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Helper addresses and dhcp

How does a dhcp server respond to a dhcp request, which has been forwarded to the dhcp server by a router using helper addresses. My point is,.... the initial dhcp lease offer from the server is still a broadcast, so how does that get through the router back to the dhcp client.

I am attempting (unsuccessfully so far) to acquire a dhcp lease from a dhcp server across a cisco 3620 router.


Re: Helper addresses and dhcp

Here is a small description of what happens between the client and server.

DHCP client sends a DHCP discover packet.

src ip -, dst ip -

The router receives the packet, inserts its ip address in the Gateway address field ( the address of the interface on which the DHCP broadcast was received). Due to the helper-address configuration, the DHCP discover packet is converted from a broadcast to unicast.

src of packet - ip add of router interface on which broadcast was received,

dst ip of packet - ip add of dhcp server

gateway field inside DHCP discover packet - ip add of router interface on which broadcast was received,

The DHCP server receives the DISCOVER packet, and The DHCP offer packet is unicast (not broadcast) back to the router's ip address.

The router will broadcast the DHCP OFFER packet back into the clients lan.

From this we can see that, the DHCP operation works transparent to the client and server.

All you need is a helper-address configuration on the routers lan interface, connected to client.

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Re: Helper addresses and dhcp

Hi shanky

Thanks for the elaboration on the dhcp process.

I do have a helper address on the lan interface connected to the client, but still do not pick up an ip from the server.

Do I need to specifically allow certain ports for Microsoft dhcp using an ip forward-packet, or are the default 8 ports which are allowed by ip helper-address good enough.


Re: Helper addresses and dhcp

No, by default, 8 udp protocols are allowed when you enable a helper-address.

For troubleshooting,

start with physical layer, see if the port on switch to which, client, server, router etc are connected, are up.

Try configuring a static ip on the client and see if you can ping, the router, dhcp server etc. or try bypassing the router and see if you are getting an ip from dhcp server (with the same switch being used)

Check whether on the switch to which the client is connected, the port has Portfast enabled.

Check if the scope has been defined correctly on the DHCP server

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Re: Helper addresses and dhcp

Way to go Shanky... Also check whether the dhcp processing is enabled on the router. I mean, check for "no ip bootp server" in the configuration. If you have this configured, router will not entertain any of the broadcasts such as dhcp request. So put "ip bootp server" and it will start working again.

Also a slight inside, dhcp "forwarding" and "relaying" are two things. Cisco routers are supposed to do only the "relaying" (when used with the command helper address). Relaying works as Shanky explained in the first reply. Now for forwarding, the router won't change the packet...




Re: Helper addresses and dhcp


If the DHCP server is on subnet "A" and you have DHCP clients on subnet "B" across the router, then you need to have a DHCP scope on the "A" server for the users of subnet "B" to get their addresses from. This will be a separate DHCP scope, different from the one which services the DHCP clients on subnet "A". Many people don't realize that one DHCP server can service multiple subnets, including remote ones.

The IP address range in each scope will need to be a subset of the addresses that would work on the subnet for which it's intended. Default gateway should be a scope option, unique to each scope. DNS and WINS servers can be specified as global options if you want; advantage is, they will apply in all scopes, and you will only have to create them once.

Hope this helps.

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