Need advice regarding VLANs and DHCP

[Jonathan Riley]

Hi all,

Happy new year,

So, my question...and fogive me if this is a stupid question, I've only just started studying networking, for extra credits to complete one of my assignments I was wondering if it's possible for the hosts on three vlans on a subnet network to get their respective ip address from DHCP pools on a single server on a separate vlan?

For the assignment we are allowed to show a DHCP server for each vlan, but I want to answer it as if it were a real life scenario and in reality I couldn't imagine a company using multiple servers for just three departments.  I could see virtual servers being implemented for that kind of situation but I was wondering if it's possible for one server with multiple DHCP pools to provide addresses to three vlans? 

Thanks in advance

[Reza Sharifi · Accepted solution]

Hi,

You are correct. Most organizations have one DHCP server that serves multiple subnets/vlans and also have a DHCP server as backup.  It would be very costly to deploy one DHCP server per vlan/subnet.  So, one DHCP server usually servers all the vlans.

HTH

[Jonathan Riley]

Thanks for replying.

If you don't mind, just one more question..

Out of interest exactly how would it work, as I only understand that a DHCP request is a broadcast message and the host accepts the first response,

Is the broadcast message sent to the router and then forwarded to the server, how does the server know which pool to use?

I can find details of DHCP from a switch or router, but I haven't fund anything that discusses pointing the DHCP discovery message to a server.

Thanks for your time

[Jon Marshall · Accepted solution]

Jonathan

In the last company i worked for we had two DHCP servers for the entire network. For each vlan it would split in half and each DHCP server would be responsible for one half of the scope. That way you could not get the same IP being handed out to different clients.

The server knows which pool to use because the request sent from the router has the IP address of the interface the L3 switch/router received the broadcast on ie. if there is a vlan interface with an ip helper-address configured the broadcast from the clients gets to the L3 vlan interface and then the L3 switch sends a request to the DHCP server with the IP address of the vlan interface so the DHCP server knows which scope to use.

Jon

[Jonathan Riley]

Thanks very much for your time and reply.

I can see now how it works.

Thanks again