Routers IP – V4 and V5 Routing Information Protocol (RIP) Open shortest path first (OSPF)

What is a router? A router is a device that connects multiple computers together. Not to be confused with a switch Routers transmit packets of data across different networks whilst switches can only transmit packets of data across the same network.

IP addresses An identifier assigned to each computer on a network. This address is logical and dynamic (it can change and does not physically tell you where the computer is)

Routing tables Every router on a network will build a table of possible routes to other machines. This table is known as the routing table. These tables are used to route traffic across a computer network.

RIP – Routing information protocol RIP is a routing protocol for exchanging routing tables between routers. Routers share their tables with each other so that they know where to pass packets of information. RIP looks for the route that requires the least amount of “Hops”. You add a “hop” for each device that a packet passes through at layer 3 of the OSI model. Each router in a network will share part of its routing table during periodic updates (about every 30 seconds). Only the neighbours are sent the updates.

RIP – hop count RIP uses hop counts to determine the best possible route. This may mean that at times packets of data are sent down slower routes. This is due to the fact the number of hops is usesed and not speed or time. Rip will discard any packet that takes more than 15 hops

RIP - Timers Rip makes use of timers to ensure everything works. These include… + Routing update timer Every 30 sends routing table updates are shared (A small random number is added to the 30- seconds to reduce collisions) + route timeout A packet can not travel indefinitely. After a set amount of time the packet is discarded and considered “timeout” + Route-flush timer After a predetermined amount of time the route in the table is “flushed”. A timer deletes the route after a set amount of time to ensure bad routes are not maintained.

RIP RIP is normally only useful for small networks. It does not scale well as it uses a maximum of 15 hops It does not react fast to topology updates (slow convergence) It uses large amounts of update packets

Open Shortest Path First (OSPF) As with RIP OSPF allows routes to build routing tables that indicate which route a packet of information should take. Like RIP routers will share OSPF data about the routes that have been learned. OSPF routers rely on cost to compute the shortest path through the network between themselves and other network detestations. The cost for OSPF is not based solely on “hops” built can be used based on connection speeds or reliability. Every router in an OSPF has a full picture of its “Area”. New updates are sent within seconds rather than waiting for routes to expire. It uses Dijkstra’s algorithm to determine the weight of each route to react when new pathways are added.

Dijkstra’s Algorithm A Level Only We use Dijkstra’s shortest path algorithm to help us find the shortest path between vertices in a weighted graph. a b c d e f g h 12 9 8 14 4 3 5 © ZigZag Education, 2016

The distance is only updated if a shorter route is found. Worked Example A Level Only We start by finding the distance from the starting vertex (A) to its neighbours. Next we repeat the process with the vertex with the shortest distance from the starting vertex. The distance is only updated if a shorter route is found. Node Shortest Distance Previous Node A B C D E F G A a 4 f B 5 A 5 e g C 5 G 3 5 4 D 9 C 2 5 3 E 7 G b c d F 4 A 3 4 G 3 A © ZigZag Education, 2016

Common Uses A Level Only There are a number of uses for the shortest path algorithm. Finding the shortest route to drive between two addresses. Finding the shortest route for packets to take between two devices in a network. 10587521 © ZigZag Education, 2016