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How Your Cayman 2E-H Works
C-12
Timing out – When a router adds a dynamic route to its routing
table, it starts a timer for that route. This timer is restarted
whenever the router receives another RIP packet advertising the
router. If the router does not see the route advertised for three
minutes, it considers the route unreachable and deletes it from
its routing table.
Link down – When a PPP link goes down, the router is informed
immediately by the network layer. The router does not time out
routes accessible over a PPP link when the link goes down.
Instead, how a router handles a route available over a PPP
interface when the link goes down depends on whether route
flushing is turned on:
If route flushing is turned on, the router begins advertising
the unreachable route as having a metric of 16 (telling other
interfaces and routers that those routes are no longer
reachable) for two minutes. At the end of two minutes, the
unavailable dynamic route is removed from the routing
table. If the deleted dynamic route replaced a less-efficient
static route, the static route is re-activated.
Route flushing is especially important when a port is
configured as a dial-in PPP interface, since the router will
not be able to dial out to re-establish a link after it goes
down.
If route flushing is turned off, the router does not change the
route's status or metric when the link goes down. Instead, it
maintains its routing table entry and freezes the RIP timer
for the route. When the link comes back up, the router
continues aging the route without restarting its RIP timer.
Turning route flushing off is most appropriate for dial-out
PPP interfaces, since you may want the router to hold onto
routes it acquired dynamically over a PPP link and not time
them out after the link closes. If the router receives a packet
destined for a network on the other side of the down PPP
link, it dials out to re-establish the link.
Cayman 2E-H User's Guide
November 2000
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