5-10 CHAPTER 5: ROUTING WITH IPX
LANplex 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 and the number of hops for each
destination. In RIP, each router that a packet must travel through to reach a
destination equals one hop.
Selecting the Best Route
On large networks, there may be multiple routes to a single network. The
criteria that should be used by the routers in selecting the “best route” to a
network when choosing between alternate routes are listed below:
■Select the route that requires the lowest number of ticks
■If multiple routes exist with the number of ticks equal, select the route that
also has the lowest number of hops
■If multiple routes exist with both ticks and hops equal, the router is free to
choose any of the routes as the “best” route
Service Advertising
Protocol (SAP)
The Service Advertising Protocol allows servers (for example, file 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. This allows clients on the network to determine what services
are available on the network and to obtain the internetwork address of the
nodes (servers) where they can access those services.