CE231
Undergraduate Examinations 2018
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(a)
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Question 1
(a)
Briefly describe the dynamic host control protocol (DHCP). Furthermore, comment on its
[10%]
use in IPv6.
(b)
A company wishes to have all clients connected to a common physical infrastructure of
[10%]
Ethernet switches and cables but maintain logical separation for security so that the traffic
of guest users is kept separate from the traffic of administrative users. Propose a
mechanism that achieves this aim and explain how it operates.
(c)
The IPv4 based Internet is said to have run out of addresses. Explain how it is possbile that
[10%]
there can be more clients using the IPv4 Internet than the number of individual IPv4
addresses and describe the mechanism used to allow this. Furthermore, how can IPv6
provide a solution to this problem?
(d)
The data centre network shown in Figure 1 is designed with redundancy in every part
[10%]
except the Access Layer, such that a single failure of any component allows the network
function to continue. Propose protocols that allow the network to automatically cope with
such a failure and state the function that they perform; you do not need to describe how the
protocols operate.
To Internet Service Provider
CoreR1 CoreR2
DistS1 DistS2
AccessS1 AccessS2
Access
Layer
Server1 Server2 Server3 Server4
Figure 1.
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Question 2
The Internet
RC2
via Internet
Service
Provider (ISP)
RC1
RISP
RC3
Branch C
RW2
RA2
LAN1
RA1
LAN2
RW3
RW1
LAN3
RW4
RA3
LAN4
RB2
RB1
WAN
Branch A
RB3
Branch B
Figure 2.
LAN Number of hosts
2000
20
120
440
Table 1.
A company network is shown in Figure 2 consisting of three branches (Branch A, B and C) interconnected through a wide area network (WAN) which also connects to the Internet through an Internet service provider (ISP). Only one ISP router (RISP) is shown and this is under the control of the ISP, all other routers are under control of the company. Only Branch A is shown fully, the other branches may contain other routers or local area networks (LANs). The number of hosts in each LAN of Branch A are given in Table 1 (where a router interface counts as one of the hosts).
(Question 2 continues over the page)
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(Question 2 continued)
(a) Design the IP subnets of Branch A for the four LANs and the three router links (ignore the [12%] subnet linking WAN and Branch A). Your subnets must use addresses starting from 10.0.0.0
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.
The network architect specifies that you must use the multi-area OSPF routing protocol in such a way that it minimises the size of the routing tables.
(b) Design a suitable multi-area OSPF deployment by specifying:
[6%]
the number and location of suitable OSPF areas;
the area border routers (ABRs) and which area they belong to; and, the autonomous system border router (ASBR).
(c) By referring to the suitable OSPF link state advertisements (LSAs), describe how the
[12%]
routing protocol operates for the network you have designed and consequently, how it reduces the number of routing table entries compared to single-area OSPF.
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Question 3
The switches that make up a single LAN of a company’s network are shown in Figure 3 (on the
next page). Each switch has a large number of access ports connected to workstations that are
not shown. The network is connected to the Internet through a single router. Most of the traffic
in the network is from each of the workstations to the Internet. The switches are running
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).
(a) By describing the operation of STP, determine the STP role of each switch port (except port [17%] r). Mark the STP role on a copy of Figure 3 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.
The administrator of the network, that you have just explained, finds that the convergence of the network may take 30 seconds or more.
(b) Explain why the convergence of the network is slow and propose a method to considerably [13%] improve this convergence time. Explain how the method you propose operates and how
long it is likely to take to converge.
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(Figure for Question 3)
To Internet
G0/0
Switch 4
a
b
r
Switch 5
b
a
a
b
Switch 1baSwitch 2baSwitch 3
Figure 3.
Record your answer on the copy of Figure 3 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