cancel
Showing results for 
Search instead for 
Did you mean: 
cancel
793
Views
4
Helpful
8
Replies

a DHCP question

wuh
Level 1
Level 1

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 8

Richard Burts
Hall of Fame
Hall of Fame

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

HTH

Rick

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,

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

HTH

Rick

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

thanks,

Han,

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

HTH

Rick

I'd agree with this.

thanks,

Han,

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.

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

Getting Started

Find answers to your questions by entering keywords or phrases in the Search bar above. New here? Use these resources to familiarize yourself with the community: