Eg AF12 = 001 100 which is a decimal value of 12. Here’s another one. AF43 = 100 110 (38)
    EF, which it equal to IPprec of 5 is assigned a major class of 5 and a drop pecedence of 3 (this is odd, I don’t know why this was done) The decimal value for EF is DSCP 46 or 101 110.

    QUESTION 8
    Refer to the exhibit. When applying this hierarchical policy map on the on the tunnel1 interface, you measure high jitter for traffic going through class 1234. What is the most likely cause of this jitter?
    A.    The configuration of a hierarchical policy map on a tunnel interface is not supported.
    B.    Class 5555 and class 5554 are both taking up 100% of the bandwidth, leaving nothing for class 1234.
    C.    The burst size for the traffic shaping is wrongly configured to 15000; this would require an interface capable of sending at 150Mb/s.
    D.    The burst size for the traffic shaping has been wrongly configured; it should be set as low as possible.
    E.    The burst size for the traffic shaping has been wrongly configured; it should be set as high as possible.
    Answer: D

    QUESTION 9
    Refer to the exhibit. When applying this policy map on the tunnel1 interface, you see packet loss for the TCP class starting at around 100000 b/s, instead of the configured 150000 b/s.
    What is the most likely cause of the discrepancy?
    A.    The violate-action command should not be configured.
    B.    The current configuration of the load-interval command on the tunnel interface is preventing proper policing calculations.
    C.    The burst size is too low.
    D.    Policing on tunnel interfaces is not supported.
    E.    The CIR keyword is missing in the policer.
    Answer: C

    QUESTION 10
    Refer to the exhibit. As a network administrator, you have configured a dual-rate, dual-bucket policer in accordance with RFC 2698 on the serial interface of you router, connecting to your provider. The SLA with your provider states that you should only send AF31 (limited to 150 kb/s), AF32 (limited to 50 kb/s)and AF33 (best effort). Your service provider claims you are not conforming to the SLA.
    Which two things are wrong with this configuration? (Choose two.)
    A.    The configuration of a service policy on half-duplex Ethernet interfaces is not supported.
    B.    The class class-default sub-command of the policy-map limit command should be set to the DSCP default.
    C.    The violate action is wrong.
    D.    This policer configuration is not implementing RFC 2698 dual-bucket, dual-rate.
    E.    The policer is configured in the wrong class.
    Answer: CE

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