Shared DHCP pool for multiple VLAN interfaces

Simon dhcp1 at thehobsons.co.uk
Thu Nov 25 17:04:28 UTC 2021


Fabian Druschke <fabian at knogle.industries> wrote:

> Now i'm facing an issue which doesn't seem to be that simple.
> 
> In my network topology i got a bunch of different VLANs, VLAN 405-480, and VLAN 700-750.
> 
> The DHCP server should serve leases to requests, coming from these interfaces. But they should share a single DHCP range only, 10.20.0.0/16.
> 
> Is it possible to accomplish this goal, serving DHCP leases on all these VLAN subinterfaces, and serving DHCP leases for this single subnet?

The short answer is “it depends” !

The fuller answer is probably not very short at all.

The first thing you need to clarify is the network topology - just saying “I have VLANS” is no more informative than “I have a number of LANS”. Specifically, how are they connected, how is the addressing done, and most importantly how do you differentiate traffic and route it to the correct VLAN ?

I ask the latter as your question makes it sound like you have a number of separate networks (VLANs), all sharing the same IP subnet (10.20.0.0/16). That’s a tricky one to pull off reliably. I can see a situation where (e.g.) a host with address 10.20.10.20/16 in VLAN 405 talks via a router at 10.20.0.1/16 to communicate with another host at 10.20.10.21/16 which also talks via a router at 10.20.0.1/16. As general IPv4 addressing goes, that just isn’t going to work - principally because IPv4 doesn’t have the concept of on-link and off-link prefixes like IPv6 does, and hence the two devices would not know they needed to go via the router to talk to each other. Unless the router does a lot of proxy-ARP which itself could be interesting with that size of network.

So more information needed before anyone can usefully answer your query.

Simon



More information about the dhcp-users mailing list