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5-14 CHAPTER 5: ROUTING WITH IPX
automatically change in response to network topology changes, you
should only manually configure a small number of servers.
Dynamic Routes Using SAP. The automated method of adding and
removing services help 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
but instead 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 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 (for example, a server has
gone down, been brought up, or is accessible through a better route), the
agent must update its server information table and inform other SAP agents
of these changes.
To relay this 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 that
occurred. This information is also reflected in all future periodic broadcasts.
SAP Aging. Router SAP agents implement an aging mechanism to handle
those conditions (for example, hardware failure, power glitch, power
outage) that cause a SAP agent to go down suddenly without sending a
DOWN broadcast. SAP agents maintain a timer for each entry in their server
information table that keeps track of how much time has elapsed since
information was received concerning a particular table entry. Since this
information is either new or changed, the SAP agents that receive this
information immediately pass it on and the change is quickly permeated
throughout the internetwork.