5.5 Static and Default Routing
5.5.9 Snapshot routing model
Snapshot routing uses the client-server design model. When snapshot routing is configured, one router is designated as the snapshot server and one or more routers are designated as snapshot clients. The server and clients exchange routing information during an active period. At the beginning of the active period, the client router dials the server router to exchange routing information. At the end of the active period, each router takes a snapshot of the entries in its routing table. These entries remain frozen during a quiet period. At the end of the quiet period, another active period begins, and the client router dials the server router to obtain the latest routing information. The client router determines the frequency at which it calls the server router. The quiet period can be as long as 100,000 minutes (approximately 69 days).

When the client router transitions from the quiet period to the active period, the line might be down or busy. If this happens, the router would have to wait through another entire quiet period before it could update its routing table, which might severely affect connectivity if the quiet period is very long. To avoid having to wait through the quiet period, snapshot routing supports a retry period. If the line is not available when the quiet period ends, the router waits for the amount of time specified by the retry period and then transitions to an active period once again.

The retry period is also useful in dial-up environments in which there are more remote sites than interface lines. For example, the central site might have one PRI (with 23 B channels available) but might dial more than 23 remote sites. In this situation, there are more dialer map commands than available lines. The router tries the dialer map commands in order and uses the retry time for the lines that it cannot immediately access.