280
NE2552E Application Guide for ENOS 8.4
Drops
invalid
ARP
packets
and
sends
a
syslog
message
with
details
about
each
dropped
packet.
DAI
determines
the
validity
of
an
ARP
packet
based
on
valid
IP
‐
to
‐
MAC
address
bindings
stored
in
a
trusted
database,
the
DHCP
snooping
binding
database.
This
database
is
built
by
DHCP
snooping
if
DHCP
snooping
is
enabled
on
the
VLANs
and
on
the
switch.
As
shown
in
if
the
ARP
packet
is
received
on
a
trusted
interface,
the
switch
forwards
the
packet
without
any
checks.
On
untrusted
interfaces,
the
switch
forwards
the
packet
only
if
it
is
valid.
For
hosts
with
statically
configured
IP
addresses,
static
DHCP
snooping
binding
entries
can
be
configured
with
a
big
lease
time.
Figure 29.
Dynamic
ARP
inspection
at
work
Interface Trust States and Network Security
DAI
associates
a
trust
state
with
each
interface
on
the
switch.
In
a
typical
network
configuration,
you
configure
all
switch
ports
connected
to
host
ports
as
untrusted
and
configure
all
switch
ports
connected
to
switches
as
trusted.
With
this
configuration,
all
ARP
packets
entering
the
network
from
a
given
switch
bypass
the
security
check.
The
trust
state
configuration
should
be
done
carefully:
configuring
interfaces
as
untrusted
when
they
should
be
trusted
can
result
in
a
loss
of
connectivity.
In
,
assume
that
both
Switch
A
and
Switch
B
are
running
DAI
on
the
VLAN
that
includes
Host
1
and
Host
2.
If
Host
1
and
Host
2
acquire
their
IP
addresses
from
the
DHCP
server
connected
to
Switch
A,
only
Switch
A
has
the
DHCP
IP
‐
to
‐
MAC
binding
of
Host
1.
Therefore,
if
the
interface
between
Switch
A
and
Switch
B
is
untrusted,
the
ARP
packets
from
Host
1
are
dropped
by
Switch
B.
Connectivity
between
Host
1
and
Host
2
is
lost.
ARP
Packets
ARP
Packets
Untrusted
Interface
DHCP
Snooping/
Binding
DB
Tr
u
s
te
d
Interface
Invalid
Packet
DAI
Valid
Packets
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