Networks and Networking AICT003-3-2

Local Area Network Technologies

Topics and Structure of the lesson

• Bridge• Hub• Switch• Router

Learning Outcomes

• At the end of this lecture YOU should be able to:– Describe the function of the following devices:

• Repeater• Bridge• Router• Hub• Switch

Key Terms you must be able to use:

If you have mastered this topic, you should be able to use the following terms correctly in your assignments and exams:

Signals

• A signal may experience attenuation as the distance it has to travel increases.

• This is not acceptable when networks reach distances covering several kilometers.

• The amount of attenuation that affect the signals along this channels would introduce very high data corruption rates.

5

Devices

• In order to link multiple networks, devices are required to determine where a particular packet is headed.

• These devices are responsible for careful routing of packets such that it would not end up in the wrong network.

6

Overview

hub

• The most basic of these devices is the repeater.

• The function of the repeater is to simply regenerate the signal which it receives and retransmits a refreshed signal back into the network towards the sink.

• A repeater has no other function except for its duty to increase the physical length of a network.

8

Connecting Ethernet Segments

• Repeater / Hub: Regenerates (strengthens) the signal and copies the incoming bit-stream to all outputs

10 Mb/s 4 hubs max

100 Mb/s 2 hubs max

1 Gb/s 1 hub max• Bridge / Switch: Filters MAC addresses to isolate local traffic

– Count starts over with switches• Router: Forwards packets based on (Layer 3) network

addresses rather than (Layer 2) MAC addresses– can link dissimilar LANs - ie LANs using different

physical/data link technologies

Traditional LAN Design: Hub & Router

Originally all network devices on a local-area network (LAN) tapped into a single physical (repeatered) or logical (hubbed) cable

The Ethernet LAN can be considered a collision domain, because all packets are visible to all devices on the LAN and are free to collide, given the CSMA/CD scheme used by Ethernet.

LANs were then interconnected with routers to overcome distance limitations

Bridges & Switches

The bridge learns the location of devices by looking at the source address in each frame, and develops a switching table

For subsequent frames, If the destination address is in the table, the frame is forwarded only to one port

MAC Address Port

08-00-07-06-41-B9 1

00-00-0C-60-7C-01 2

00-80-24-07-8C-02 3

• If the destination address is not in the table the frame is sent to all ports, except the port on which the frame was received

• Multicast and Broadcast frames are also “flooded” to all ports

Bridges and Switches are used to control the size of collision domains

Thus all the bridged segments together form a single broadcast domain, and each port of a bridge is a collision domain.

Bridge vs. Switch

Bridges Switches

Software-based Hardware-based (port-level ASICs)

Relatively slow Comparatively fast

Typically up to 16 ports Possibly hundreds of ports

Only forwards complete frames

Can start sending a frame before it is completely received (cut-through)

Only one forwarding path Supports multiple, parallel forwarding paths

One spanning-tree per bridge

Supports one spanning-tree per VLAN(We’ll talk about this in Week 9)

A switch works exactly like a bridge, but is faster and more capable - application-specific integrated circuits (ASICs) handle forwarding

Traditional LAN Design

• Originally campus networks consisted of a single local-area network (LAN) to which new users were added. This LAN was a logical or physical cable into which the network devices tapped. The Ethernet LAN can be considered a collision domain, because all packets are visible to all devices on the LAN and are free to collide, given the CSMA/CD scheme used by Ethernet.

• When the collision domain of the LAN became congested, a bridge was inserted. A LAN bridge is a store-and-forward packet switch. The bridge segments the LAN into several collision domains, and therefore increases the available network throughput per device.

• Bridges flood broadcasts, multicasts, and unknown unicasts to all segments. Therefore, all the bridged segments in the campus together form a single broadcast domain.

Table 4-8. Maximum Size of a Broadcast Domain

Protocol Maximum Number of Workstations

IP 500

NetWare 300

AppleTalk 200

NetBIOS 200

Mixed 200

If IP users are running multimedia applications with high-bandwidth and low-delay requirements, and/or a high level of broadcast or multicast packets, the maximum number of workstations should be reduced to 200.

General rule:Limit the size of broadcast domains to under 200 workstations (or other devices like IP phones)

Bridging

• In theory, the amount of broadcast traffic sets a practical limit to the size of the broadcast domain. In practice, managing and troubleshooting a bridged campus becomes increasingly difficult as the number of users increases. One misconfigured or malfunctioning workstation can disable an entire broadcast domain for an extended period of time.

• When designing a bridged campus, each bridged segment corresponds to a workgroup. The workgroup server is placed in the same segment as the clients, allowing most of the traffic to be contained. This design principle is referred to as the 80/20 rule and refers to the goal of keeping at least 80 percent of the traffic contained within the local segment.

Layer 2 Switching

• Layer 2 switching is hardware-based bridging. The frame forwarding is handled by specialized hardware, usually application-specific integrated circuits (ASICs). Layer 2 switches are replacing hubs at the wiring closet in campus network designs because the performance advantage is dramatic.

• The high performance of Layer 2 switching has led to some network designs that increase the number of hosts per subnet, which means a flatter design with fewer subnets or logical networks in the campus.

• However, broadcast domains built with Layer 2 switches still experience the same scaling and performance issues as large bridged networks.

Layer 3 Routing

• A router is a packet switch that is used to provide connectivity between broadcast domains. Routers forward packets based on network addresses rather than Media Access Control (MAC) addresses. These internets are more scalable than flat bridged networks, because routers summarize reachability by network number.

• Routing protocols offer:– Load balancing across many equal-cost paths (in the Cisco

implementation)– Optimal or lowest-cost paths between networks– Fast convergence when changes occur– Summarized (and therefore scalable) reachability information

(more details coming soon)

Layer 2 and Layer 3 Switching

• Switching is the process of taking an incoming frame from one interface and delivering it out through another interface. – Routers use Layer 3 switching to route a packet– Switches use Layer 2 switching to forward frames.

• The difference between Layer 2 and Layer 3 switching is the type of information inside the frame that is used to determine the correct output interface. – With Layer 2 switching, frames are switched based on

MAC address information. – With Layer 3 switching, frames are switched based on

network-layer information.

Switch

• By scanning the address of a network packet, a switch can route the packet directly towards the destination, thereby it reduces collisions on the Ethernet.

• The other advantage is that each packet does not use up the entire capacity of the Ethernet. This would, then, allow for simultaneous packet transmissions.

• In this sense, a switch would function better than a hub in Ethernet networks.

19

Transparent Bridging (Switching)

An end system sends a frame to a destination without knowing whether the destination is local or on the other side of a transparent bridge

The bridge learns the location of devices by looking at the source address in each frame, and develops a switching table

MAC Address Port

1

2

3

08-00-07-06-41-B9

00-00-0C-60-7C-01

00-80-24-07-8C-02

Transparent Bridging (Switching)

• If the destination address is in the table, the frame is forwarded to the port

• If the destination address is not in the table the frame is sent to all ports (except the port on which the frame was received)

• Multicast and Broadcast frames are also “flooded” to all ports

Switch A

Station A1 Station A2 Station A3

Network A

Switch B

Station B1 Station B2 Station B3

Network B

First. Imagine two switches that are not connected to each other in any way. Switch A connects stations in Network A and Switch B connects stations in Network B,

When Station A1 sends a broadcast, Station A2 and Station A3 receive the broadcast, but none of the stations in Network B receive the broadcast, because the two switches are not connected.

Virtual LANs (VLANs)

• An emulation of a standard LAN that allows data transfer to take place without the traditional physical restraints placed on a network

• A set of devices that belong to an administrative group• VLANs can be configured on a single switch or across multiple

switches• Designers use VLANs to constrain broadcast traffic

– a misconfigured or malfunctioning workstation can introduce errors that will impact or disable the entire domain. A jabbering network interface card (NIC) may flood the entire domain with broadcasts. A workstation with the wrong IP address can become a black hole for packets. Problems of this nature are difficult to localize.

VLANs versus Real LANs

Switch A

Station A4 Station A5 Station A6

Switch B

Station B4 Station B5 Station B6

Station A1 Station A2 Station A3 Station B1 Station B2 Station B3

LAN A

LAN A

LAN B

LAN B

VLANs versus Real LANs

Switch A

Station B1 Station B2 Station B3

Switch B

Station B4 Station B5 Station B6

Station A1 Station A2 Station A3 Station A4 Station A5 Station A6

VLAN B

VLAN A

VLAN B

VLAN A

Switch A

Station B1 Station B2 Station B3

Switch B

Station B4 Station B5 Station B6

Station A1 Station A2 Station A3 Station A4 Station A5 Station A6

VLAN B

VLAN A

VLAN B

VLAN A

Both switches contain stations that are members of VLAN A and VLAN B.

Problem: what to do with the forwarding of broadcast, multicast, or unknown-destination frames from a member of a VLAN on one switch to the members of the same VLAN on the other switch?

Switch A

Station B1 Station B2 Station B3

Switch B

Station B4 Station B5 Station B6

Station A1 Station A2 Station A3 Station A4 Station A5 Station A6

VLAN B

VLAN A

VLAN B

VLAN A

All frames going from Switch A to Switch B take the same interconnection path (called a trunk link or simply a trunk)

As a frame leaves Switch A, a special header is added to the frame, called the VLAN tag. The VLAN tag contains a VLAN identifier (ID) that specifies to which VLAN the frame belongs.

What is IOS?Internetwork Operating System

• Operating System of all Cisco Devices

• A derivative of BSD UNIX

• Custom built by Cisco for each platform

• Pre-packaged and static. Complete IOS is upgraded.

• Features available in different versions (for a price!)

• GUI’s available, but 90%+ of users still prefer command-line configuration.

• IOS is designed to be hardware independent.

Switch Overview

Color System Status

Off System is not powered on.

Green System is operating normally.

Amber System is receiving power butis not functioning properly.

Switches contain CPU, RAM, Operating System

POST

When switched on – System LED

indicates status

Status LED Overview

• Mode button toggles between different port modes

• Port mode displayed on Port Mode LEDs

• Individual Port status above each port

Port Status 1

Port Status 2

User Mode Commands

Default Configuration

• When powered up without configuration, the default name is Switch.

• No passwords have been configured.• All switch ports are part of VLAN 1• Switch has no IP address• Show version shows the IOS version and

the configuration register.

• First,– Remove any VLAN info (delete flash: vlan.dat)– Erase startup– Reload

• Follow router configuration method to configure hostname, line passwords, set a default gateway, etc.

• Set IP address for management VLAN1 for telnet access.

• Fast Ethernet ports default to auto-speed and auto-duplex, or they can be set manually.

• Management of a switch can also be done using a GUI interface as long as HTTP service is turned on.

Actual Configuration

Switch Configuration 1• Erase any existing settings

– VLAN database– Configuration in NVRAM

• Reload the switch

Switch Management• Connect to switch via console connection• Give switch IP address on management VLAN 1

Switch(config)#interface vlan 1Switch(config-if)#ip address 192.168.1.2 255.255.255.0

• Use web-browser based managementSwitch(config)#ip http server

Defining port speed / duplex

• Speed:–Interface config mode: – speed [10 | 100 | auto]

• Line mode is either full duplex or half duplex.–Interface config mode: – duplex [auto | full | half]

–auto option only for fixed Fast Ethernet TX ports.

VLAN Configuration• Entering VLAN Database

– Switch#vlan database– Switch(vlan)#

• Adding VLANs– Switch(vlan)#vlan 15 name Accounting

• Deleting VLAN database– Switch#delete flash:vlan.dat

• Assigning ports to a VLAN– Switch(config)#interface FastEthernet 0/12– Switch(config-if)#switchport access vlan 15

• Verify VLAN Configuration– Switch#show vlan

Summary

• Repeater• Bridge• Hub• Switch• Router

Question and Answer Session

Q & A