User`s guide

How Your Cayman 3220-H Works Cayman 3220-H User’s Guide
C-12 December 2000
If the router has a static route and a dynamic route with the same
metric, it uses the static route and discards the dynamic route. If a
router has one dynamic route to a remote network but learns of
another with the same metric, it retains the existing route and
discards the new route.
Under some circumstances, you may want your router to use the
same path consistently to reach a remote network, even if another,
more efficient route to the remote network exists. To accomplish this,
set up a static route for the preferred pathway and assign it a metric
of 1 (even if the actual destination to that network is more than 1).
This will ensure the route cannot be replaced by a dynamic route.
Dealing with
Unavailable Routes
A router acquiring dynamic routes from other gateways needs to be
able to identify routes that are no longer available. A router considers
a route unavailable when the route times out or a PPP link goes
down:
 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.