CE231
Undergraduate Examinations 2017
Computer and Data Networks
Time allowed: Three hours
Candidates are permitted to bring into the examination room:
Calculator – Casio FX-83GT PLUS or Casio FX-85GT PLUS only
The following items are provided:
Answer sheet for Question 3(b)
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Question 1
(a)
A company has an internal network that contains many client workstations and a single
[10%]
HTTP server providing its web-presence. The company is using an Internet connection
from an Internet service provider that only provides one single IP address but wants all the
workstations to access the Internet and requires Internet users to be able to access the HTTP
server. Propose a solution that meets the given requirements and describe how it operates.
(b)
The IEEE 802.1Q protocol is said to provide virtual local area networks (VLANs). Briefly
[10%]
describe how VLANs operate using 802.1Q and what support for VLANs is required in
routers and switches.
(c)
Wireless LAN shares some similarities with wired Ethernet, however, it requires more
[10%]
complex media access control to cope with collisions. Explain why this is so and how the
collision control differs between the two systems.
(d)
An organisation is likely to use both static and dynamic routing within its network. Explain
[10%]
why this is so with regard to both security and resiliency.
3 CE231
Question 2
A company network is shown in Figure 1. Workstations (not shown on the diagram) are connected to Switches 1, 2 and 3 respectively. These workstations need high speed access to the Database server, DB, that is connected to the router R1. The original design specification required the network to have a fully redundant design, except for the connections of individual workstation to access layer switches.
(a) Explain why the network, as shown in Figure 1, does not meet the design specification for
[5%]
redundancy and propose a solution that does meet the specification.
The company ignores your advice and continues to use the design shown in Figure 1, rather than your proposed solution in Part (a). The switches are all using the default configurations; this means that spanning tree and port modes are all in their default state. The MAC address of a switch is given by ab:cd:ef:00:0x where x is replaced by the switch number, i.e. Switch 1 has a MAC address of ab:cd:ef:00:01.
(b)
Determine the STP roles of the switch ports shown in Figure 1. You should record your
[15%]
answer on the separate, loose, answer sheet showing a copy of Figure 1. Use the boxes on
this sheet to indicate the STP role with a single letter (i.e. D for designated, N for
non-designated, and, R for root). Note that only the switch ports with square boxes use STP
and thus are the only ports that have STP roles. In your answer booklet you must give a
description of how you have determined the STP roles. Make sure you attach the loose
answer sheet to your answer booklet using the provided tag.
Base-line traffic measurements show that the traffic levels to and from Switch 3 are reaching
saturation, while traffic on all other network ports are operating within normal parameters.
Additionally, you are told that every link shown has a capacity of 1 Gb/s, except for the
connections to the router, R1, which all have a capacity of 10 Gb/s.
(c)
Explain why the switching in this network is sub-optimal and propose how you would
[10%]
change only the configurations in the switches to reduce the congestion in Switch 3 and
improve the switching paths. Explain how your solution operates.
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R1
b
a
DB
Switch 4 d
a
b
c e
d
Switch 5
a
c b
abababSwitch 1Switch 2Switch 3Access ports
Access ports
Access ports
Figure 1.
Record your answer on the copy of Figure 1 provided on the loose sheet and attach that sheet to your answer booklet. Do NOT use this question paper for recording your answer, anything written on the question paper will not be awarded marks.
5 CE231
Question 3
A company has a headquarters (HQ) with four LANs containing the following number of hosts:
Admin LAN with 10 hosts Staff LAN with 2000 hosts
Management LAN with 60 hosts
BuildingManagement LAN with 400 hosts.
The subnet allocation for these LANs requires that:
the IP addresses should start from 155.245.0.0
the subnets must be contiguous from the starting address
the smallest possible number of addresses should be allocated to each subnet.
(a)
Design the subnet allocation for HQ given the requirements above. For each LAN you
[8%]
should list the:
subnet address
subnet mask in dotted decimal form (e.g. 255.255.255.0)
the last valid host address in the subnet.
The company expands over a number of years such that they have 100 branches worldwide,
each branch has 50 internal networks. The branches are connected through a wide area network
(WAN) which has 200 links.
(b)
Estimate how many routing entries are required in each router, assuming that address
[1%]
summarisation is not used by the routing protocol.
(c)
Propose a routing protocol that can use network address summarisation in the given
[16%]
scenario and explain how the routing protocol operates. In particular you should describe
how it summarises addresses such as those you have designed in Part (a). You should state
any assumptions you make about the network design.
(d)
Assuming the solution you present in Part (c) is operating, determine and explain which
[5%]
routers have the largest number of routing entries. Using your answer estimate the number
of routing entries in these routers.
END OF PAPER CE231