Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. If you'd prefer to explore, try our test area to get started. And see here for current known issues.

New Member

DHCP updates and grey hair

I have a DHCP server in one building and users in other buildings on different subnets using it to get addresses. The problem is that when a computer moves from one subnet to another the DHCP server continues to issue the address from the old subnet. The only way to get it to renew with a correct address is to let the lease expire and then request a new address. This dosen't happen with computers that are straight out of the box, only ones moving between subnets. I have tried many different configurations on my router but I haven't hit the right one yet. Any help is appreciated...

  • Other Network Infrastructure Subjects
New Member

Re: DHCP updates and grey hair

First, I want to say - Grey hair happens, DHCP or not!

A high altitude view of what is supposed to happen: A device sends a broadcast for an IP address. If the DHCP server is local, the server responds and the router drops the packet. If the server is remote and the router is configured to forward the requset, the router routes it to the next hop. The DHCP request has a field in it for the router IP address. So before routing the packet the router places its address (the address local to the requesting device) in this field. When the packet reaches the DHCP server, the server looks at the router field to determine the scope to use, offers an address and sends the packet back.

When a device that has previously been issued an address is rebooted, it will ask to use the same address. If it is still available, the server will renew it. If there is no answer from the server, some devices will continue to use the address and continue to attempt to renew it. Other devices will use automatic addressing. Others - who knows? If they cannot reuse the address, maybe because they are on a different subnet, they should drop the address and make a new request. That is high level of what is supposed to happen.

Anyway; besause you indicate that they eventually get an address (after the lease expires), I will assume that your server has the proper scopes built, the proper options are present in those scopes, the router has the proper helper address, and the router can route to all of the subnets. The questions you need to answer are these: Are the devices properly attempting renewal? Is the router properly forwarding the request? Is the DHCP server receiving the request? Is the server properly responding to the request? Are the responses from the DHCP server getting back to the device?

Debugs and a sniffer may be necessary to answer these questons. I know I didn't tell you what is wrong, but I hope I was able to give you some insight as to where to look. When you know how it is supposed to work, its a lot easier to fix when it doesn't!

This widget could not be displayed.