OSI Model

Switches

point to point bridges two types

store & forward = entire frame received the decision made, and can handle frames with errors

cut-through = starts to transmit frame as destination address received, cannot handle errored frames but are the fastest bridges

latency = data transmission delay created by frame or packet processing

DATA LINK LAYER (BRIDGE)

Application

Presentation

Session

transport

Network

Data link

Physical

Application

Presentation

Session

transport

Network

Data link

Physical

Data link

Physical

Data link

Physical

BRIDGE

EXAMPLE: MAC Bridge

Bridge Operation

How a Bridge works! listens to all traffic

checks source and destination addresses

builds a routing table

forwards packets on the following basis

if destination not in routing table send to all segments

if destination in routing table forward to that segment only

bridges segment networks hence reducing network traffic in resulting segments

Spanning Tree Algorithm (SPA) can be used to prevent data going in a continuous loop

Source Segment Nodes

A

D

F

1, 2, 3

5, 45, 68

34, 233

Bridge Protocol Data Units

bridges announce themselves to each other using special MAC frames - BPDUs

a bridge is elected root bridge they discover other bridges and paths

(including how many hops away from each other)

spanning tree algorithm used to disable loops

spanning tree can be turned off and bandwidth used by BPDUs saved

Multiple LANs

Routing options

• Transparent

• Fixed

• Spanning Tree

Bridges That Use A Spanning Tree Algorithm

Spanning Tree Algorithm Used To Dynamically Discover A Loop-free Tree In Topology

Bridges Exchange Configuration Messages To Calculate A Spanning Tree

When the Topology Changes, the Spanning Tree is Re-calculated

Spanning Tree Algorithm Frame Forwarding Address Learning Loop Resolution

NETWORK LAYER (ROUTER)

Application

Presentation

Session

transport

Network

Data link

Physical

Application

Presentation

Session

transport

Network

Data link

Physical

Data link

Physical

Network

Data link

Physical

Network

ROUTER

EXAMPLE: IP Router

Routers I operates at the Network layer of the OSI model interconnect networks meets need for best path can filter, isolate and connect segments better traffic managers than bridges and do not

pass broadcast storms has interfaces on multiple networks can share routing information with other routers uses NETWORK NUMBERS to forward packets

Routers II forwards packets between networks and transforms

packets as required to meet standards for each network

routers maintain a TABLE as follows to determine route all known network addresses how to connect to other networks possible paths between routes costs of sending data over paths

router selects best route based costs and available paths

APPLICATION LAYER GATEWAY

Application

Presentation

Session

Transport

Network

Data link

Physical

Application

Presentation

Session

Transport

Network

Data link

Physical

Data link

Physical

Network

Transport

Data link

Physical

Network

Transport

APPLICATION GATEWAY

Presentation

Session Session

EXAMPLES: DBMS, E-MAIL, WP

Presentation