D-Link NetDefend DFL-210 User Manual page 144

4.2.5. Proxy ARP
minimum value allowed is 100 ms.
• Sample
The number of polling attempts used as a sample size for calculating the Percentage Loss and
the Average Latency. This value cannot be less than 1.
• Maximum Failed Poll Attempts
The maximum permissible number of polling attempts that fail. If this number is exceeded then
the host is considered unreachable.
• Max Average Latency
The maximum number of milliseconds allowable between a poll request and the response. If this
threshold is exceeded then the host is considered unreachable. Average Latency is calculated by
averaging the response times from the host. If a polling attempt receives no response then it is
not included in the averaging calculation.
The Reachability Required option
An important option that can be enabled for a host is the Reachability Required option. When this
is selected, the host must be determined as accessible in order for that route to be considered to be
functioning. Even if other hosts are accessible, this option says that the accessibility of a host with
this option set is mandatory.
Where multiple hosts are specified for host monitoring, more than one of them could have
Reachability Required enabled. If NetDefendOS determines that any host with this option enabled
is not reachable, Route Failover is initiated.
HTTP Parameters
If the HTTP polling method is selected then two further parameters can be entered:
• Request URL
The URL which is to be requested.
• Expected Response
The text that is expected back from querying the URL.
Testing for a specific response text provides the possibility of testing if an application is offline.
If, for example, a web page response from a server can indicate if a specific database is
operational with text such as "Database OK", then the absence of that response can indicate that
the server is operational but the application is offline.
A Known Issue When No External Route is Specified
With connections to an Internet ISP, an external network route should always be specified. This
external route specifies on which interface the network which exists between the NetDefend
Firewall and the ISP can be found. If only an all-nets route is specified to the ISP's gateway, route
failover may, depending on the connected equipment, not function as expected.
This issue rarely occurs but the reason why it occurs is that ARP queries arriving on a disabled route
will be ignored.
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Chapter 4. Routing