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DHCP issue

I am running DHCP and routing and remote access on the same machine.   Done of the machines on my network can hear the DHCP .  I am thinking that maybe the firewall feature of routing and remote access is bloking it.  Is there a setting that must be set to allow DHCP and routng services to work together?
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Longstreet_1861
Asked:
Longstreet_1861
1 Solution
 
SCCMCanuckNetwork AnalystCommented:
Hey Long,

You will need to add and configure the DHCP Relay Agent in Routing and Remote Access, The following document explains how to set it up:

http://technet.microsoft.com/en-us/library/cc781416.aspx

Let me know how you make out :)
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MrMintanetCommented:
Provide more details, please.  Let's start with the server OS.
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Aaron StreetInfrastructure ManagerCommented:
on the router you have to set up a helper address

for instance say your server has an ipaddress of 10.1.1.5 255.255.255.0

and you set up a scope for 10.2.2.0 - 255 mask 255.255.255.0

then on the router that holds the default gateway to the 10.2.2.0 network you set up a DHCP helper. All this does if forward any DHCP requests that it recives (all devices in the 10.2.2.0 network will get the request as it is a broadcast) to the DHCP servers IP address, and tags them with the Network address it has come from.

the DHCP server will recive this packet and becasue it has a scope in the range of 10.2.2.0 it will reply with an ipaddress.

this way a broadcast packet is encapsulated by the router, sent to the DHCP server, the offer of the ip address is recived back and this is forwarded to the client.

in this way you can have multipy subnets set on your DHCP server. and each router that has a DHCP helper address insures that the right scope of address are assigend to the correct network segment.

Rember a DHCP request from a client machine is a Broadcast packet. Routers do not forward broadcasst packets so unless the DHCP server is on the same physical subnet to the client the Packet will not reach it. by setting up a helper address you are gettign round the prroblem by using the router to relay the packet to the DHCP server as a unicast IP packet, which can be forwarded across the routers.

any more information let me know

in the case of a cisco router the command is

#ip helper address 125.356.254.152  (where the ip address is the ip address of the DHCP server)

and thats it!! all you need to do si make sure the DHCP server has a scope with the correct range.

when DHCP server recives a packet it first see's if it is encapsulated (comes from a DHCP relay) if not it is from the local network and if the DHCP server has a scope in the same address range as its own IP address it will respond with an IP adrress from that range..

if it is encplasated it will have been taged with a ip network address from the network it came from. in this case it will cehck if it has a scope in that network range and respone.

if none of these are true it will ignore the packet.
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