D-Link DFL-2500 - Manual (Page 95)

methods must be chosen:
Interface Link Status NetDefendOS will monitor the link status of the interface
specified in the route. As long as the interface is up, the route is
diagnosed as healthy. This method is appropriate for monitoring
that the interface is physically attached and that the cabling is
working as expected. As any changes to the link status are
instantly noticed, this method provides the fastest response to
failure.
Gateway Monitoring If a specific gateway has been specified as the next hop for a
route, accessibility to that gateway can be monitored by sending
periodic ARP requests. As long as the gateway responds to these
requests, the route is considered to be functioning correctly.
Setting the Route Metric
When specifying routes, the administrator should manually set a route's Metric. The Metric is a
positive integer that indicates how preferred the route is as a means to reach its destination. When
two routes offer a means to reach the same destination, NetDefendOS will select the one with the
lowest Metric value for sending data (if two routes have the same Metric, the route found first in the
routing table will be chosen).
A primary, preferred route should have a lower Metric (for example "10"), and a secondary, failover
route should have a higher Metric value (for example "20").
Multiple Failover Routes
It is possible to specify more than one failover route. For instance, the primary route could have two
other routes as failover routes instead of just one. In this case the Metric should be different for each
of the three routes: "10" for the primary route, "20" for the first failover route and "30" for the
second failover route. The first two routes would have Route Monitoring enabled in the routing
table but the last one (with the highest Metric) would not since it has no route to failover to.
Failover Processing
Whenever monitoring determines that a route is not available, NetDefendOS will mark the route as
disabled and instigate Route Failover for existing and new connections. For already established
connections, a route lookup will be performed to find the next best matching route and the
connections will then switch to using the new route. For new connections, route lookup will ignore
disabled routes and the next best matching route will be used instead.
The table below defines two default routes, both having all-nets as the destination, but using two
different gateways. The first, primary route has the lowest Metric and also has Route Monitoring
enabled. Route Monitoring for the second, alternate route isn't meaningful since it has no failover
route.
Route # Interface Destination Gateway Metric Monitoring
1 wan all-nets 195.66.77.1 10 On
2 wan all-nets 193.54.68.1 20 Off
When a new connection is about to be established to a host on the Internet, a route lookup will result
in the route that has the lowest Metric being chosen. If the primary WAN router should then fail,
this will be detected by NetDefendOS, and the first route will be disabled. As a consequence, a new
route lookup will be performed and the second route will be selected with the first one being marked
as disabled.
Re-enabling Routes
Even if a route has been disabled, NetDefendOS will continue to check the status of that route.
Should the route become available again, it will be re-enabled and existing connections will
4.2.3. Route Failover Chapter 4. Routing
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