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wuh
Community Member

a DHCP question

Hi guys, I have a question about DHCP.What rules do a DHCP use when it assigns IPs to clients? Are they based on the vlan the attached port configured in?

I was troubleshooting yesterday. I found at our distribution layer switch where Inter-vlan occurs was trying to assign an IP from a different Vlan to the PC attached to port that was configured in a different vlan.

certianly, the PC wont work. but I dont understand why the server trying to give a different IP?

What is the reason?

thanks,

Han,

8 REPLIES
Hall of Fame Super Gold

Re: a DHCP question

Han

DHCP does (or should) assign addresses that are specific for the subnet (or VLAN) from which the DHCP request originated. If the origin was local then DHCP should know the VLAN and subnet and if the request was not local then DHCP looks at the gateway address in the request. If the DHCP server is serving for several sources (several VLANs) then it should have a unique set of addresses (scopes) for each origin.

Without knowing a bit more about your environment (how are the VLANs defined, what scopes are configured in the DHCP server, what address was it assigning, what range of address should it have assigned, what VLAN does the address being assigned belong to) we can not tell what caused this problem.

HTH

Rick

wuh
Community Member

Re: a DHCP question

Rick:

thanks for first. Let me redescribe it.

The port that the new pc was attached was configured as vlan 100. But at the distribution layer switch i could see the debug message say that the DHCP server replied the request from the MAC for an IP from vlan 200.

Can you just give me an imagined scenario in which the settings result this? I just couldn't imagine.

Han,

Hall of Fame Super Gold

Re: a DHCP question

Han

What address range should be used for VLAN 100 and what range should be used for VLAN 200?

It might also help to know what is functioning as the DHCP server? Is it a dedicated server, or is it the switch/router, or is it something else?

HTH

Rick

wuh
Community Member

Re: a DHCP question

There are other ports in both vlan works fine. We use QIP.

thanks,

Han,

Hall of Fame Super Gold

Re: a DHCP question

Han

Is it possible that there is something in QIP that thinks that a particular IP address is reserved for the MAC address of that PC? That could be a scenario that would explain the behavior.

HTH

Rick

wuh
Community Member

Re: a DHCP question

I'd agree with this.

thanks,

Han,

Community Member

Re: a DHCP question

does the DHCP server sit on a different subnet than the client? if so, you may need to go into the vlan interface for the client (i.e. int vlan 100) and add the ip helper-address x.x.x.x command to point it to the DHCP server.

wuh
Community Member

Re: a DHCP question

thanks, but there are ports from both vlan and work fine.

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