From: John Cuccia 

On Wed, 02 Apr 2003 20:09:59 -0500, Mike '/m'  wrote:

>On Wed, 2 Apr 2003 19:34:37 -0500, "Geo."
 wrote:
>
>>Since they are really 3 network
>>segments each segment would have to have a dhcp server so they can get IP
>>addresses in their netblock.
>
>Or one DHCP server with 3 NICs in it, and a wonderful config file?
>
>(my last reading of the dchp docs suggested that you can have one server
>handling multiple NICs and subnets)

One server can handle multiple subnets if the routers connecting those NICs
are configured to forward DHCP broadcasts to the subnet on which the DHCP
server is located.  Configure the DHCP server with a scope for each subnet
for which it will be supplying addresses.

The Cisco config directive is "IP Helper-Address x.x.x.x":

http://www.cisco.com/univercd/cc/td/doc/product/software/ssr83/rpc_r/48383.htm#
xtocid670625
Excerpt:
There are circumstances in which you want to control which broadcast
packets and which protocols are forwarded. You do this with helper
addresses and the forward-protocol commands.

The ip helper-address interface subcommand tells the router to forward UDP
broadcasts, including BootP, received on this interface. (UDP is the
connectionless alternative to TCP at the Transport Layer.) Use the ip
helper-address interface subcommand to specify the destination address for
forwarding broadcast packets. Full command syntax follows.

ip helper-address address
no ip helper-address address
The address argument specifies a destination broadcast or host address to
be used when forwarding such datagrams. You can have more than one helper
address per interface. You remove the list with no ip helper-address.

If you do not specify a helper address command, the router will not forward UDP
broadcasts.

Example:
This example defines an address that act as a helper address.

ip helper-address 121.24.43.2



The ip forward-protocol global configuration command allows you to specify
which protocols and ports the router will forward. Its full syntax is
listed next.

ip forward-protocol {udp|nd|spanning-tree} [port] no ip forward-protocol
{udp|nd|spanning-tree} [port]

The keyword nd is the ND protocol used by older diskless SUN workstations.
The keyword udp is the UDP protocol. A UDP destination port can be
specified to control which UDP services are forwarded. By default both UDP
and ND forwarding are enabled if a helper address has been defined for an
interface. If no ports are specified, these datagrams are forwarded, by
default:

Trivial File Transfer (TFTP)
Domain Name System
IEN-116 Name Server
Time service
NetBios Name Server
NetBios Datagram Server
Boot Protocol (BootP) client and server datagrams TACACS service

Use the no ip forward-protocol command with the appropriate keyword and
argument to remove the protocol.

Example:
The example below first defines a helper address, then uses the ip
forward-protocol command to specify forwarding of UDP only.

interface ethernet 1
ip helper-address 131.120.1.0
ip forward-protocol udp

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