Serving two subnets on same VLAN

Simon Hobson dhcp1 at
Mon Sep 26 07:13:16 UTC 2011

Norman Elton wrote:
>  > Can you just clarify that when you say primary and secondary?
>Multiple L3 interfaces (IP addresses) on the same L2 VLAN.

>Normally, only the "primary" interface does DHCP relaying, so the 
>DHCP server would only assign addresses from the pool containing IP 
>address of that primary interface.

No. DHCP is a **broadcast** protocol and is done by physical 
interface, it is not done by IP address. When you receive a packet 
from addressed to there is no subnet 
information in there, so you need to unlearn everything you thing 
about a client "belonging" to an IP subnet.

But, if you don't correctly specify your network topology, then the 
server can't work properly. As you have realised, without a shared 
network statement, the server will only ever serve addresses 
appropriate for whatever IP address the relay agent inserts into the 
packets (typically the primary address of the interface).

>  It sounds like the shared-network stanza will let the DHCP server 
>know that other L3 interfaces are available. I will test this in 
>depth tomorrow and post back if there are problems!

It does, and it will - that's the reason the shared-network statement exists.

Simon Hobson

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