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 l 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 tableforward 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 ADFADF 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 l 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 l EXAMPLES: DBMS, , WP Presentation