After startup, the router gratuitously sends a Response message out every RIP-enabled interface every 30
seconds, on average. The Response message, or update, contains the router's full routing table with the
exception of entries suppressed by the split horizon rule. The update timer initiating this periodic update
includes a random variable to prevent table synchronization.[6] As a result, the time between individual
updates from a typical RIP process may be from 25 to 35 seconds. The specific random variable used by
Cisco IOS, RIP_JITTER, subtracts up to 15% (4.5 seconds) from the update time. Therefore, updates
from Cisco routers vary between 25.5 and 30 seconds (Figure 5.1). The destination address of the update
is the all-hosts broadcast 255.255.255.255.
Several other timers are employed by RIP. Recall from Chapter 4, "Dynamic Routing Protocols," the
invalidation timer, which distance vector protocols use to limit the amount of time a route can stay in a
routing table without being updated. RIP calls this timer the expiration timer, or timeout. Cisco's IOS
calls it the invalid timer. The expiration timer is initialized to 180 seconds whenever a new route is
established and is reset to the initial value whenever an update is heard for that route. If an update for a
route is not heard within that 180 seconds (six update periods), the hop count for the route is changed to
16, marking the route as unreachable.
Another timer, the garbage collectionor flushtimer, is set to 240 seconds–60 seconds longer than the
expiration time.[8] The route will be advertised with the unreachable metric until the garbage collection
timer expires, at which time the route is removed from the routing table. Figure 5.2 shows a routing table
in which a route has been marked as unreachable, but has not yet been flushed.
This command applies to the entire RIP process. If the timing of one router is changed, the timing of all
the routers in the RIP domain must be changed. Therefore, these timers should not be changed from their
default values without a specific, carefully considered reason.
RIP employs split horizon with poison reverse and triggered updates. A triggered update occurs whenever
the metric for a route is changed and, unlike regularly scheduled updates, may include only the entry or
entries that changed. Also unlike regular updates, a triggered update does not cause the receiving router to
reset its update timer; if it did, a topology change could cause many routers to reset at the same time and
thus cause the periodic updates to become synchronized. To avoid a "storm" of triggered updates after a
topology change, another timer is employed. When a triggered update is transmitted, this timer is
randomly set between 1 and 5 seconds; subsequent triggered updates cannot be sent until the timer
expires.
NOTE
Silent hosts
Some hosts may employ RIP in a "silent" mode. These so-called silent hosts do not generate RIP updates,
but listen for them and update their internal routing tables accordingly. As an example, using routed with
the -q option enables RIP in silent mode on a UNIX host.