D-Link DFL-260E User Manual page 175

4.2.3. Route Failover
route by route basis. To enable route failover in a scenario with a preferred and a backup route, the
preferred route will have route monitoring enabled, however the backup route does not require this
since it will usually have no route to failover to. When route monitoring is enabled for a route, one
of the following monitoring methods must be chosen:
Interface Link Status
Gateway Monitoring
Automatically Added Routes Need Redefining
It is important to note that the route monitoring cannot be enabled on automatically added routes.
For example, the routes that NetDefendOS creates at initial startup for physical interfaces are
automatically added routes. The reason why monitoring cannot be enabled for these routes is
because automatically created routes have a special status in an NetDefendOS configuration and are
treated differently.
If route monitoring is required on an automatically created route, the route should first be deleted
and then recreated manually as a new route. Monitoring can then be enabled on the new route.
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
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.
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.
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Chapter 4. Routing