ET100A LAN-WAN Bridge
13
Theory of Operation
A
bridge
is
used
to
connect
networks
locally
or
remotely
such
that
they
appear
to
the
user
to
be
the
same
network.
An
Ethernet
LAN
bridge
will
connect
two
LAN
segments
at
the
Data
Link
Layer
(ISO
Layer
2).
At
this
layer,
the
MAC
(Media
Access
Control)
addresses,
are
used
for
low
level
addressing
to
send
information
to
devices.
The
bridge
builds
tables
of
MAC
addresses
for
each
network
segment
based
on
the
source
and
destination
addresses
of
the
packets
it
receives
and
forwards,
then
filters
the
traffic
not
destined
for
the
remote
network.
The
Ethernet
‐
WAN
bridge
will
connect
two
remote
Ethernet
networks
over
bit
stream
interfaces
such
as
that
of
synchronous
modems
or
DSU/CSUs.
One
method
to
do
this
is
to
use
HDLC,
an
international
standard
set
by
the
ISO,
a
set
of
protocols
for
carrying
data
over
a
link
with
error
and
flow
control.
Another
method
uses
PPP
and
a
third
uses
Cisco®
HDLC.
The
ET100A
utilizes
both
Ethernet
Bridging
and
encapsulation
to
provide
a
connection
between
LANs
over
bit
stream
architectures.
The
LAN
side
of
the
ET100A
receives
an
Ethernet
packet
and
examines
its
destination
MAC
address.
If
it
knows
the
MAC
is
on
the
local
network
then
it
simply
drops
the
packet.
Otherwise,
if
it
knows
the
packet
destination
is
on
the
remote
side,
or
if
it
cannot
be
determined
because
its
MAC
cannot
be
found
in
the
table,
then
it
forwards
it.
During
forwarding,
the
packet
is
processed
for
transmission
across
the
WAN
link.
Here
is
where
the
Ethernet
packet
in
encapsulated.
Summary of Contents for ET100A
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