CE231
Undergraduate Examinations 2014
COMPUTER AND DATA NETWORKS
Time allowed: Three hours
Candidates are permitted to bring into the examination room:
Hand-held, non-programmable calculators (containing no textual information).
Candidates must answer ALL questions
The questions are NOT of equal weight:
Part of the answer for Question 3 (b) must be written on the provided answer sheet. You must attach this answer sheet to your answer booklet using the tag provided.
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Candidates must answer ALL questions.
Question 1
(a)
Distance vector protocols, such as RIP, are said to suffer from routing loops under
[10%]
certain conditions. Briefly explain how a routing loop may occur in a distance vector
protocol and how distance vector protocols avoid routing loops.
(b)
Classless inter-domain routing (CIDR) replaced classful addressing in the Internet
[10%]
from 1993. Explain why this change was required and how CIDR achieves the
necessary solution.
(c)
Explain how a router with a single Ethernet port can be used as a router between many
[10%]
subnets in an IEEE 802.1Q based network.
(d)
Spanning tree protocol (STP) IEEE 802.1D requires that switches wait 30 seconds
[10%]
before a newly connected port is moved from the blocking state to the forwarding state.
By referring to the processes in STP, explain why this delay is required. Furthermore,
propose how this delay can be shortened.
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Question 2
Figure 1
A small company network is shown above in Figure 1. The company has three sites: HQ,
Branch1 and Branch2. All of the server resources are at the HQ site and it is important
that both Branch1 and Branch2 have high-speed connectivity to HQ. The networks
internal to each site are high-speed Ethernet local area networks (LANs) using a single
subnet at each site. The wide area network (WAN) connections between the sites have
the capacities shown on Figure 1. The connection between Branch1 and HQ is designed
only as an emergency management connection in case one of the other WAN connections
fails.
The network is to use the address range 192.168.1.0-192.168.1.255 and router interfaces
on each LAN must use the last address in each subnet.
(a) Specify suitable addresses for each of the networks in the scenario such that your
[13%]
design uses the smallest range of addresses for each subnet. Your answer must
specify the subnet addresses, masks and IP addresses of each router interface.
The network designer considers using either RIPv2 or OSPF as the routing protocol in the
network.
(b) Compare RIPv2 and OSPF and thus propose the most suitable routing protocol for
[17%]
the given scenario. Your answer must include sufficient explanation of both
protocols to justify the comparison and choice.
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Question 3
The network shown in Figure 2 (on the following page and loose sheet) provides connectivity for an organisation operating in a single building according to a hierarchical network model. The organisation has 1000 desktop computers (not shown in Figure 2) that are connected to the four high-port density switches in the Access layer. A central server resource, S1, is a high capacity server farm providing all the centralised resources for the desktop computers. The seven Ethernet switches are all running spanning tree protocol (STP) IEEE 802.1D providing connectivity for a single subnet.
(a)
Figure 2 shows that the switches are organised into three layers with Switch 01 in
[6%]
Layer 1 and Switches 02 and 03 in Layer 2. State the names usually given to Layer 1
and Layer 2 and briefly describe the roles of each of the three layers.
Each of the switches has a switch MAC address of AA:BB:CC:DD:EE:XX where the XX
is replaced by the switch number. Each switch is using its default configuration.
(b)
Determine the STP role of each port of the switches shown in Figure 2. A copy of
[18%]
the network is available on a loose sheet for you to record your answer. Mark each
port’s role in the appropriate box using a single letter: r for root, d for designated and
n for non-designated. Attach your filled in sheet to the answer booklet. In your answer booklet, explain how you have derived the port roles for switches 01, 02 and 04.
The network administrator has become aware that users of the network are finding the network slow since a recent increase in the number of desktop computers. The network links have the following capacities:
•
S1 to R1 and R1 to Switch 01 have 10 Gb/s
•
Connections between any of the switches are each 1Gb/s
•
Access port connections are 100 Mb/s
The switches and router are all modern systems designed for high-capacity campus
networks. It has been noted that the maximum traffic to the server is only 1 Gb/s and that
there is a large amount of broadcast traffic in the network.
(c)
The organisation needs to improve the network performance by changing the
[6%]
network configuration but without changing any of the hardware or cabling. Propose
how this can be achieved and briefly explain how your proposals achieve the improvement in performance.
Question 3 continues…
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Question 3 (continued)
Figure 2
Record your answer on the copy of Figure 2 provided on the loose sheet and attach that sheet to your answer booklet, do NOT use this question paper for recording your answer.
(However, you may use the diagram on this question paper for rough working.)
END OF EXAM PAPER CE231