| 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.
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