9
High Availability
160
Increasing heartbeat frequency
Heartbeats
are
not
sent
at
smaller
intervals
than
a
tenth
of
a
second
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.
Failover time
The
time
for
failover
is
typically
within
seconds,
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
units
in
a
cluster
are
aware
of
the
shared
IP
addresses.
However,
ARP
queries
for
the
shared
IP
address,
or
any
other
IP
address
published
via
ARP
configuration
or
through
Proxy
ARP,
are
answered
by
the
active
unit
only.
The
hardware
address
of
the
shared
IP
address
and
other
published
addresses
are
not
related
to
the
actual
MAC
addresses
of
the
Ethernet
interfaces.
Instead,
a
new
MAC
address
is
constructed
by
the
SEG.
The
first
part
of
the
constructed
address
is
always
10:00:00
.
The
second
part
is
based
on
the
configuration
and
contains
the
cluster
ID.
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
a
failover
occurs.
When
a
cluster
member
discovers
that
its
peer
is
not
operational,
it
broadcasts
gratuitous
ARP
queries
on
all
interfaces
using
the
shared
MAC
address
as
the
sender.
This
allows
switches
to
re
‐
learn
within
milliseconds
where
to
send
packets
destined
for
the
shared
address.
Therefore,
the
only
failover
delay
is
in
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.
