dhcp problems

[Tony201100]

Hi there,

I have a branch network that recently had its IP address changed. The scenario is as follows:

old IP & mask

10.171.81.64/26

g/w: 10.171.81.65

new IP & mask

10.173.109.0/24

g/w: 10.173.109.1

1 cisco router & 1 HP2710 switch

switch IP: 10.173.109.2

All desktops are dhcp enabled and are getting IP addresses from our dhcp servers; however they are still getting the old addresses.

Both new & old scopes are configured on the dhcp servers. As a test I mac reserved a pc in the new scope but it never activates. The new scope is hence empty and the old scope is showing all rerservations & address leases.

I can ping the new g/w & switch from a pc in the branch. The router is managed by our service provider & I do not have access to it. the switch is managed by us.

Please advice what the problem is steps to troubleshoot.

Many thanks.

[kcnajaf]

HI Tony,

After changing the ip address are you using the same L2 vlans with the new ip range? Or is it a different L2 vlan for new ip range?

If so whether the switch port where the PC's are connected is configured to the new VLAN? Also check if there is a IP helper address configured on the router for the new ip range.

Hope this helps.

Regards

Najaf

Please rate when applicable or helpful !!!

[Tony201100]

Hi Mfurnival,

Thanks for your reply.

I can't deactivate the old scope because it is still active with  dhcp clients. I have tried setting up static IP to no good. Doing this I can't ping let alone connect to it remotely.

Regards.

[mfurnival]

Why are the old and new scopes activated simultaneously? Why not deactivate the old scope? If a PC has an IP address from the old scope it will just try and renew this address from the server when it is set to expire - if the server still has that scope still active then it will let the address be renewed.

Can you set a static IP address and gateway on a PC and get out?

[s.kanth]

Old and new DHCP scope created on same serer or on different server.

If new DHCP scope created on diffent server, then new server IP has to be configured on the router interface (IP helper

Thanks

Sri

[Richard Burts]

There are some things that we do not know that might be helpful in trying to figure out the cause of the problem. For example it would be good to know if the DHCP server is local at the remote site or is the server at HQ. This brings up a related question of whether the remote clients are sending a broadcast request and receiving a local response or is the client broadcast request being forwarded by the router to the DHCP server at HQ? It might also be helpful to know whether the switch has any vlans configured and operating or whether it has all ports as access ports in a common broadcast domain/vlan.

The symptoms suggest that something has not been changed or has been changed in a way that is not effective. We do not have enough information to really understand the problem. But I will take a guess based on what I believe fits the facts that we have available at this point. My guess is that the provider made the change to the new subnet addressing by configuring the new address as a secondary address on the router interface. This is a somewhat common way to accommodate wanting to have both the old address and the new address active so that a gradual transition can be made. If the old address is still primary then that is the address that will be used to forward the DHCP request and the DHCP server will continue to use the old scope.

There are a few things that you might try to troubleshoot the issue. You might try a traceroute from a PC connected to the switch to the address of the DHCP server. Look at the response that is generated by the router, which is probably the first hop from the PC. Does the traceroute reponse come from the old address or the new address?

But I would suggest that the best troubleshooting that you can do is to ask the provider who manages the router to give you some details of how the router interfaces are configured, especially whether there is secondary addressing, and if so which address is primary.

HTH

Rick

[Tony201100]

Hi Richard,

Thanks for your reply.

The DHCP server is at the HO and the clients are in a branch on a different subnet so yes the DHCP broadcast is relayed via a router in this case.

Tracert comes from the old IP subnet. I can ping the new gateway as mentioned in my enquiry.

I am now thinking whether to use confifure a superscope which will include the 2 subnets. Will this work? As noted the 2 subnets are using different masks.

Regards.

[Tony201100]

Hi Sri,

Thanks for your reply.

Both old and new scopes are created on the same server and I was told that IP helpers have been setup on the router.

Regards

[Richard Burts]

I believe that a superscope is not likely to be an effective solution.

It is helpful to know that response to tracert comes from the old subnet. That does suggest that the provider has configured the new subnet as a secondary address on the interface. And in that case the packet generated by the helper address will have the old subnet as its source address. Which means that the DHCP server will use the old scope and not the new. I suggest that you check with the provider about that.

If it is the case that the router interface is configured with secondary addresses you might ask them if they would change it so that the new subnet is the primary address and the old subnet is the secondary address. In this configuration the old leases will continue to work. But new requests will get addresses from the new subnet.

HTH

Rick