6-10 CHAPTER 6: ROUTING WITH IPX
Selecting the Best Route
Large networks contain many possible routes to each destination. A
router performs the following steps to find the best route toward a
destination:
■ If one route requires the lowest number of ticks, the router selects it as
the best route.
■ If multiple routes require the same lowest number of ticks, the router
selects the route that requires the lowest number of hops as the best
route.
■ If multiple routes require the same lowest number of ticks and hops,
the router may select any of them as the best route.
Service Advertising
Protocol in IPX
With the Service Advertising Protocol (SAP), file, print, application, and
gateway servers broadcast their addresses and services throughout the
intranetwork. Services are added and removed dynamically. A server
advertises itself at startup; and if a server is shut down, it broadcasts that
its services are no longer available.
Internetwork Service Information
Using SAP, IPX routers maintain a database of internetwork service
information. Client workstations 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 services.
Before it can initiate a session with a file server, a workstation must know
a server’s network address.
SAP Packet Structure
SAP packets support the following functions:
■ Workstation requests for the name and address of the nearest server
of a certain type
■ Router requests for the names and addresses of all the servers, or of
all servers of a certain type, on the intranetwork
■ Responses to workstation or router requests
■ Periodic server and router SAP broadcasts
■ Changed server information broadcasts
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