Serving two subnets on same VLAN

Simon Hobson dhcp1 at
Sun Sep 25 07:37:35 UTC 2011

Norman Elton wrote:
>Assume that a router is configured with three interfaces:
> (primary interface)
> (secondary interface)
> (secondary interface)
>The router is configured to be a DHCP relay; however, only sends
>requests from its primary interface (

Norman Elton wrote:

>  > That's what the shared-network statement is used for:
>Interesting, I always thought the relay would have to send requests
>from the individual subnet, even in a shared-network. I will
>experiment with this.

Can you just clarify that when you say primary and secondary, you are 
talking about the same physical port/VLAN on the router ?
Assuming yes, then shared-network is what you need and it "just 
works". If you think about it, the router cannot use the different 
subnet addresses because it has no knowledge of which subnet a client 
is supposed to be in - if you think about it, how can it ?

If you want specific clients to use a particular subnet, then you 
will need to use classing or host entries with fixed addresses to do 
so. The server will not automatically assign clients to a specific 
subnet - it just treats all available addresses as equal.

Simon Hobson

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