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    CCIE R&S 350-001 Q&As – QoS Voice (16-20)

    By admin | June 18, 2014

    Tagged with:

    Section 9 – Implement Quality of  Service (QoS)

    QUESTION 16
    Which two protocols can have their headers compressed through MQC? (Choose two.)
    A.    RTP
    B.    RTSP
    C.    HTTP
    D.    TCP
    E.    UDP
    Answer: AD

    Explanation:
    RTP or TCP IP header compression is a mechanism that compresses the IP header in a data packet before the packet is transmitted. Header compression reduces network overhead and speeds up transmission of RTP and TCP packets.
    Cisco IOS software provides a related feature called Express RTP/TCP Header Compression. Before this feature was available, if compression of TCP or RTP headers was enabled, compression was performed in the process-switching path. Compression performed in this manner meant that packets traversing interfaces that had TCP or RTP header compression enabled were queued and passed up the process to be switched. This procedure slowed down transmission of the packet, and therefore some users preferred to fast-switch uncompressed TCP and RTP packets.
    Now, if TCP or RTP header compression is enabled, it occurs by default in the fast-switched path or the Cisco Express Forwarding-switched (CEF-switched) path, depending on which switching method is enabled on the interface. Furthermore, the number of TCP and RTP header compression connections was increased.
    If neither fast-switching nor CEF-switching is enabled, then if TCP or RTP header compression is enabled, it will occur in the process-switched path as before.
    The Express RTP and TCP Header Compression feature has the following benefits:
    It reduces network overhead.
    It speeds up transmission of TCP and RTP packets. The faster speed provides a greater benefit on slower links than faster links.

    QUESTION 17
    On the basis of the definitions of different services in various RFCs, traffic with Expedited Forwarding per-hop behavior should be marked as which of these?
    A.    IP ToS of 0xEF
    B.    IP experimental ECN
    C.    DSCP decimal 5
    D.    Binary value of 101110
    Answer: D
    Explanation:
    The assured forwarding (AF) model is used to provide priority values to different data applications. The Expedited Forwarding (EF) model is used to provide resources to latency (delay) sensitive real-time, interactive traffic. The EF model uses one marking — DSCP 46. DSCP 46 is backward compatible with an IP Precedence value of 5 as seen in the following binary pattern:
    101110 = DSCP 46
    The EF marking of 46 does NOT follow the drop preference rules of the assured forwarding model. Please do NOT think that the 11 means high drop preference. The EF model is used for voice over IP media traffic (RTP) by default in most vendors phones. Cisco IP Phones mark signaling packets (SCCP or SIP) to CS3 (24), while media (RTP) is marked to EF (DSCP 46) by default. All EF traffic is normally mapped to the priority queue (PQ) on Cisco switches and routers. The priority queue guarantees three critical services:
    Packet Loss
    Delay
    Jitter (delay variation)
    The three most significant bits of 101 are only considered if IP Precedence was being used. The binary digits of 4 2 1 are used to factor the 101 binary pattern when only three digits are under consideration. The DSCP binary pattern of 101110 (46) uses six digits or binary values-32 16 8 4 2 1.
    It is good to know how to convert a DSCP decimal value to an entire ToS octet (byte) values as well. The ToS byte uses all eight bits, while the DSCP is only using the leading six digits. The EF pattern discussed above, will becomes 10111000 when considering the entire octet. Notice the two least significant zeros that were added to the 101110 binary pattern. Many network management utilities will only allow administrators to configure or display the entire ToS byte. A ping –V from a Microsoft operating system requires setting the entire ToS byte. An extended ping from a Cisco router will also allow administrators to see the entire ToS byte. Sniffer Pro LAN and Wire Shark sniffers show the entire ToS field as well. IP accounting shows the entire ToS byte, while Netflow shows the ToS byte in hexadecimal format. The ToS byte value for EF is as follows:
    128 64 32 16 8 4 2 1
    1 0 1 1 1 0 0 0
    A DSCP value of 46 results in a ToS byte value of 184. Although you can mark a ping with a ToS value of 184, the ICMP (ping) traffic will probably not be mapped to the proper application class. In the next blog, we will learn QoS models for using markings for different application classes.

    QUESTION 18
    For the following items,what is the mathematical relationship between the committed information rate (CIR), committed burst (Bc), and committed rate measurement interval (Tc)?
    A.    CIR = TC / Bc
    B.    CIR = Be / Tc
    C.    Tc = CIR / Bc
    D.    Tc = Bc / CIR
    Answer: D
    Explanation:
    Terminologies: The term CIR refers to the traffic rate for a VC based on a business contract.
    Tc is a static time interval, set by the shaper.
    Committed burst (Bc) is the number of bits that can be sent in each Tc.
    Be is the excess burst size, in bits. This is the number of bits beyond Bc that can be sent after a period of inactivity.

    QUESTION 19
    Which two statements best describe CBWFQ? (Choose two.)
    A.    The CBWFQ scheduler provides a guaranteed minimum amount of bandwidth to each class.
    B.    CBWFQ services each class queue using a strict priority scheduler.
    C.    The class-default queue only supports WFQ.
    D.    Inside a class queue, processing is always FIFO, except for the class-default queue.
    Answer: AD

    QUESTION 20
    You work as a network technician at Company.com, study the exhibit provided. You are implementing this QoS configuration to improve the bandwidth guarantees for traffic towards two servers, one with the IP address 5.5.5.5 and the other with the IP address 5.5.5.4. Even after the configuration is applied, performance does not seemto improve. Which will be the most likely cause of this problem?
    A.    The policy map mark has been applied on a half-duplex Ethernet interface; this is not supported.
    B.    The policy map queue is configured on the wrong interface; it is applied on the serial interface whereas traffic is going over the tunnel interface.
    C.    The class maps are wrongly configured
    D.    The ip nbar protocol-discover command cannot be configured together with a service policy output on the serial interface.
    E.    This is probably a software bug
    Answer: C

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