D-Link DFL-260E User Manual page 176

4.2.3. Route Failover
enabled. Route monitoring for the second, alternate route is not meaningful since it has no failover
route.
Route #
1
2
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
automatically be transferred back to it.
Route Interface Grouping
When using route monitoring, it is important to check if a failover to another route will cause the
routing interface to be changed. If this could happen, it is necessary to take some precautionary steps
to ensure that policies and existing connections will be maintained.
To illustrate the problem, consider the following configuration:
Firstly, there is one IP rule that will NAT all HTTP traffic destined for the Internet through the wan
interface:
Action
NAT
The routing table consequently contains the following default route:
Interface
wan
Now a secondary route is added over a backup DSL connection and Route Monitoring is enabled for
this. The updated routing table will look like this:
Route #
1
2
Notice that Route Monitoring is enabled for the first route but not the backup, failover route.
As long as the preferred wan route is healthy, everything will work as expected. Route Monitoring
will also be functioning, so the secondary route will be enabled if the wan route should fail.
There are, however, some problems with this setup: if a route failover occurs, the default route will
then use the dsl interface. When a new HTTP connection is then established from the intnet
network, a route lookup will be made resulting in a destination interface of dsl. The IP rules will
then be evaluated, but the original NAT rule assumes the destination interface to be wan so the new
connection will be dropped by the rule set.
In addition, any existing connections matching the NAT rule will also be dropped as a result of the
Interface Destination
wan all-nets
wan all-nets
Src Iface Src Net
lan lannet
Destination
all-nets 195.66.77.1
Interface Destination
wan all-nets
dsl all-nets
176
Gateway
195.66.77.1
193.54.68.1
Dest Iface
wan
Gateway Metric
10
Gateway
195.66.77.1
193.54.68.1
Chapter 4. Routing
Metric Monitoring
10 On
20 Off
Dest Net Parameters
all-nets http
Monitoring
Off
Metric Monitoring
10 On
20 Off