Different Types of Routing Technologies.

Dynamic Routing

In a dynamic routing environment, routers use special routing protocols to communicate. The purpose of these protocols is simple; they enable routers to pass on information about themselves to other routers so that other routers can build routing tables. There are two types of routing protocols usedthe older distance vector protocols and the newer link state protocols.

Distance Vector Routing

The two most commonly used distance vector routing protocols are both called Routing Information Protocol (RIP). One version is used on networks running TCP/IP. The other, sometimes referred to as IPX RIP, is designed for use on networks running the IPX/SPX protocol.

RIP works on the basis of hop counts. A hop is defined as one step on the journey to the data's destination. Each router that the data has to cross to reach its destination constitutes a hop. The maximum number of hops that RIP can accommodate is 15. That is to say that in a network that uses RIP, all routers must be within 15 hops of each other to communicate. Any hop count that is in excess of 15 is considered unreachable.

Distance vector routing protocols operate by having each router send updates about all the other routers it knows about to the routers directly connected to it. These updates are used by the routers to compile their routing tables. The updates are sent out automatically every 30 or 60 seconds. The actual interval depends on the routing protocol being used. Apart from the periodic updates, routers can also be configured to send a triggered update if a change in the network topology is detected. The process by which routers learn of a change in the network topology is known as convergence.

Although distance vector protocols are capable of maintaining routing tables, they have three problems. The first is that the periodic update system can make the update process very slow. The second problem is that the periodic updates can create large amounts of network trafficmuch of the time unnecessarily as the topology of the network should rarely change. The last, and perhaps more significant, problem is that because the routers only know about the next hop in the journey, incorrect information can be propagated between routers, creating routing loops.

Two strategies are used to combat this last problem. One, split horizon, works by preventing the router from advertising a route back to the other router from which it was learned. The other, poison reverse (also called split horizon with poison reverse), dictates that the route is advertised back on the interface from which it was learned, but that it has a metric of 16. Recall that a metric of 16 is considered an unreachable destination.

Link State Routing

Link state routing works quite differently from distance vector-based routing. Rather than each router telling each other connected router about the routes it is aware of, routers in a link state environment send out special packets, called link state advertisements (LSA), which contain information only about that router. These LSAs are forwarded to all the routers on the network, which enables them to build a map of the entire network. The advertisements are sent when the router is first brought onto the network and when a change in the topology is detected.

Of the two (distance vector and link state), distance vector routing is better suited to small networks and link state routing to larger ones. Link state protocols do not suffer from the constant updates and limited hop count, and they are also quicker to correct themselves (to converge) when the network topology changes.

On TCP/IP networks, the most commonly used link state routing protocol is the Open Shortest Path First (OSPF). On IPX networks, the NetWare Link State Protocol (NLSP) is used. Table 1 summarizes the distance vector and link state protocols used with each network protocol.

Table 1 Routing Protocols
Network Protocol	Distance Vector	Link State
TCP/IP	RIP	OSPF
IPX/SPX	RIP*	NLSP

IPX RIP

Sometimes, to distinguish between the versions of RIP for IP and IPX, the version for IPX is referred to as IPX RIP.

Gateways

Any device that translates one data format to another is called a gateway. Some examples of gateways include a router that translates data from one network protocol to another, a bridge that converts between two networking systems, and a software application that converts between two dissimilar formats. The key point about a gateway is that only the data format is translated, not the data itself. In many cases, the gateway functionality is incorporated into another device.

Gateways and Default Gateways

Don't confuse a gateway with the term default gateway, the term default gateway refers to a router to which all network transmissions not destined for the local network are sent.

CSU/DSU

A Channel Service Unit/Digital Service Unit (CSU/DSU), sometimes called Data Service Unit, is a device that converts the digital signal format used on LANs into one used on WANs. Such translation is necessary because the networking technologies used on WANs are different from those used on LANs.

The CSU/DSU sits between the LAN and the access point provided by the telecommunications company. Many router manufacturers are now incorporating CSU/DSU functionality into their products.