DHCP Issue when moving a node from one VLAN to the other

[jhshin]

We have two vlans. VLAN10 - 192.168.0.0/24 and VLAN 11 - 192.168.11.0/24. Using IP helper-address, we have one dhcp server sitting on VLAN 10 and it works find when you plug a new computer into any network. The problem occurs when we move one machine from one vlan to the other. iIt apears that he machine that got 192.168.0.111 for instance from vlan 10 still tries to get the same ip from VLN 11, which results in freeze-up.

the workaround we've found so far is the following. we issue ipconfig /release before we move a computer, remove that ip from the dhcp server and move the computer to a differnet vlan, which is very very cumbersom and we look stupid. please help!!

[Pari Thiagasundaram]

Im not an expert on this. But here is a sugggestion.

1) Reduce the DHCP timers. Let it assign ip addresses only for 4 hours or even less.

2) when you are moving your PCs, make sure it is turned off and turned back "on" after 4 hours (in our case). So in that case, it would have already released the ip addresses !.

Can you let us know if this helped?

[andrew.butterworth · Accepted solution]

Are the two DHCP scopes in the same DHCP Superscope? If they are you need to split them into two separate SDHCP scopes. Superscopes are not for simplifying the configuration of the DHCP server they group together IP networks that share the same broadcast domain - i.e. secondary addressing. I have seen this many times and the cause is always the same - DHCP Superscopes.

HTH

Andy

[jhshin]

YES! It did it. Thanks Andy! You da man CCIE

[xanob]

Hi Andrew,

Sorry for any inconvenience!! I would llike to know if you can detail what do you mean with "split them into 2 separate SDHCP scopes (2 super dhcp scopes, I understand). Do you mean (if it´s suposse that I would have 2 scopes, A and B on a only superscope), to move B to other Superscope different, so I will have 2 Superscopes?

And other question, please, I have 5 scopes (no superscope created) and our Cisco L3 switch we have configured VLAN´s interfaces each of one with IP helper address, one for each scope. Problem it´s that if we change vlan, device it´s not getting IP address for that vlan and it remains with an IP of previous vlan.

In this case our situation it´s not with one superscopes with 5 scopes...

Do you know why it can be?

Sorry for any inconvenience

Thanks in advance

[scottmac]

DHCP addresses are provided and tracked by the MAC address. Since the PC is hitting the same DHCP server, the server sees the same MAC and assigns the old IP address.

You could make clearing the DHCP server (recovering addreses) part of the moving process (disconnect, clear the server, reconnect on the new net) or, keep a/some spare NICs of the same type around and change the NIC when you move the PC ... not the way I'd want to do it, but it'd work.

The object of the game would be to clear the MAC<->IP address association between the time you disconnect from the old network and reconnect to the new one.

Good Luck

Scott

[andrew.butterworth]

Scott, that isn't quite correct. The DHCP server associates a MAC Address AND a Source Subnet when remote networks/subnets are used. The 1st-hop router configured with the IP Helper command inserts it's IP Address into the DHCP Discover & Request packets so the DHCP Server knows the source IP network the DHCP client is from.

If you move a DHCP Client from one IP network to another the DHCP server knows it has moved because the IP Helper will be different and will offer an IP address from the relevent DHCP Scope. You can legitimately have leases in multiple DHCP Scopes from the same MAC address. If you use Superscopes then the DHCP Server 'groups' DHCP scopes together and can offer IP addresses from different networks/subnets that it thinks are all on the same broadcast domain.

Andy