Objectives

After completing this chapter you will be able to:

Describe the different types of bridging: Transparent, Source Route and Translate

Describe how Transparent Bridges operateOutline the advantages and disadvantages of bridgingDescribe the fundamentals of LAN switches

OSI Model

Repeater

Bridge

Router

Data

Data

Data

Data

101100011110101010010

Data

DataT

A

N

S

P

D

Data

Data

Data

Data

101100011110101010010

Data

DataT

A

N

S

P

D

SYSTEM A SYSTEM B

Repeaters v Routers / Bridges v Routers

REPEATER

Single Broadcast DomainSingle Collision Domain

Network Segment A

C.D. B.D.

BRIDGE orSWITCH

Single Broadcast DomainTwo Separate Collision Domains

Network Segment A

C.D. B.D.

ROUTER

Two Separate Broadcast DomainsTwo Separate Collision Domains

Network Segment A

C.D. B.D.Network Segment A Network Segment B Network Segment B

Different Types of Bridging

Source Route BridgesDeveloped by IBM for use in Token Ring Networks.

The entire route to a destination is predetermined prior to sending data.

Transparent Bridges

Developed by Digital Equipment Corporation (DEC) for use in Ethernet

networks.Frames are forwarded one hop at a time towards the destination

Source Route Bridging

2

3

1

4

5

6

7

8

Discovery

Response

Transparent Bridging

Source Address tableAddress Interface

1 A2 A3 A4 B5 B6 B7 C8 C9 C

A

B 6

5

4

2

3

1BRIDGE

9

8

7

C

Transparent Bridge Operation - Example

AB

6

5

4

2

3

1

BRIDGE

9

8

7

C

Source 1 - Destination 4

Source 4 - Destination 1

Bridging Loops

Packet with unknown destination

Bridge 1 Bridge 2

Preventing Loops

FORWARDING

BLOCKING

FORWARDING

FORWARDING

Packet with unknown destination

Spanning Tree Protocol

A C

JI

H

FEG

D

B2 31 4

98

765

Root Bridge

BLOCKING

LAN

Bridge

Symbols:

Comparison of Source Route and Transparent Bridges

Transparent bridges are connection-less whereas source-route bridging provides connection-oriented networking.

Transparent bridges are completely invisible to the hosts and are fully compatible with all existing 802 products.

With Source-Route bridging, the network manager must manually install the LAN and bridge numbers.

One of the few advantages of source-route bridging is that, in theory, it can use optimal routing.

Source route bridges are more expensive, especially if they use VLSI chips in their interface cards.

Transparent bridges learn about bridge and LAN failures and other topology changes quickly and automatically.

LAN Switches

High Speed Backplane

Switch

HUB

Each network device, for example, a file server, has dedicated bandwidth.

VLANs

Switch

HUB

HUBHUB HUB

Advantages of Bridging

Bridges can connect networks running different protocols without requiring additional software.

Bridges form logically single networks. A bridge makes the movement of network devices, e.g. PCs, within the network easy.

Bridges are simple to install.Bridges are cheaper than routers.The presence of a bridge is transparent to users from the

instant it is first installed, and bridges adapt automatically to network changes.

Disadvantages of Bridging

Bridges cannot load-share traffic over two paths to a single destination, because the STA ensures that one of these paths will block all traffic.

Bridges cannot prevent a ‘broadcast storm’.Bridges do not provide significant support for fault

isolation or other distributed management capabilities.