Where there are only a few networks and routers, it is easy to enter a few static routes. However, if a larger number of routes are required, manual entry can become quite time consuming. Dynamic routing, on the other hand, lets routers create their own routing tables, and communicate these tables to other routers. Dynamic routers are set up through the “Routing and Remote Access” MMC Snap-in:
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Dynamic routers work by discovering their surroundings and communicating it with other nearby routers. RIP is a simple protocol, it’s simple and easy to configure, however it has a few performance issues that restrict is usefulness on medium-to-large networks. When first initialized RIP routers start off with an empty routing table. They then start sending out announcements to other routers advertising which networks they can see. Routers on other networks hear these announcements and add the routes to their own routing tables. These routers in turn will advertise their new routes which will then be received by other routers.
RIP is a dynamic routing protocol that is best used on medium sized networks with 5 or more subnets. It is used to remove the administrative overhead of maintaining routing tables. Microsoft’s RIP implementation supports both version 1 and version 2 of RIP. The main difference between the two versions is in the way updates are sent: RIP v1 routers send a broadcast every 30 seconds, whilst RIP v2 uses multicasts only when routes are changed. RIP v1 routers are unable to receive RIP v2 multicasts. RIP v2 routers can be configured to use broadcasts for backward compatibility.
RIP routers use Distance Vector Routing. Distance Vector routing protocols send their entire routing table to their nearest neighbors at regular intervals. Those neighbors then filter through the received routing table to learn which direction to send traffic to reach a given network. Distance Vector routing protocols use a distance measurement (metric) to determine the best path.
RIP routers will only maintain routing tables for those networks that are within 32 “hops” (for the round trip – there and back). This is because each hop adds 1 to the metric, thus a route which goes over more than 32 hops has a metric of more than 16 and is therefore invalid. One problem that plagues routers is “routing loops”, occurring when a router (or route) goes offline:
…prevents routing information being sent back to the router it came from. This reduces network traffic.
…cause the metric for an offline route to increase to 16, which triggers an update and prevents other routers from using this route until it comes back online.
This algorithm prevents any route with a metric over 16 being used. This avoids routing loops and count to infinity errors.
Silent RIP Host
In this case, RIP version 2 is configured to only accept routes, and not advertise its own.