IPX Routing

SAP Operation for an IPX Router

SAP operation for an IPX router consists of the following processes:

Initialization If the IPX router is also a hosting service, such as a Novell NetWare server or Windows 2000 Server–based computer, it broadcasts a SAP packet onto each attached network informing adjacent routers of its own services. The IPX router then broadcasts a SAP general request for all services onto each attached network. The responses to the SAP general request are used to build the SAP table.

Ongoing Maintenance Every 60 seconds, the IPX router broadcasts its SAP Table using split horizon on all attached networks. Adjacent IPX routers receive the advertised services and update their SAP tables appropriately.

Administrative Router Shutdown If an IPX router is brought down properly through an administrative action, it sends a SAP broadcast on all attached networks indicating that the services previously available through the router are no longer available. The SAP Agent sets the hop count for the services to 16 to indicate that these services are unreachable. Adjacent routers propagate this change throughout the IPX internetwork with a triggered update.

Downed Link If a link corresponding to one of the router's interfaces goes down and this failure is detected by the interface hardware and indicated to the router, SAP services learned through the interface are unreachable. The unreachable SAP services are advertised with a hop count of 16 in a triggered update. Note that most LAN-based interface hardware does not currently detect media faults and therefore this change is not immediately sensed and propagated. Many WAN adapters, however, have the ability to sense that a link to the WAN service provider is down.

Downed Router If a router goes down due to a power outage or other hardware failure, it does not have the ability to inform the adjacent routers that the services available through the router are no longer available. To prevent the lingering existence of inaccessible services in SAP tables, each learned entry in the SAP table of each IPX router has a default time-out value of three minutes. If the entry is not refreshed in three minutes, it is given a hop count of 16 and eventually removed. Therefore, if an IPX router goes down, it takes up to three minutes for the adjacent routers to time out the entries in their SAP table for the services that were available through the downed router. The adjacent routers then broadcast those changes through a triggered update.