Serving two subnets on same VLAN
dhcp1 at thehobsons.co.uk
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 0.0.0.0 addressed to 255.255.255.255 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.
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