Amer Networks E5Web GUI - Manual (Page 703)

• The destination MAC address is the Ethernet multicast address corresponding to the shared
hardware address and this has the form:
11-00-00-00-nn-mm
Where nn is a bit mask made up of the interface bus, slot and port on the master and mm
represents the cluster ID,
Link layer multicasts are used over normal unicast packets for security. Using unicast packets
would mean that a local attacker could fool switches to route heartbeats somewhere else so
the inactive system never receives them.
Failover Time
The time for failover is typically about one second which means that clients may experience a
failover as a slight burst of packet loss. In the case of TCP, the failover time is well within the
range of normal retransmit timeouts so TCP will retransmit the lost packets within a very short
space of time, and continue communication. UDP does not allow retransmission since it is
inherently an unreliable protocol.
Shared IP Addresses and ARP
Both master and slave know about the shared IP address. ARP queries for the shared IP address,
or any other IP address published via the ARP configuration section or through Proxy ARP, are
answered by the active system.
The hardware address of the shared IP address and other published addresses are not related to
the actual hardware addresses of the interfaces. Instead the MAC address is constructed by cOS
Core from the Cluster ID in the form 11-00-00-C1-4A-nn where nn is derived by combining the
configured Cluster ID with the hardware bus/slot/port of the interface. The Cluster ID must be
unique for each cOS Core cluster in a network.
As the shared IP address always has the same hardware address, there will be no latency time in
updating ARP caches of units attached to the same LAN as the cluster when failover occurs.
When a cluster member discovers that its peer is not operational, it broadcasts gratuitous ARP
queries on all interfaces using the shared hardware address as the sender address. This allows
switches to re-learn within milliseconds where to send packets destined for the shared address.
The only delay in failover therefore, is detecting that the active unit is down.
ARP queries are also broadcast periodically to ensure that switches do not forget where to send
packets destined for the shared hardware address.
HA with Anti-Virus and IDP
If a cOS Core cluster has the Anti-Virus or IDP subsystems enabled then updates to the Anti-Virus
signature database or IDP pattern database will routinely occur. These updates involve
downloads from the external Clavister databases and they require cOS Core reconfiguration to
occur for the new database contents to become active.
A database update causes the following sequence of events to occur in an HA cluster:
1. The active (master) unit downloads the new database files from the Clavister servers. The
download is done via the shared IP address of the cluster.
2. The active (master) node sends the new database files to the inactive peer.
Chapter 11: High Availability
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