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| Question ID 18511 | Which two options are reasons for TCP starvation? (Choose two.)
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| Option A | The use of tail drop
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| Option B | The use of WRED
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| Option C | Mixing TCP and UDP traffic in the same traffic class
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| Option D | The use of TCP congestion control
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| Correct Answer | C,D |
Explanation Explanation: It is a general best practice to not mix TCP-based traffic with UDP-based traffic (especially Streaming-Video) within a single service-provider class because of the behaviors of these protocols during periods of congestion. Specifically, TCP transmitters throttle back flows when drops are detected. Although some UDP applications have application-level windowing, flow control, and retransmission capabilities, most UDP transmitters are completely oblivious to drops and, thus, never lower transmission rates because of dropping. When TCP flows are combined with UDP flows within a single service-provider class and the class experiences congestion, TCP flows continually lower their transmission rates, potentially giving up their bandwidth to UDP flows that are oblivious to drops. This effect is called TCP starvation/UDP dominance. TCP starvation/UDP dominance likely occurs if (TCP-based) Mission-Critical Data is assigned to the same service-provider class as (UDP-based) Streaming-Video and the class experiences sustained congestion. Even if WRED or other TCP congestion control mechanisms are enabled on the service-provider class, the same behavior would be observed because WRED (for the most part) manages congestion only on TCP-based flows. Reference: http://www.cisco.com/c/en/us/td/docs/solutions/Enterprise/WAN_and_MAN/QoS_SRND/Qo S-SRND-Book/VPNQoS.html
| Question ID 18512 | What is the cause of ignores and overruns on an interface, when the overall traffic rate of
the interface is low?
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| Option A | a hardware failure of the interface
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| Option B | a software bug
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| Option C | a bad cable
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| Option D | microbursts of traffic
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| Correct Answer | D |
Explanation Explanation: Micro-bursting is a phenomenon where rapid bursts of data packets are sent in quick succession, leading to periods of full line-rate transmission that can overflow packet buffers of the network stack, both in network endpoints and routers and switches inside the network. Symptoms of micro bursts will manifest in the form of ignores and/ or overruns (also shown as accumulated in input error counter within show interface output). This is indicative of receive ring and corresponding packet buffer being overwhelmed due to data bursts coming in over extremely short period of time (microseconds). You will never see a sustained data traffic within show interfaces input rate counter as they are averaging bits per second (bps) over 5 minutes by default (way too long to account for microbursts). You can understand microbursts from a scenario where a 3-lane highway merging into a single lane at rush hour the capacity burst cannot exceed the total available bandwidth (i.e. single lane), but it can saturate it for a period of time. Reference: http://ccieordie.com/?tag=micro-burst