CE231
Undergraduate Examinations 2016
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)
Virtual LANs (VLANs) are frequently used within campus style networks. Describe how
[10%]
they operate and thus explain why they are useful in this type of network.
(b)
Briefly describe the stages required for a wireless LAN host to securely connect to a
[10%]
wireless access point.
(c)
State three essential settings for an IP connected host. Propose how these can automatically
[10%]
be set for a host and briefly describe how this mechanism operates.
(d)
Consider a network constructed from two Ethernet LANs that are connected together using
[10%]
a single IP router. On each LAN a single host is connected to the router through an Ethernet
switch. Explain how a packet is sent from the host in one Ethernet network to the host in
the other Ethernet network. Your explanation should include how switching or routing decisions are made in the sending host, the intermediate Ethernet switches and the IP router.
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Question 2
A network at a location called Branch is made up of local area networks (LAN)s with the following number of hosts in each LAN:
LAN
Number of hosts
A
200
B
100
C
500
D
50
(a)
Given a range of addresses 192.168.0.0 – 192.168.3.255, allocate IP subnets for each of
[12%]
these LANs using the minimum possible number of addresses in each subnet. For each
LAN you should list the:
subnet address;
subnet mask in dotted decimal form (e.g. 255.255.255.0);
and, the last valid host address in the subnet.
(b)
Calculate how many subnets are left in the range you have been allocated for networks that
[2%]
are constructed from point-to-point links. You must explain your answer.
(c)
Determine the summary address for the address range 192.168.0.0-192.168.3.255. Your
[1%]
answer should be given as a network address and subnet mask in dotted decimal form.
The network Branch is one of many remote networks that are connected together using a wide area network (WAN). The WAN is a single network made up of a number of routers. The whole network is to use a single interior-gateway routing protocol. The routing protocol should make use of summarisation as much as possible to reduce the routing table sizes.
(d) Propose a routing protocol for use in this network and explain how it operates including
[15%]
how it minimises the size of the routing tables.
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Question 3
The data center network shown in Figure 1 uses a redundant design.
(a) Explain why spanning tree protocol (STP) is needed in this network and briefly describe
[5%]
how it determines the root bridge.
The switches are all using the default spanning tree configuration and 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. Every link shown has a capacity of 1 Gb/s.
(b)
Determine the STP roles of the switch ports shown in Figure 1. You should record your
[15%]
answer on the loose copy of Figure 1 using the boxes to indicate the STP role with a single
letter (e.g. 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 sheet to your answer booklet using the provided tag.
The two WAN links are provided by a service provider. The primary WAN link is attached to
R1 and is normally activated. The WAN link to R2 is a backup connection and is normally off
but is automatically activated by the service provider if the link to R1 fails. The network should
have the default route set to the WAN through R1 unless the link fails, in that case the default
route should automatically switch over to the link over R2.
(c)
Propose how the network can be configured to automatically update the default route in
[4%]
case of a WAN failure and briefly explain how the mechanism operates.
Normally, servers connected to the Access ports use R1 as their default gateway; if R1 fails they should automatically switch to using R2 as their default gateway.
(d) In the case that R1 fails, propose how the network can be configured to automatically
[6%]
switch the default gateway of the servers from R1 to R2.
R1
Switch 4
a
a b
Switch 1
Access ports
WAN
c
c
R2
b
b
a
a
feefSwitch 5bddaccb
abbaSwitch 2
Switch 3
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.
END OF PAPER CE231