RIP and OSPF Do Not Start Dial On Demand Connections (235492)

SUMMARY

Routing Information Protocol (RIP) and Open Shortest Path First (OSPF) are protocols designed to detect unsuccessful network connections and report these network problems to neighboring routers through time-out mechanisms. This means that these protocols are designed to believe that network connections are permanent unless they are actually down.

When you are using Dial On Demand (DOD) interfaces within RRAS, it is understood that these connections are up only when needed, and are disconnected when no traffic is being routed.

MORE INFORMATION

It is necessary to add static routes in the route table to trigger a DOD interface to make a connection. This is the opposite of how RIP and OSPF operate, because RIP and OSPF do not require static routes to operate. In fact, RIP and OSPF are designed to prevent an administrator from having to add static routes to a route table. RIP and OSPF simply listen for dynamic route updates and add new routes to the route table as appropriate.

Therefore, RRAS dynamic routing protocols such as RIP and OSPF do not dial a DOD connection when a normal networking path does not succeed. Neither RIP nor OSPF in Windows NT 4.0 RRAS has the capability to force the dialing of a DOD connection if another route does not work. When a route or network path does not succeed, you must still have static routes available to force the dialing of a DOD connection. To keep the static routes from interfering with normal RIP and OSPF operation, simply add static routes with higher associated costs than the normal network connection.

You can identify the priority order for routes learned by the RRAS server. In the RRAS Administration tool, under IP Routing, right-click Summary, and then click Properties. On the Preference Level tab, make sure that static routes are listed as less preferred than RIP or OSPF. The lower the number, the higher the preference. This way, when a network route does not work and times out, the RRAS server sees the higher-metric static route and forces a dial-up connection to the other network. Once the connection is up and authenticated, both RIP and OSPF operate as if the DOD connection was a permanent LAN connection.

A drawback to this strategy is that when the original network route becomes available again, the DOD connection may stay up indefinitely. Traffic is routed across the original network interface as it is supposed to be, but the DOD connection does not drop because of the idle time-out properties. This occurs because RIP and OSPF believe that the DOD connection is permanent and continue to send packets along these paths to keep themselves updated. This can prevent idle time-out events from occurring and can result in the connections staying up indefinitely.

Unless they are carefully configured, RIP and OSPF packets do not allow an idle time-out mechanism to occur on these DOD connections. This can result in additional expense if you are dialing long distance or using ISDN lines.