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- CCNA Routing and Switching 200-105
96.
Which of the following QoS marking fields could remain with a packet while being
sent through four different routers, over different LAN and WAN links? (Choose
two answers.)
- A.CoS
- B.IPP
- C.DSCP
- D.MPLS EXP
- Answer & Explanation
- Report
Answer : [B and C]
Explanation :
Explanation :
| The Class of Service (CoS) field exists in the 802.1Q header, so it would be used only on trunks, and it would be stripped of the incoming data link header by any router in the path. The MPLS EXP bits exist as the packet crosses the MPLS network only. The other two fields, IP Precedence (IPP) and Differentiated Services Code Point (DSCP), exist in the IP header, and would flow from source host to destination host. |
97.
Which of the following are available methods of classifying packets in DiffServ on
Cisco routers? (Choose three answers.)
- A.Matching the IP DSCP field
- B.Matching the 802.1p CoS field
- C.Matching fields with an extended IP ACL
- D.Matching the SNMP Location variable
- Answer & Explanation
- Report
Answer : [A, B and C]
Explanation :
Explanation :
| In general, matching a packet with DiffServ relies on a comparison to something inside the message itself. The 802.1p CoS field exists in the data link header on VLAN trunks; the IP DSCP field exists in the IP header; and extended ACLs check fields in message headers. The SNMP Location variable does not flow inside individual packets, but is a value that can be requested from a device. |
98.
Which of the following behaviors is applied to a low latency queue in a Cisco router
or switch?
- A.Congestion management
- B.Shaping
- C.Policing
- D.Priority scheduling
- E.Round robin scheduling
- Answer & Explanation
- Report
Answer : [D]
Explanation :
Explanation :
| Low Latency Queuing (LLQ) applies priority queue scheduling, always taking the next packet from the LLQ if a packet is in that queue. To prevent queue starvation of the other queues, IOS also applies policing to the LLQ. Most traffic chosen for LLQ classification requires low delay, jitter, and loss, so using a congestion management tool makes little sense. Likewise, applying shaping to an LLQ slows the traffic, and makes no sense with the presence of a policing function already. |
99.
Think about a policing function that is currently working, and also think about a
shaping function that is also currently working. That is, the current bit rate of traffic
exceeds the respective policing and shaping rates. Which statements are true about
these features? (Choose two answers.)
- A.The policer may or may not be discarding packets.
- B.The policer is definitely discarding packets.
- C.The shaper may or may not be queuing packets to slow down the sending rate.
- D.The shaper is definitely queuing packets to slow down the sending rate.
- Answer & Explanation
- Report
Answer : [A and D]
Explanation :
Explanation :
| Policers monitor the bit rate and take action if the bit rate exceeds the policing rate. However, the action can be to discard some packets, or to re-mark some packets, or even to do nothing to the packets, simply measuring the rate for later reporting. For shaping, when a shaper is enabled because the traffic has exceeded the shaping rate, the shaper always queues packets and slows the traffic. There is no option to remark the packets or to bypass the shaping function. |
100.
A queuing system has three queues serviced with round robin scheduling and one low
latency queue that holds all voice traffic. Round robin queue 1 holds predominantly
UDP traffic, while round robin queues 2 and 3 hold predominantly TCP traffic.
The packets in each queue happen to have a variety of DSCP markings per the QoS
design. In which queues would it make sense to use a congestion avoidance (drop
management) tool? (Choose two answers.)
- A.The LLQ
- B.Queue 1
- C.Queue 2
- D.Queue 3
- Answer & Explanation
- Report
Answer : [C and D]
Explanation :
Explanation :
| Drop management relies on the behavior of TCP, in that TCP connections slow down sending packets due to the TCP congestion window calculation. Voice traffic uses UDP, and the question states that queue 1 uses UDP. So, queues 2 and 3 are reasonable candidates for using a congestion management tool. |
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