Cayman Systems 2E-W, Руководство пользователя

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Cayman 2E-W User’s Guide How Your Cayman 2E-W Works
November 2000
C-11
Selecting the Most Efficient Route
The efficiency of a route is expressed in terms of the route's metric,
or hop count, which measures the number of routers a packet must
pass through to reach its destination. A route to a network connected
directly to the router has a metric of 1, a route to a network
reachable through one other gateway has a metric of 2, and so on.
Routes with metrics greater than 16 are considered unreachable and
are discarded.
The PPP interface tries to use the most efficient path to reach a
remote destination network. When only one route (static or
dynamic) to a remote network is available, the router uses that route
to reach the network. When more than one route to a network is
available (for example, when the router has one route to a network
but learns of another one from a new router), the router selects the
more efficient route (that is, the one with the lower metric, or hop
count) and discards the less-efficient one.
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.
NOTE: 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: