11
I-WLAN
184
Sending the P-CSCF address
Following
completion
of
an
IKE
negotiation,
clients
send
a
DHCP
request
to
the
SEG
through
the
now
established
IPsec
tunnel.
The
SEG
will
respond
to
this
with
the
P
‐
CSCF
(SIP
server)
address.
The
option
exists
for
the
SEG
to
either
intercept
and
respond
to
the
client
DHCP
requests
it
receives
or
to
ignore
them.
The
recommendation
is
to
intercept
them
by
enabling
the
setting
Intercept
DHCPINFORM
messages
from
clients
that
request
P
‐
CSCF
information
.
When
this
intercept
option
is
enabled,
it
is
also
required
to
supply
a
Fake
DHCP
Server
address.
This
IP
address
is
then
used
in
two
instances:
•
It
is
sent
to
clients
during
the
IKE
negotiation
of
the
tunnel.
Clients
therefore
know
the
address
to
which
to
send
their
DHCPINFORM
messages
and
therefore
only
unicast
their
DHCPINFORM
messages.
The
SEG
will
only
intercept
DHCPINFORM
messages
destined
for
the
fake
DHCP
Server
address.
•
It
will
be
sent
by
the
SEG
in
the
response
packet
when
responding
to
a
client
DHCPINFORM
message.
If
the
GGSN
does
not
supply
a
P
‐
CSCF
address
then
it
responds
with
a
0.0.0.0
address.
Support for multiple GGSNs
It
is
possible
to
use
multiple
GGSNs
networked
through
a
single
SEG.
This
can
be
implemented
in
the
following
ways:
•
Using
DNS
Round
‐
robin
It
is
possible
to
have
a
different
GGSN
IP
handed
out
in
a
round
‐
robin
fashion.
This
is
done
with
the
following
steps:
1. Enable
the
option
RoundRobinDNS
on
the
GTP
tunnel
object.
2. Have
the
multiple
GGSN
IP
addresses
configured
on
the
DNS
server.
This
option
is
provides
a
way
load
balancing
between
the
GGSN
servers.
•
Using
Multiple
IPsec/GTP
Pairs
It
is
also
possible
to
have
multiple
pairs
of
IPsec/GTP
tunnels,
where
each
pair
uses
a
different
GGSN.
Since
the
APN
must
be
different
for
each
GGSN,
the
Subject
Alternative
Name
field
of
the
certificates
used
for
each
pair
must
be
different
and
refer
to
the
relevant
GGSN.
I-WLAN use case
A
setup
example
is
fully
described
in
the
I
‐
WLAN
chapter
of
the
SEG
‐
100
Getting
Started
Guide
.