3Com 2500 manual Selecting the Best Route

6-10 CHAPTER 6: ROUTING WITH IPX

system uses RIP (one of the most widely used IGPs), to dynamically build its routing tables.

RIP operates in terms of active and passive devices. The active devices, usually routers, broadcast their RIP messages to all devices in a network; they update their own routing tables when they receive a RIP message. The passive devices, usually hosts, listen for RIP messages and update their routing tables; they do not send RIP messages.

An active router sends a RIP message every 60 seconds. This message contains both the network number for each destination network and the number of hops to reach it. In RIP, each router that a packet must travel through to reach a destination equals one hop.

Selecting the Best Route

Large networks have multiple routes to a single network. The routers use these criteria to select the best “route” to a network when choosing between alternate routes:

■Select the route that requires the lowest number of ticks.

■If multiple routes exist with an equal number of ticks, select the route that also has the lowest number of hops.

■If multiple routes exist with both ticks and hops equal, choose any of the routes as the “best” route.

Service Advertising The Service Advertising Protocol (SAP) allows servers (for example, file Protocol servers, print servers, and gateway servers) to advertise their addresses and

services. Through the use of SAP, adding and removing services on an internetwork becomes dynamic. As servers are booted up, they advertise their services using SAP. When they are brought down, they use SAP to indicate that their services are no longer available.

Internetwork Service Information

Using SAP, routers create and maintain a database of internetwork service information. Clients on use this data to determine what services are available on the network and to obtain the internetwork address of the nodes (servers) where they can access desired services.