Amer Networks E5Web GUI - Manual (Page 266)

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 established, 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 change in the destination interface. Clearly, this is undesirable.
To overcome this issue, potential destination interfaces should be grouped together into an
Interface Group and the Security/Transport Equivalent option should be enabled for the group
so that connections can be moved between interfaces. The interface group is then used as the
destination interface when setting policies. For more information on interface groups, see
Section 3.4.8, “Interface Groups”.
Gratuitous ARP Generation
By default cOS Core generates a gratuitous ARP request when a route failover occurs. The reason
for this is to notify surrounding systems that there has been a route change. This behavior can be
controlled by the advanced setting Gratuitous ARP on Fail.
4.2.4. Host Monitoring for Route Failover
Overview
To provide a more flexible and configurable way to monitor the integrity of routes, cOS Core
provides the additional capability to perform Host Monitoring. This feature means that one or
more external host systems can be routinely polled to check that a particular route is available. It
is similar in concept to the monitoring used for the Server Load Balancing feature in cOS Core (see
Section 10.4.5, “SLB Server Monitoring”) but there are important differences.
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
network.
• 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 failover if response latency times become unacceptable using the existing
route.
Enabling Host Monitoring
Host monitoring can be enabled as a property of a Route object and a single route can have
multiple hosts associated with it for monitoring. Multiple hosts can provide a higher certainty
that any network 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 This is the period of time after startup or after
reconfiguration of the Clavister Security Gateway which
cOS Core will wait before starting monitoring. This waiting
period allows time for all network links to initialize once the
Chapter 4: Routing
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