Routing OverviewIP routing at its most basic level allows a network packet to travel from its source address to its destination address. In a basic LAN there is normally only one network and traffic can be sent directly to the machine it is intended for. In larger, more complicated networks there are often multiple subnets which are separated by routers. The process of routing allows a network packet to reach its destination by examining a routing table.

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The biggest network of all, the Internet, is bound together by routers, e.g. Your e-mail may pass through several routers before reaching its destination. A router contains a list

of the networks it is able to reach. Remote network packets are forwarded to routers which will pass the packet onto the correct destination (or another router).Routing is the method computers use to transmit data packets from a machine on one subnet to a machine on a different subnet. This is done via a router:

Each computer is on a different network……but all computers can talk to each other through the router!

A computer with an IP address of 10.10.10.1/24 and a computer with an IP address of 10.10.11.1/24 would be on different networks.

A ping from the 10.10.10.1 host to the 10.10.11.1 host would not reach its target.

To access the distant machine a router must be enabled and configured with the correct entries.

Routing between networks can be achieved by using a combination of the following Hardware and Software solutions:

Solution 1: A multi-homed Windows Server (one with more than one network card) can function as a router.

Each NIC must be bound to the IP address of the network it is attached to. The Routing and remote Access service must be enabled on the server.

In TCP/IP Networking, the Default Gateway is the router which connects a network segment to other segments & possibly the rest of the world!

Solution 2: A dedicated, hardware Router. The router has several interfaces which can be used to link it up to different networks.

The router would normally be configured through a web browser or by using telnet.

Solution 3: Software Routing. There are two types: manually entered static routes and dynamic routing protocols, such as RIP. Routing works by using a routing table. A

routing table is a database that stores route information. A route table is very similar to a road map. The route print command can be used to view a computer’s routing table.

Dynamic RoutingWhere 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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RIP Routing

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:

Split Horizon

…prevents routing information being sent back to the router it came from. This reduces network traffic.

Triggered Updates

…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.

Poison Reverse

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.

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