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6-14 CHAPTER 6: ROUTING WITH IPX
Static Servers. A static server is one you manually configure in the server
information table. Static servers are useful in environments where no
routing protocol is used or where you want to override some of the servers
generated with a routing/server protocol. Because static servers do not
automatically change in response to network topology changes, you
should manually configure only a small number of relatively stable servers.
Dynamic Routes Using SAP. An automated method of adding and
removing services helps you keep up with a changing network
environment, allowing servers to advertise their services and addresses
quickly and reliably. SAP provides this automated method.
As servers are booted up, they advertise their services using SAP. When
servers are brought down, they use SAP to indicate that their services are
no longer available.
The information that these servers broadcast is not directly used by clients;
rather it is collected by a SAP agent within each router on the server’s
segment. The SAP agents store this information in the server information
table. Clients can then use the table to contact the nearest router or file
server SAP agent for server information.
Server Information Maintenance
When a router’s SAP agent receives a SAP broadcast response indicating a
change in the internetwork server configuration, the agent must update its
server information table and inform other SAP agents of these changes.
Examples of such a change are when a server is disconnected or becomes
accessible through a better route.
To relay this changed information to the rest of the internetwork, the SAP
agent immediately sends a broadcast to all of its directly connected
segments except the segment from which the information was received.
This broadcast packet contains information regarding the server change.
The change information is also reflected in all future periodic broadcasts.
SAP Aging. Router SAP agents implement an aging mechanism to handle
conditions that cause a SAP agent to go down suddenly without sending a
DOWN broadcast. Examples of such changes are a hardware failure, power
interruptions, and power surges. A SAP agent maintains a timer for each
entry in its server information tables that keeps track of how much time has