D-Link NetDefend DFL-210 User Manual page 87

4.2.2. Route Failover
be controlled by the advanced setting RFO_GratuitousARPOnFail.
Host Monitoring
Overview
To provide a more flexible and configurable way to monitor the integrity of routes, NetDefendOS
provides the additonal capability to perform Host Monitoring as a means to monitor a route. This
feature means that one or more external host systems can be routinely polled to check that a particu-
lar route is available.
The advantages of Host Monitoring are twofold:
• In a complex network topology it is more reliable to check accessibility to external hosts. Just
monitoring a link to a local switch may not indicate a problem in another part of the internal net-
work.
• Host monitoring can be used to help in setting the acceptable Quality of Service level of internet
response times. Internet access may be functioning but it may be desirable to instigate route fail-
over if response latency times become unacceptable using the existing route.
Enabling Host Monitoring
As part of Route Properties Host Monitoring can be enabled and a single route can have multiple
hosts associated with it for monitoring. Multiple hosts can provide a higher certainty that any net-
work problem resides in the local network rather than because one remote host itself is down.
In association with Host Monitoring there are two numerical parameters for a route:
Grace Period
Minimum Number of Hosts
Available
Specifying Hosts
For each host specified for host monitoring there are a number of property parameters that should be
set:
A host can be polled using a different protocol which can be one of:
• Method - The method by which the host is to be polled. This can be one of:
• ICMP - ICMP "Ping" polling. An IP address must be specified for this.
• HTTP - A normal HTTP server request using a URL. A URL must be specified for this as
well as a text string which is the beginning (or complete) text of a valid response. If no text
is specified, any response from the server will be valid.
• Interval - The interval in milliseconds between polling attempts.
This is the period of time after startup or after reconfiguration
of the D-Link Firewall which NetDefendOS will wait before
starting Route Monitoring. This waiting period allows time
for all network links to initialize once the firewall comes on-
line.
This is the minimum number of hosts that must be considered
to be accessible before the route is deemed to have failed. The
criteria for host accessibility are described below
74
Chapter 4. Routing