Cisco Network Devices

Chapter 6

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Chapter Objectives

At the end of this Chapter you will be

able to:Identify and explain various Cisco networking devices

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Network Devices

1. Network Hub

A network hub or concentrator is a device for connecting multiple twisted pair or fiber optic Ethernet devices together, making them act as a single network segment. Hubs work at the physical layer (layer 1) of the OSI model, and the term layer 1 switch is often used interchangeably with hub. The device is thus a form of multiport repeater. Network hubs are also responsible for forwarding a jam signal to all ports if it detects a collision.

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2. The Switch

A switch is a layer 2 network device that forwards frames using MAC addresses in the header of frames. It is used to improve network performance by:-Segmenting the network and creating separate collision domains.Reducing competition for bandwidth.In a switch frame forwarding is handled by specialized hardware called "Application Specific Integrated Circuit" (ASIC). powered by DJ 4

Steps of Switch FunctioningLearning  When switch starts, the MAC address table has no entry. When a node transmits data on its wire the MAC address of the node is learned by Switch Port connected to that node. In this way all the MAC addresses are learned by respective ports and these entries remain in the cache for a specific time. If during this specific time no new frame arrives from a node MAC address entry for that node is dropped from cache. Forwarding & Filtering When a MAC address for a port is learnt, packets addressed to that MAC address are forwarded only to the port associated with it, using one of the Switching Methods. Loop Avoidance Switches and Bridges use Spanning Tree Protocol (STP), specified by IEEE 802.1d, to prevent loops.

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Switching Methods

Store & Forward:  In this method complete frame is received by the switch. CRC(Cyclic Redundancy Check)l , source address and destination address are checked. This method has following features:-

Highest latency (delay in forwarding of frame) but may vary depending upon the length of frame.

Highest error checking.

Lowest frame forwarding speed.

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Cut Through:  In this method forwarding starts as soon as destination address of the frame is received in header. Also known as WIRE SPEED. This method has following features:-

Lowest latency.

Lowest error checking.

Highest frame forwarding speed.

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Fragment Free (Modified Cut Through):  In this method forwarding starts as soon as first 64 bytes of the frame are received as fragmentation occurs usually in first 64 bytes. This method has following features:-

Latency approx 60m Sec.

Sufficient error checking.

Moderate frame forwarding speed.

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3.The Bridge

It is a layer 2 device used to connect different network types or networks of the same type. Packets having destination address on the same network segment are dropped. Bridges use "Store and Forward" method to inspect the whole packet. 

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Advantages and Disadvantages of Bridge

Advantages:

Reliability

Manageability

Scalability

Disadvantages: 

A bridge cannot filter out broadcast traffic.

It introduces 20 to 30 % latency. 

Only 2 networks can be linked with a bridge.

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Collision Domain

Take a look at the diagram below. In it, all computers are connected to a hub, and as such, are part of the same collision domain. Collisions are obviously a bad thing, since when they occur, systems much resend their data, which leads to less-than-optimal throughput

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Conti…. In the past, in order to make better use of the network and reduce

the number of collisions, companies would implement a network bridge. The purpose of the bridge was to segment the network in a number of smaller collision domains, as shown below.

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Conti…

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4. ROUTER Routers, by default, break up a broadcast domain, which is the set

of all devices on a network segment that hear all broadcasts sent on that segment.

Two advantages of using routers in your network are They don’t forward broadcasts by default. They can filter the network based on layer 3 (Network layer) information (i.e., IP address). Four router functions in your network can be listed as Packet switching Packet filtering Internetwork communication Path selection

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ROUTER

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THANK YOU

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