IPX Routing
IPX Routing |
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The RIP operation for an IPX router consists of the following processes:
Initialization At startup, the IPX router broadcasts a RIP packet on each of its attached networks informing adjacent routers of the network numbers that are directly attached to the IPX router. Adjacent IPX routers process the broadcast and add appropriate entries to their routing tables. The initializing IPX router also broadcasts a RIP general request on all of its attached networks. The adjacent IPX routers respond to the RIP general request by sending their routing tables to the initializing IPX router, which uses them to build its own routing table.
Ongoing Maintenance Every 60 seconds, the IPX router broadcasts its routing table (using split horizon) to all attached networks. Adjacent IPX routers receive the announced routes and update their routing tables accordingly.
Administrative Router Shutdown If an IPX router is brought down through an administrative action, it sends a RIP broadcast on all locally attached networks. In this broadcast all routes available through the router have a hop count of 16, indicating that these routes are no longer available. (All IPX routes must have a hop count of less than 16 to be considered reachable.) Neighboring routers propagate this change throughout the IPX internetwork through 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 routing process, IPX routes learned through the interface are no longer reachable. The unreachable IPX network numbers are announced, with a hop count of 16, in a triggered update. Note that most LAN-based interface hardware does not detect media faults, and therefore, the downed link is not immediately sensed. Many WAN adapters, however, have the ability to sense that a link to the WAN service provider is down.
Downed Router An IPX router that goes down due to a power outage or other hardware or software failure does not have the ability to inform neighboring routers that routes once available through it are now unavailable. To prevent the lingering existence of unavailable network numbers in routing tables, each learned entry in the IPX routing table has a time-out value of three minutes (default value). If the entry is not refreshed in three minutes, it is set to unreachable (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 neighboring routers to time out routing table entries for routes that had been available through the downed router. The adjacent routers then broadcast those changes through a triggered update.