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VLAN DHCP Request Problems..

Chris Lakey
Occasional Visitor

VLAN DHCP Request Problems..

Hi All,

Recently I segmented my network into several vlans:

Business1_VLAN 100
Business2_VLAN 200
Business3_VLAN 300
Business4_VLAN 400

In windows server terms they are seperate businesses with seperate/independant 'forests'. What this means is that they are completely self contained in the resources they access ie. they each have their own DHCP servers.

I am experiencing issues for several (Business3_VLAN) users connected to a 2524 switch. The switch ports in question on the 2524 are all 'Business3_VLAN Untagged.
The switch uplinks to a HP 2800 switch. The uplink is Untagged on the DEFAULT_VLAN and Tagged on Business1_VLAN, Business2_VLAN, Business3_VLAN etc..

DHCP requests are failing on reboot, but seem to have contacted the correct DHCP server by the time the user logs in.

For Example:
The user's 'Roaming Profile' cannot be found when they log on - due to network problems -, however full network connectivity (network shares, internet, email etc) is realised once windows has started. Checking the DHCP lease time confirms that the lease was effectively issued seconds before windows desktop was ready to go.

Example 2:
If I log in as user bob.brown - windows won't be able to locate profile due to network issues. Bob's desktop then loads up.
If I log bob off and log joe.brown on, joe's profile is fine as the workstation had already negotiated the DHCP lease.

I've been reading a little on the forums and come across the notion that I should be using an 'ip helper'/'dhcp helper'?? but i believe that this needs to be installed on the routing switch?

I actually use my firewall router (pFsense) to do all of my routing so I don't *think* that this applies to me..

I'm running out of idea's, especially as this is only affecting a few users.

I looked at the 2524's logs and port 6 kept on going up and down (flapping). I had disabled lacp last week as this seemed to be causing a few issues.

Why would a port flap - I have the same problem if I plug a desktop and laptop into this particular port..?

Apologies for the lengthy question.

Thoughts are welcome and appreciated.

Cheers - Lakey.

Jonathan Axford
Trusted Contributor

Re: VLAN DHCP Request Problems..

Hi Lakey,

If all of your VLANs have there own DHCP servers then there is no need to have an IP HELPER configured on your Layer 3 device.

This command is used to forward DHCP broadcasts between VLAN boundaries and is useful in Single DHCP server/Multiple VLAN setups.

Are all of your Speed/Duplex settings correct? (Match between host and switch) a port may be flapping due to physical issues such as cables, duplex mismatches.

Are the DHCP servers located on the same physical switches as the clients?
Where there is a will there is a way...
Matt Hobbs
Honored Contributor

Re: VLAN DHCP Request Problems..

What firmware are you running and are you running spanning-tree?

The original firmware on the 2524 only supported regular 802.1D spanning-tree which will block the port for 30 seconds.

If you haven't done so already, update the firmware and check that it is using RSTP instead. (802.1w)

For that port 6, it may just be faulty. Double check the cabling first though.