CE231
Undergraduate Examinations 2015
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 2 (a)
Graph paper (available on the invigilator’s desk)
CE231
2
Question 1
(a)
Wireless local area networks use CSMA/CA as opposed to CSMA/CD that is used in wired
[10%]
Ethernet. Expand each of the abbreviations and thus explain why CSMA/CA, as opposed to
CSMA/CD, is used in wireless local area networks
(b)
Explain the changes that the IEEE 802.1Q VLAN protocol makes to the standard Ethernet
[10%]
frame and thus how VLANs operate.
(c)
A small company is supplied with one IP address by an Internet service provider but wishes
[10%]
to have many internal PCs that can access the Internet. Propose a mechanism that allows
this and briefly describe how it operates.
(d)
Routing can be configured either manually, using static routes, or automatically, using a
[10%]
routing protocol. Explain why in practice both types of routing are often used and propose
an example for each type of routing to aid your description.
3 CE231
Question 2
The campus network, shown in Figure 1 on the following page, represents part of a larger network. All links between the switches shown on Figure 1 have a capacity of 1 Gb/s. The links to the router both have a capacity of 10 Gb/s. Switches S1 and S2 are access switches with 48 clients connected to each of them; the client ports are not shown. S1 and S2 are relatively cost-effective access switches with a single power-supply. Switches S3, S4 and S5 are distribution layer switches with redundant power-supplies and purchased with high-reliability and high-switching capacity capabilities. It is expected that, at a later date, further access switches will be added. R1 provides all layer-3 routing in the network.
All of the switches are in their default state except for the fact that all the switch ports shown have been configured as IEEE 802.1Q trunk ports. As the switches are otherwise in the default state, no spanning-tree protocol (STP) priorities have been set on the switches. The MAC address of each switch is 09CD:1234:123X where X is replaced with the switch number; i.e. S1 has a MAC address of 09CD:1234:1231. The switches are using the default STP: IEEE 801.D. The majority of the traffic in the network is between the clients and the Server Farm.
(a)
Determine the STP role of each switch port shown in the network. A copy of the network is
[18%]
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 each port role.
(b)
Consider the virtual topology you have determined from the spanning-tree in (a). Explain
[6%]
why this virtual topology is sub-optimal with regard to the distribution of traffic in the
network. Propose a solution that gives better distribution of traffic and explain how it works.
(c)
Following the changes that you have proposed in (b), determine the STP roles of the four
[6%]
ports that connect the pair of links between S5 and S4.
CE231
4
Server
Farm
2
S5
1
6
1
1
S3
3
4
5
S4
2
2
3
1
2
2
3
S1
S2
3
1
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. (However, you may use the diagram on this question paper for rough working.)
5 CE231
Question 3
The network, on the following page in Figure 2, shows part of a large company network. The company has many distributed branches, two of which – Branch1 and HQ – are shown in Figure
The company connects its branches using a wide area network (WAN) shown in the diagram. Although the WAN connections are provided by a network operator the company operates its own backbone network using a number of routers, two of which are shown in the WAN part of the Figure. The company has decided to use OSPF for its network routing protocol.
The company uses private IP addressing for its internal network with addresses in the range 192.168.0.0-192.168.255.255 reserved for WAN links and a larger 10.0.0.0-10.255.255.255 range used for the branches. You will be designing the IP address allocation for HQ only. The WAN link between W1 and R1 has been allocated as shown on Figure 2. The addresses in the branches are to be allocated in contiguous blocks so that their addresses can be easily summarised. The number of hosts in each LAN are specified in Table 1 on the following page.
(a)
The addresses in HQ are to use a contiguous block starting at 10.1.0.0 and provide subnets
[14%]
that are just large enough for the four LANs and the router links. Design the IP subnet
allocation for HQ; for each HQ subnet specify the:
subnet address
subnet mask (a four “dotted” decimal number)
first possible host address
last useable host address in the subnet
(b)
Determine the smallest, single, address summarisation that can encompass all of the HQ
[2%]
addresses that you have allocated in (a). For this summarised address, also determine the
addresses that are not allocated to subnets in (a) by stating the first and last unused address.
It is to be assumed that these unused addresses will be used for future expansion.
(c)
OSPF is said to be a link state protocol. Describe what is meant by this term and thus how
[7%]
OPSF uses link state advertisements (LSAs) to determine the routing tables.
(d)
Propose, and explain, how the OSPF configuration can be designed to ensure that the
[7%]
routing tables in the network are kept as simple as possible. As part of your explanation
state how the routing table in B2 (in Branch1) would represent the networks in HQ and any
configuration required in R1. Your answer should briefly describe the LSAs that would be
used in this design.
(Figure 2 and Table 1 are overleaf)
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B3 Branch1
B1
G1
G0
B2
LAN1
R2 LAN2
192.168.1.0/30
W2
W1
R1
LAN3
WAN
HQ
R3
LAN4
Figure 2
HQ LAN
Number of hosts
LAN1
100
LAN2
50
LAN3
3000
LAN4
20
Table 1
CE231-5-FY Answer Sheet for Question 2 (a)
Server
Farm
2
S5
1
6
1
1
S3
3
4
5
S4
2
2
3
1
2
2
3
S1
S2
3
1
Figure 1
Record your answer on the copy of Figure 1 provided above and attach this sheet to your answer booklet.