M2M
Cellular
Gateway
Index
skipping
is
used
to
reserve
slots
for
new
function
insertion,
when
required.
284
Scenario
Application
Timing
Above
diagram
illustrates
the
security
gateway
in
headquarters
playing
the
GRE
client
role.
In
fact,
the
GRE
tunnel
establishment
can
be
started
from
either
site.
The
GRE
tunnel
is
established
by
starting
from
GRE
client,
the
Security
Gateway
2
in
Network
‐
B.
All
client
hosts
behind
the
Security
Gateway
2
or
the
mobile
device
can
access
the
resources
in
the
Intranet
of
Network
‐
A
at
headquarters
via
this
established
GRE
tunnel.
Usually,
these
hosts
at
GRE
client
peer
access
the
Internet
directly
via
the
WAN
interface
of
Security
Gateway
2.
Only
the
packets
whose
destination
is
in
the
dedicated
subnet
to
Network
‐
A
will
be
transferred
via
the
GRE
tunnel.
But
if
GRE
client
peer
is
configured
to
all
packets
are
delivered
via
the
GRE
tunnel,
as
shown
in
the
diagram
by
configuring
the
GRE
tunnel
is
the
default
gateway
at
GRE
client
peer,
the
Internet
accessing
packets
will
be
also
sent
to
the
Security
Gateway
1
in
Network
‐
A
and
be
re
‐
transferred
to
the
Internet.
That
means
the
Internet
accessing
of
GRE
Client
peer
is
also
controlled
by
the
Security
Gateway
1,
the
LGRE
VPN
server.
Scenario
Description
GRE
Tunneling
is
similar
to
IPSec
Tunneling,
client
requesting
the
tunnel
establishment
with
the
server.
Both
the
client
and
the
server
must
have
a
Static
IP
or
a
FQDN.
Any
peer
gateway
can
be
worked
as
either
a
client
or
a
server,
even
using
the
same
set
of
configuration.
GRE
Tunneling
protocol
is
used
for
establishing
a
GRE
VPN
tunnel.
If
the
GRE
server
at
HQ
supports
DMVPN
Hub
function,
like
Cisco
router
as
the
VPN
concentrator,
the
GRE
client
at
branch
office
can
activate
the
DMVPN
spoke
function
here
since
it
is
implemented
by
GRE
over
IPSec
tunneling.
The
GRE
Client’s
“Default
Gateway/Remote
Subnet”
setting
determines
how
the
