D-Link NetDefend DFL-1660, User Manual

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11.2. HA Mechanisms
This section discusses in more depth the mechanisms NetDefendOS uses to implement the high
availability feature.
Basic Principles
D-Link HA provides a redundant, state-synchronized hardware configuration. The state of the active
unit, such as the connection table and other vital information, is continuously copied to the inactive
unit via the sync interface. When cluster failover occurs, the inactive unit knows which connections
are active, and traffic can continue to flow after the failover with negligible disruption.
The inactive system detects that the active system is no longer operational when it no longer detects
sufficient Cluster Heartbeats. Heartbeats are sent over the sync interface as well as all other
interfaces.
Heartbeat Frequency
NetDefendOS sends 5 heartbeats per second from the active system and when three heartbeats are
missed (that is to say, after 0.6 seconds) a failover will be initiated. By sending heartbeats over all
interfaces, the inactive unit gets an overall view of the active unit's health. Even if sync is
deliberately disconnected, failover may not result if the inactive unit receives enough heartbeats
from other interfaces via a shared switch, however the sync interface sends twice as many heartbeats
as any of the normal interfaces.
Heartbeats are not sent at smaller intervals because such delays may occur during normal operation.
An operation, for example opening a file, could result in delays long enough to cause the inactive
system to go active, even though the other is still active.
Disabling Heartbeat Sending on Interfaces
The administrator can manually disable heartbeat sending on any interface if that is desired. This is
not recommended since the fewer interfaces that send heartbeats, the higher the risk that not enough
heartbeats are received to correctly indicate system health.
The exception to this recommendation is if an interface is not used at all. In this case, it can be
advantageous to disable heartbeat sending on that interface. The reason for this is that NetDefendOS
would otherwise send heartbeats on the disabled interface and this can contribute to a false picture
of system health since these heartbeats are always lost. A "false" failover could therefore be the
result.
Heartbeat Characteristics
Cluster heartbeats have the following characteristics:
• The source IP is the interface address of the sending firewall.
• The destination IP is the broadcast address on the sending interface.
• The IP TTL is always 255. If NetDefendOS receives a cluster heartbeat with any other TTL, it is
assumed that the packet has traversed a router and therefore cannot be trusted.
• It is a UDP packet, sent from port 999, to port 999.
• The destination MAC address is the Ethernet multicast address corresponding to the shared
hardware address and this has the form:
11.2. HA Mechanisms Chapter 11. High Availability
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