© Copyright Lenovo 2017
Chapter 23: Basic IP Routing
401
Dynamic Host Configuration Protocol
Dynamic
Host
Configuration
Protocol
(DHCP)
is
a
transport
protocol
that
provides
a
framework
for
automatically
assigning
IP
addresses
and
configuration
information
to
other
IP
hosts
or
clients
in
a
large
TCP/IP
network.
Without
DHCP,
the
IP
address
must
be
entered
manually
for
each
network
device.
DHCP
allows
a
network
administrator
to
distribute
IP
addresses
from
a
central
point
and
automatically
send
a
new
IP
address
when
a
device
is
connected
to
a
different
place
in
the
network.
DHCP
is
an
extension
of
another
network
IP
management
protocol,
Bootstrap
Protocol
(BOOTP),
with
an
additional
capability
of
being
able
to
dynamically
allocate
reusable
network
addresses
and
configuration
parameters
for
client
operation.
Built
on
the
client/server
model,
DHCP
allows
hosts
or
clients
on
an
IP
network
to
obtain
their
configurations
from
a
DHCP
server,
thereby
reducing
network
administration.
The
most
significant
configuration
the
client
receives
from
the
server
is
its
required
IP
address;
(other
optional
parameters
include
the
“generic”
file
name
to
be
booted,
the
address
of
the
default
gateway,
and
so
forth).
DHCP
relay
agent
eliminates
the
need
to
have
DHCP/BOOTP
servers
on
every
subnet.
It
allows
the
administrator
to
reduce
the
number
of
DHCP
servers
deployed
on
the
network
and
to
centralize
them.
Without
the
DHCP
relay
agent,
there
must
be
at
least
one
DHCP
server
deployed
at
each
subnet
that
has
hosts
needing
to
perform
the
DHCP
request.
Note:
The
switch
accepts
gateway
configuration
parameters
if
they
were
not
configured
manually.
The
switch
ignores
DHCP
gateway
parameters
if
the
gateway
is
configured.
DHCP Relay Agent
DHCP
is
described
in
RFC
2131,
and
the
DHCP
relay
agent
supported
on
CN4093s
is
described
in
RFC
1542.
DHCP
uses
UDP
as
its
transport
protocol.
The
client
sends
messages
to
the
server
on
port
67
and
the
server
sends
messages
to
the
client
on
port
68.
DHCP
defines
the
methods
through
which
clients
can
be
assigned
an
IP
address
for
a
finite
lease
period
and
allowing
reassignment
of
the
IP
address
to
another
client
later.
Additionally,
DHCP
provides
the
mechanism
for
a
client
to
gather
other
IP
configuration
parameters
it
needs
to
operate
in
the
TCP/IP
network.
In
the
DHCP
environment,
the
CN4093
acts
as
a
relay
agent.
The
DHCP
relay
feature
enables
the
switch
to
forward
a
client
request
for
an
IP
address
to
two
BOOTP
servers
with
IP
addresses
that
have
been
configured
on
the
switch.
When
a
switch
receives
a
UDP
broadcast
on
port
67
from
a
DHCP
client
requesting
an
IP
address,
the
switch
acts
as
a
proxy
for
the
client,
replacing
the
client
source
IP
(SIP)
and
destination
IP
(DIP)
addresses.
The
request
is
then
forwarded
as
a
UDP
Unicast
MAC
layer
message
to
two
BOOTP
servers
whose
IP
addresses
are
configured
on
the
switch.
The
servers
respond
as
a
UDP
Unicast
message
back
to
the
switch,
with
the
default
gateway
and
IP
address
for
the
client.
The
destination
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