iap 20 - switching and bridging

8/14/2019 IAP 20 - Switching and Bridging

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Lecture 21 Switching and Bridging


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Lecture Objectives

To understand the functioning of layer 2devices

Broadcast and Collision Domains

Differentiating between a bridge and aswitch

To understand different bridging and

switching techniques To examine the different types of

bridges and switchesCS-3302-3 Internet Architecture and

Protocols, PUCIT, University of thePunjab, Pakistan


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Overview of Link Layer

Main functions:

Data flow control

Error control

Physical addressing

Managing access to physical media

Framing

Examples of link layer protocols: Token Ring (802.5), Ethernet(802.3),

FDDI.CS-3302-3 Internet Architecture and



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CS-3302-3 Internet Architecture and


Internetworking Devices


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Network Domains

Domain is referred to a specific area. In Networks those computers which come in a

single domain can by default communicate

with each other. Broadcast Domain

A range in which if a broadcast is done, all hearit.




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A group of devices in the network that receivesone anothers broadcast messages.

A broadcast domain is a set of NICs for which abroadcast frame sent by one NIC is received byall other NICs in the same broadcast domain.




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Collision Domain

Collision Domain A collision domain is a set of network interface

cards (NICs) for which a frame sent by one NICcould result in a collision with a frame sent by

any other NIC in the same collision domain. group of devices connected to the same

physical media such that if two devices accessthe media at the same time ,the result is thecollision of two signals




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Network Domains

Any device with more broadcast domains isbetter, as it can segment the network intomultiple network segments, reducing thebroadcasts and thus improving the efficiency in

terms of time and load. Hubs

Network becomes Half Duplex with Hubs

Hubs supports a single collision domain (assume that atthe back plane/motherboard, there is a single road

available for the frames to travel)




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Switch Network becomes Full Duplex with Switches

Switch supports multiple collision domains (assume thatat the back plane/motherboard, there are as many roadsare available for the frames to travel as many devicesare attached to the switch)




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LAN Switching

LAN switches forward Ethernet framesthey justhave to decide when to forward them and whennot to.

Most switch logic relates somehow to the source

and destination MAC addresses inside theEthernet frame headers of the frames sentthrough the LAN.

Switch logic is also dependent on the type of MACaddresses used.




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Working of a switch

A switch listens for frames that enter allits interfaces.

After receiving a frame, about a switchdecides whether to forward a frame and,if so, out which port(s).




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MAC address Table

Switches reduce network overhead byforwarding traffic from one segment to anotheronly when necessary.

To decide whether to forward a frame, theswitch uses a dynamically built table called abridge table or MAC address table.

The switch examines the address table to

decide whether it should forward a frame.




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Similarities

Both are called Layer 2 devices (DataLink Layer)

Both filters the traffic on the basis ofMAC address

Each segment has its own collisiondomain

All segments are in the samebroadcast domain.




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Bridges and Switches

The basic 5 operations: Learning

Forwarding

Flooding

Filtering

Aging




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Switches / Bridges

Forwarding Decisions: If Source and Destination LANs are

same, discard the frame

If Source and Destination LANs aredifferent, forward the frame

If Destination LAN is unknown, use

flooding.




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Bridges/Switches Operations

LearningThe switch learns MAC addresses by

examining the source MAC address of

each frame it receives. Switches build the MAC address table by

listening to incoming frames and

examining the frames source MACaddress.




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If a frame enters the switch, and thesource MAC address is not in the addresstable, the switch creates an entry in the

table.The MAC address is placed in the table,

along with the interface in which theframe arrived.

By learning, the switch can make goodforwarding choices in the future.




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Flooding

Bridges forward LAN broadcast frames,and unknown unicast frames, out all

ports.The switch floods the frame with the hope

that the unknown device will be on someother Ethernet segment, it will reply, and

the switch will build a correct entry in theaddress table.




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Filtering A switch generally does filtering along

with forwarding

There are two cases When the source and destination devices

exist on the same interface (port)

ofbridge/switch, then the bridge/switch willfilter the frame and doesnt pass it to anyother interface




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If source and destination are on differentinterfaces, and the destination port is known,then the frame is forwarded to only thedestination interface and it will be filtered or not

sent on the remaining interfaces.




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Bridge


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Learning Bridges


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LAN Switch Logic Summary

The following list provides a quick review of thebasic logic a switch uses: 1. A frame is received.

2. If the destination is a broadcast or multicast, forward

on all ports except the port on which the frame was received.

3. If the destination is a unicast, and the address is notin the address table, forward on all




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ports except the port on which the frame was received.

4. If the destination is a unicast, and the address is inthe address table, and if the associated

interface is not the interface on which the frame arrived,

forward the frame out the one correct port.

5. Otherwise, filter (do not forward) the frame.




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Switches

Layer 2 device

Usually connect individual computers in a LAN

Supports multiple ports (modular switches)

Gives dedicated bandwidth as compared to Hubsshared bandwidth

Like bridges, perform filtering




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Types of Switches

ATM switches Provides high speed switching facility

Used to connect multi-LAN networks

LAN switches Use to interconnect multiple LAN

segments or individual computers WAN

switches

WAN switches




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Types of InternetworkingDevices

Considering the Number of Ports inInternetworking Devices, all thedevices can be categorized as: Fixed Port Devices

In which the number of ports are fixed andcannot be increased

Modular Devices In which the number of ports can be

increase according the the requirements.




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Comparison Bridges

Contains limited numberof ports. (maximum no. ofports is 16)

Only uses store-and-forward switching

Generally used to connectmultiple LAN segments

Bridges only run 1spanning tree algorithm.

Bridge only analyze oneframe at a time

Switches

Contains multiple ports.Even new modularswitches support modulesin which we can increasethe no. of ports easily

Two switching techniquescan be used

Used to connectindividual devices in thesame LAN

Switches can run multiplespanning tree algorithms

Switch can analyzemultiple frames at a time.




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Layer 3 Switches

A layer 3 switch performs the work of arouter as well as a switch.

Layer 3 switches are modular switches,

we can add an RSM (Router SwitchModule) with it and then it can be usedto perform the work of a router I.e.communication over WAN as well as LAN




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Layer 3 or Routing switches, however,perform many of the layer 3 functionsusually reserved for routers. And because

the routing is implemented in hardwarerather than software, it is faster. Thedownside of routing switches is that theyare not as powerful or as flexible as full-

fledged routers.




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Switches

Loop prevention

The switch creates a loop-freeenvironment with other bridges by using

Spanning Tree Protocol (STP). Havingphysically redundant links helps LANavailability, and STP prevents the switchlogic from letting frames loop around the

network indefinitely, congesting the LAN.




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References

Cisco ICND 2004 by Cisco Press TCP/IP Protocol Suite

2nd Edition, Behrouz A. Forouzan, McGraw-Hill Inc.

Computer Networks 4th Edition, Andrew S. Tananbaum

Computer Networking; A Top DownApproach Featuring the Internet 3rd Edition: Jim Kurose and Keith Ross

http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/


Protocols PUCIT University of the

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