CE231
Undergraduate Examinations 2019
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)
Explain why network address translation (NAT) is required in the Internet. Furthermore,
[10%]
explain how it operates.
(b)
A data-centre has two gateway routers, each with a link to the Internet to support
[10%]
redundancy (in case one router fails). However, the servers in the data-centre only support
one gateway at a time. Propose how the servers can be automatically directed to the correct
gateway in case one fails.
(c)
A network administrator wants the IP configurations of each host to be set automatically.
[10%]
Explain how this can be achieved for a network that has both IPv4 and IPv6 clients.
(d)
IEEE 802.1Q is used on trunks between switches and between switches and routers. By
[10%]
describing the packet format of IEEE 802.1Q frames, explain how they provide the support
for virtual LANs (VLANs).
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Question 2
LANA
Company
The Internet
R2
Network
via Internet
Service
R1
LANB
Provider (ISP)
LANC
R3
LAND
Figure 1.
LAN Number of hosts
120
1020
2010
D56
Table 1.
A small company network is shown in Figure 1, consisting of four LANs and three interconnecting router links. The number of hosts in each LAN is indicated in Table 1. The network connects to the Internet through the external interface of R1. The network is using OSPF as a routing protocol.
(a)
Design the IP subnets of the company network for the four LANs and the three router links
[12%]
(ignore the subnet linking R1 to the Internet). Your subnets must use addresses starting
from 172.16.0.0 and be using as few addresses as possible in a contiguous block. For each
subnet you must list: the subnet address; the subnet mask (in 4-byte dotted decimal form);
and, the last possible host address in the subnet.
(b)
Propose how routing information towards the Internet is configured in the network.
[3%]
(c)
The company expands to many hundreds of branches, each one of which has a unique wide
[15%]
area network connection to the network in Figure 1. R1 is the gateway to the Internet for the
whole network. Propose how routing for this network should be configured and briefly
explain how this operates.
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Question 3
A data centre network is shown in Figure 2 (shown on the following page). The switches are using spanning tree protocol (STP) 802.1D (1998). The MAC address of each switch is 01:02:03:04:05:0? where ? is replaced by the switch number so that, for example, Switch 1 has a MAC address of 01:02:03:04:05:01. All ports and links are using 1 Gb/s Ethernet and switches are in their default state (i.e. no STP priorities have been set). Multiple servers (not shown) are attached to the access ports and use a large number of virtual LANs (VLANs).
(a)
The data centre network can be said to be organised in the conventional manner, in that it
[9%]
consists of three layers: Acces, Distribution, and, Core. Identify each layer in Figure 1 by
referring to the network components. Furthermore, describe the purpose of each layer.
(b)
By describing the operation of STP, determine the STP role of each switch port (except
[18%]
ports e in Switches 1 and 2). Mark the STP role on a copy of Figure 2 that you have been
issued as a separate sheet. Use the boxes on the provided sheet to indicate the STP role with
a single letter (i.e. D for designated, N for non-designated, and, R for root). Make sure that
you have described how you have derived the port roles through your description of STP.
Ensure you attach the loose answer sheet to your answer booklet using the provided
tag.
(c)
The network is found to have network throughput that is approximately half of the capacity
[3%]
compared to the number of links between switches. State why this is so, and propose a
solution.
(Note Figure 2 on the next page)
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(Figure for Question 3(b))
Internet
R1
b
b
R2
a
a
Switch 1 e
a
b
e
Switch 2
d
d
a
c
c
b
a
b
a
b
Switch 3
Switch 4
b
Switch 5
Access ports Access ports Access ports
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, anything written on the question paper will not be awarded marks.
END OF PAPER CE231-5-FY