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My question involves how you should handle a single DHCP server for a multihop network: (in our case) you have one DHCP server and a network with a "row" of routers attached to each other (and only one router attached to the DHCP server). We already looked around but only found some information one a situation like in the picture (We are not using the iSeries DHCP Server, I'm just using the picture as an example)

So, this situation is with one router; but would it be possible to have a whole network of routers (which are DCHP enabled) behind that first router and still have DHCP working right? And if this is possible, how would the DHCP server know to which subnet it should send its respond (e.g. how would the packet look, ...)?

Many thanks in advance,


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I'd think ip helper addresses on the routers would solve the problem, should ask what type of routers. – tony roth Dec 6 '11 at 21:23

but would it be possible to have a whole network of routers

This sounds possible, but it sure seems like you are building something unusual and convoluted. I suspect I might look at alternatives...

how would the DHCP server know to which subnet it should send its respond

This is part of how DHCP operates. A DHCP client starts its request with a broadcast, broadcasts do not cross routers. So a DHCP will only respond to the broadcasts with addresses from a scope that matches the IP address of the interface it was received on.

A special piece of software can be installed on an router called a DHCP agent. You must manually configure the DHCP agent with the IP address(s) of the DHCP server. When the router receives the broadcast on one of its local interfaces the agent accepts it, and then adds the ip address the request was received on, then it forwards that to the DHCP server. The DHCP server is able to to select the correct scope since it knows the address that received the request from the router.

If you cannot install a DHCP agent on your router, then it can be install on other devices on the network.

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I wouldn't really say its a piece of software its just an interface command on a router. – tony roth Dec 6 '11 at 21:28
@tonyroth, I certainly agree that there are agents built-in on many routers, but I have setup more dhcp-relay agents on Linux and Windows boxes which both require adding software. In any case the bit that does handles DHCP forwarding is certainly a separate from the TCP/IP stack. – Zoredache Dec 6 '11 at 21:55
either way it gets the job done. – tony roth Dec 7 '11 at 5:44

If these are routers you should set up a VLAN (across all your routers) and connect your DHCP-server to that VLAN. Since a VLAN builds a broadcast domain your DHCP-server should be able to answer DHCP-requests that way.

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