Technical Refresher

Session 3

Overview Difference between communication

between devices on a single logical network and communication between different logical networks.

Role of a router/gateway in such a set of logical networks.

Available protocols Exercises:

Static routes Using OSPF

Routing basics Routing: How networks discover what

paths they can use to send data packets.

Best route goals can include: Shortest hops Fastest links

Route around network failure

The role of the router Sits at connection point between

networks. Primary functions:

Send information to correct destination Send information ONLY to correct

destination and nowhere else (save network resources)

Within a logical network Machines are physically connected via

switches or hubs Each device has a Layer 2 MAC

address -- associated with Layer 3 IP address using Address Resolution Protocol

Network address and mask define boundaries of logical network

Between logical networks All traffic routed via default gateway Rules that determine where gateway

sends packet = routes A route is a tuple specifying:

Destination network (address/mask pair) Gateway (IP address)

Exercise 1: Static Routes Simulate a single ISP using static routes

throughout its network

y z x Router PC

Router PC

Router

PC

Router PC

Switch

Switch

Switch Switch

Switch

Dynamic Routing Static routing doesn’t

scale beyond a couple of devices – 36 routes in this network!

If edge routers can exchange information: only 9 routes needed.

So, networks need a way to distribute routes between routers: OSPF and iBGP!

Exercise 2: Dynamic Routing Dynamic routing using OSPF

See detailed notes in handout