bn002 yr2 comp h2015 jan08 semester3 switching basics & intermediate routing

8/7/2019 BN002 Yr2 COMP H2015 Jan08 Semester3 Switching Basics & Intermediate Routing

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INSTITUTE OF TECHNOLOGY BLANCHARDSTOWN

BN002 Yr2 - Higher Certificate in Science in Computing in Information

Technology

BN013 Yr 2 - Bachelor of Science in Computing in Information Technology BN104 Yr 2 - Bachelor of Science (Honours) in Computing

Switching Basics and Intermediate RoutingCOMP H2015

Semester 1

Internal Examiner(s): Mr. Mark Cummins

External Examiner(s): Dr. Richard StuddertMr. John Dunnion

Thursday 17th January 2008 3.30pm - 5.30pm

Instructions to candidates:

Attempt ALL PARTS of question 1 and any THREE other questions. Question 1 is worth 40 marks and all other questions are worth 20 marks each.

DO NOT TURN OVER THIS PAGE UNTIL YOU ARETOLD TO DO SO

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Attempt ALL parts of this question

Question 1

Section A:(4 marks

each)

(a) Describe full and half duplex Ethernet operation on a switch.

Half Duplex:

data sent in only one direction at a time,either sending or receiving.

Typically uses CSMA/CD becausecollisions occur.

Hubs operate using half duplex.

Full Duplex:

Data sent in both directions at sametime.

No collisions occur.

Switches prefer to operate in full duplexmode

(b) Explain four features of the role of a designated router (DR) inan OSPF network?



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Question Not Covered in course was on

the old course. Part of routing course. We

covered it but wont be asked this.

Outline four advantages of using a link state routing protocolwhen designing a network?

Question Not Covered in course wason the old course. Part of routingcourse. We covered it but wont be asked

this. Provide an example of how an administrator using VLSMs can

save IP addresses by further subnetting the address(172.16.32.0/20) after it has already been subnetted to anetwork that has only ten hosts.

If you have a subnet address172.16.32.0/20, and you need to

assign addresses to anetwork that has ten hosts. With thissubnet address, however, you haveover 4000 (2122 = 4094) host addresses, most of which will be wasted. With VLSMsyou can further subnet the address 172.16.32.0/20 to

give you more network addressesand fewer hosts per network. If, for example,you subnet 172.16.32.0/20 to172.16.32.0/26, you



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Outline the main differences between a static and a dynamicVLAN.

The two common approaches toassigning VLAN membership are asfollows:

y Static VLANs

y Dynamic VLANs

Static VLANs are also referred to asport-based VLANs. Static VLANassignments are created by assigningports to a VLAN. As a device enters thenetwork, the device automaticallyassumes the VLAN of the port. If the user

changes ports and needs access to thesame VLAN, the network administrator must manually make a port-to-VLANassignment for the new connection.

Dynamic VLANs are created through theuse of software. With a VLANManagement Policy Server (VMPS), anadministrator can assign switch ports toVLANs dynamically based on



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information such as the source MACaddress of the device connected to the

port or the username used to log ontothat device. As a device enters thenetwork, the device queries a databasefor VLAN membership.

(i) What is the primary difference between a broadcast domainand a

collision domain? (ii) List two devices that can separate collisiondomains.

y A broadcast domain is a logicaldivision of a computer network, inwhich all nodes can reach each other by broadcast at the data link layer.

y A broadcast domain can be within thesame LAN segment or it can bebridged to other LAN segments.

y In terms of current popular

technologies: Any computer connected to the same Ethernetrepeater or switch is a member of thesame broadcast domain.



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y Further, any computer connected tothe same set of inter-connected

switches/repeaters is a member of thesame broadcast domain.

y Routers and other higher-layer devices form boundaries betweenbroadcast domains.

y This is as compared to a collision

domain, which would be all nodes onthe same set of inter-connectedrepeaters, divided by switches andlearning bridges.

y Collision domains are generallysmaller than, and contained within,

broadcast domains.y While some layer two network

devices are able to divide thecollision domains, broadcastdomains are only divided by layer 3network devices such as routers or

layer 3 switches.y A collision domain is a physical

network segment where data packets



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can collide with one another whenbeing sent on a shared medium, in

particular, when using the Ethernetnetworking protocol.

y A network collision occurs whenmore than one device attempts tosend a packet on a network segmentat the same time.

y Collisions are resolved using carrier sense multiple access or a variantthereof in which the competingpackets are discarded and re-sentone at a time.

y This becomes a source of inefficiency

in the network(2 devices )Switch or router willseperate

Outline four features of Spanning Tree protocol in a switchnetwork topology?

List two reasons how Ethernet switching increases a networksavailable bandwidth?

Ethernet switching increases thebandwidth available on a network



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by creating dedicated networksegments (that is, point-to-pointconnections) and connecting

those segments in a virtualnetwork within the switch. Thisvirtual network circuit exists onlywhen two nodes need tocommunicate. This is why it iscalled a virtual circuit, it existsonly when needed and isestablished within the switch.

Because a Switched EthernetLAN uses bandwidth soefficiently, it can provide morethroughput than Ethernet LANsconnected by bridges or hubs. Ina Switched Ethernetimplementation, the available

bandwidth can reach close to100%.

(c) What is the difference between OSI layer 2 and layer 3 switching?

Traditional network switches operate atLayer 2 of the OSI model while networkrouters operate at Layer 3. This often

leads to confusion over the definition of "Layer 3 switch."



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A Layer 3 switch is a high-performancedevice for network routing. Layer 3

switches actually differ very little fromrouters. A Layer 3 switch can support thesame routing protocols as networkrouters do. Both inspect incomingpackets and make dynamic routingdecisions based on the source and

destination addresses inside. Both typesof boxes share a similar appearance.



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Answer any 3 questions from this section

(All questions carry equal marks)

Question 2(a) What are the differences between symmetric and asymmetric switching? Draw a

schematic of each?

MethodsLAN switching can be classified as:Symmetric:All ports use the same bandwidth.

Asymmetric:

Ports can be different bandwidths.Applies to most Cisco switches.Memory buffering is required



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(4 marks)

(b) Describe how a switch handles broadcast domains. In your answer describe how the

use of routers reduces network traffic by reducing the size of a broadcast domain.

Although switches filter most framesbased on Mac addresses, they do notfilter broadcast frames. A collectionof interconnected switches forms asingle broadcast domain. Only aLayer 3 entity, such as a router, or

Virtual (Vlan), can bound a layer 2broadcast domain. Routers and Vlansare used to segement both collisionand broadcast domains.



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When a device sends out a layer2

broadcast, the destination macaddress in the frame is set to all ones.By setting the destination to thisvalue, all the devices accept andprocess the broadcasted frame.The broadcast domain at Layer 2 isreferred to as the destination MAC

broadcast domain. The MACbroadcast domain consists of alldevices on the LAN that receive framebroadcasts by a host on the LAN

(8 marks)

(c)Describe in detail two modes that an Ethernet switch uses to forward frames.(8 marks)

Store-and-forward:

Only method currently used by Cisco

switches

Complete frame is stored in buffer

Performs error checking on CRC



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Frames with errors are dropped

Traffic prioritisation (Converged networks)

Cut-through: (Two variations)

Fast-forward switching:

Fastest forwarding method

Reads just the destination before

forwardingNo error checking, corrupt frames

forwarded

Fragment-free switching:

First 64 bytes stored

Partial error check

Compromise between previous twomethods

Quality of service (Converged networks)



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Questi

Outli t l t t S i P t l l i ti l f t .

S P uses the spanning-tree algor itm S to eter mine hi h switch ports on a network need to e

locked to prevent loops romoccurr ing. he S designates a single switch as the I E

and uses it as a reference point for allsubsequent calculations. In the



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figure Switch S1 is chosen through anelection process

When the STA has determined the³best´ paths emanating from the rootbridge, it configures the switch portsinto distinct port roles. The port rolesdescribe their relation in the networkto the root bridge and whether they

are allowed to forward traffic.Root portsSwitch ports closet to the root bridgeDesignated portsAll nonroot ports that are stillpermitted to forward traffic on the

networkNon designated portsAll ports configured to be in ablocking state to prevent loops.

(8 marks)

(b) VL AN Trunking Protocol (VTP) was created to solve potential operational problems in

aVL

AN switched environment. Explain the fundamentals of V

TP.

y As the size of the network for a small-or medium-sized business grows, the



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management involved in maintaining the

network grows.

VL AN Trunking Protocol (VTP) of CiscoCatalyst switches simplifies management of the VL AN database across multipleswitches.

y VTP allows a network manager to

configure a switch so that it willpropagate VL AN configurations to other switches in the network.

y VTP only learns about normal-rangeVL ANs (VL AN IDs 1 to 1005).

y Extended-range VL ANs (IDs greater than 1005) are not supported by VTP

y After a trunk is established between thetwo switches, VTP advertisements areexchanged between the switches.

y

Both the server and client leverageadvertisements from one another toensure each has an accurate record of VL AN information.



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comes from a host A. Switch A forwards the

broadcast to switch B. In turn, Switch B

forwards the broadcast out a different portback to Switch A. Switch A repeats its initial

action and a broadcast loop is created.

Bridge table corruption can occur on a

layer 2 network when ARP frames getcirculated in a loop similar to the broadcast

loop causing the ARP table entries to

Fig 21

y The Spanning Tree Protocol (STP)prevent loops while still allowing for

redundancy in a bridged network



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y Two potential problems with loops ina bridged network are broadcast

loops and bridge table corruption.y Spanning Tree Protocol dictates

which path to take through a loopednetwork in order to avoid theseproblems.

y STP enabled bridges communicate

with each other to discover redundantpaths in the network.

y This discovery results in aspanning-tree which defines a singlepath to a given destination andalternate paths in the event of a

network outage or configurationchange.

(3marks)



Question 21

Page 4 of 5

(a) Outline the main reasons why a loopback address should be configured on

routers using the OSPF routing protocol?

Question Not Covered in course was

on the old course. Part of routingcourse. We covered it but wont be askedthis.

(3 marks)

(a) Describe the election process of the Designated Router (DR) and Backup Designated

Router (BDR) in an OSPF type network? Outline in your answer two ways in which this

election can be modified.

Question Not Covered in course wason the old course. Part of routingcourse. We covered it but wont be askedthis.

(8 marks)

(a) Describe in three steps how a switch learns addresses.

(9 marks)



Question 22

Page 4 of 5

(a) The class C address 192.168.30.0/24 has been assigned and must support thenetwork shown in the diagram above. Create an addressing scheme using VLSMthat will support the network requirements. The range of addresses for each L ANand WAN connection should be clearly o utlined, as well as the relevant subnetmask.

The following guidelines must be adhered to:i. Subnet 0 must be incorporated into your design.ii. Addresses assigned to the serial interfaces should be assigned from the end

of the addresses assigned to the L AN's.

(15 marks)

(b) What is the difference between CIDR and supernetting?

CIDR uses variable length subnet masking (VLSM) to allocate ip addresses to subnetsaccording to individual need rather than by class. This type of allocation allows thenetwork/host boundary to occur at any bit of the address. Networks can be further

Scenario



Question

divided or subnetted into smaller and smaller subnets. In the diagram ISP has four customers, each with a var iable amount of IP address space. owever , all the

customer address space can be summar ized into one advertisement to ISP . he . . . / summar ized or aggregated route includes all the networks belonging to

customers a, b, c and . his type of route is known as a supernet route. supernet summar izes multiple network addresses with a mask less than the classful mask.

Propogating vlsm and supernet routes requires a classless routing protocol, because the subnet mask can no longer be deter mined by the value of the f irst octet. he subnet mask now needs to be included with the network address. lassless routing protocols include the subnet mask with the network address in the routing update.

marks)

bn002 yr2 comp h2015 jan08 semester3 switching basics & intermediate routing

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