D-Link DFL-2500 - Manual (Page 296)

11.4. High Availability Issues
The following points should be kept in mind when managing and configuring an HA Cluster.
SNMP
SNMP statistics are not shared between master and slave. SNMP managers have no failover
capabilities. Therefore both firewalls in a cluster need to be polled separately.
Using Individual IPs
The unique individual IP addresses of the master and slave cannot safely be used for anything but
management. Using them for anything else such as for source IPs in dynamically NATed
connections or publishing services on them, will inevitably cause problems, as unique IPs will
disappear when the firewall it belongs to does.
Failed Interfaces
Failed interfaces will not be detected unless they fail to the point where NetDefendOS cannot
continue to function. This means that failover will not occur if the active unit can still send "am
alive" heartbeats to the inactive unit through any of its interfaces, even though one or more
interfaces may be inoperative.
Changing the Cluster ID
Changing the cluster ID in a live environment is not recommended for two reasons. Firstly this will
change the hardware address of the shared IPs and will cause problems for all units attached to the
local LAN, as they will keep the old hardware address in their ARP caches until it times out. Such
units would have to have their ARP caches flushed.
Secondly this breaks the connection between the firewalls in the cluster for as long as they are using
different configurations. This will cause both firewalls to go active at the same time.
Invalid Checksums in Heartbeat Packets
Cluster Heartbeats packets are deliberately created with invalid checksums. This is done so that they
won't be routed. Some routers may flag this invalid checksum in their log messages.
11.4. High Availability Issues Chapter 11. High Availability
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