Hello All................I have a DHCP Superscope that has around 5 subscopes inside it. Subscopes basically are based on different VLANs such as 10, 20, 30, 40, 50, etc. 

I faced a problem recently where computers in VLAN 10 were receiving IP Address from VLAN 20. 

1.  Is this normal?

2.  Computers were unable to communicate with anyone because they were in different VLAN, but why would they get an IP Address from different VLAN?

Hi Junkie,

Are the clients and DHCP server on a single physical network segment (such as a single Ethernet LAN segment)?

If yes, DHCP server could not offer IP addresses according to the VLAN.

If not, we need to check the configuration of DHCP relay agent. For detailed analysis, we could use Network Monitor. We could analyze the captured packets to find out the problem.

Here is the guide for Network Monitor:
Network Monitor:
https://technet.microsoft.com/en-us/library/cc938655.aspx?f=255&MSPPError=-2147217396

Best Regards,

Leo

Thanks for the reply.

It is a Segmented LAN with each Location having its own VLAN and Relay Agent is configured on switch and clients are getting IP Addresses as well.

The problem is that they are getting IP Addresses from different VLAN.

There is only one DHCP Server that has Superscope and includes subscopes for different VLANs.  The way IP Addresses are being assigned is like this.  First subscope is used first then when it fills up, second scope starts giving IP Addresses.

I want it to be like this. "Request coming from 10 VLAN Computer should get IP from 10 Network Subscope from Superscope".

Because when 10 VLAN Network gets an IP Address from 11 Network Subscope, it is of no use since it is different VLAN.

How can I correct this?

Hi Junkie,

All subnets should have their own DHCP relay agent. The IP address of every DHCP relay agent should be within the same subnet of clients. When requesting IP address, the DHCP server would verify the IP address of DHCP relay agent and choose a corresponding scope.

I suppose we may try to not use superscope, ordinary scope is suitable for your deployment.

For detalied analysis, we could use Network Monitor. We could analyze the packets to check which IP addresses are sending requestes to DHCP server.

Best Regards,

Leo

So, one DHCP server should be enough for this?

All I need to do is assign DHCP Relay agent to all subnets ?But, would not it be the address of DHCP Server ? So, in essence we will be at square one with all subnets having one dhcp relay agent?

Hi Junkie,

Yes, one DHCP is enough.

Every subnet needs its own DHCP relay agent.

Best Regards,

Leo

Thanks for the reply.

But DHCP Relay Agent is the IP Address of the DHCP Server? Is it not? 

Hi Junkie,

DHCP relay agent is not the same as DHCP server.

Here is the guide for configuring:
Configure the DHCP Relay Agent:
https://technet.microsoft.com/en-us/library/cc781416(v=ws.10).aspx

Best Regards,

Leo

Hi Junkie,

A server with RRAS installed could be a DHCP relay agent.

We could configure according to the guide.

Best Regards,

Leo

Thanks for the reply.

So, you are suggesting that it to succeed, I need a relay agent based on RRAS?  Cannot it be done on the Switch/Router?  Also, would I be needing 10 RRAS Servers for 10 VLANs?

Hi Junkie,

Yes, a router could act as a DHCP relay agent if it supports.

10 VLANs need 10 DHCP relay agents.

Best Regards,

Leo

Hi Junkie,

A router can handle it.

We could ask the manufacturer for help. They could help us to achieve the goal.

Best Regards,

Leo

Thanks for the reply.

So, you are basically saying that I do not need to do anything on the DHCP Server besides having all the scopes correctly configured.  All of the rest is would be required at the Router-end?  i.e. configuring of relay agent for all the required Scopes?  Can you share any link to blog/article for such a configuration?

Hi Junkie,

Yes, that's right. DHCP relay agent is related to the router, no further configuration is required on DHCP server.

About the detailed configuration, we could search on the forum of your product. There may be some similar cases.

Best Regards,

Leo

Thanks for the reply.

I am using CISCO Multilayer Switch.

But, I am confused as to what would be configured and where.

To give a more clear picture of my problem. 

Scopes:  10.0.1.1-254  and 192.168.0.1-254

Networks: 10.0.1.0 and 192.168.0.0

Location:  Building A and Building B

I have Scopes defined for different Network Segments in my DHCP Server and IP Helper defined so that clients can contact the DHCP Server for IP Address.  My problem is that since DHCP does not distinguish where the request is coming from, it will assign the first available IP Address in the scope or subsequent scopes wherever it is free.

This causes a problem, For example when a user from 10.0.1.0 would try to get an IP Address and if there are no IP Address available in 10.0.1.0, it would get any free IP Address that is available in 192.168.0.0.  But, since the port would in VLAN 10.0.1.0, enduser will not be able to communicate at all with anyone.

I am looking to resolve it that like when user from Network 10.0.1.0 sends request for IP Address, he should get the IP Address from 10.0.1.0 network. and same for 10.2.0.0 Network users getting 10.2.0.0 IP Address.  How can this be achieved?

• Edited by Technet Junkie 23 hours 55 minutes ago

Thanks for the reply.

I am using CISCO Multilayer Switch.

But, I am confused as to what would be configured and where.

To give a more clear picture of my problem. 

Scopes:  10.0.1.1-254  and 192.168.0.1-254

Networks: 10.0.1.0 and 192.168.0.0

Location:  Building A and Building B

I have Scopes defined for different Network Segments in my DHCP Server and IP Helper defined so that clients can contact the DHCP Server for IP Address.  My problem is that since DHCP does not distinguish where the request is coming from, it will assign the first available IP Address in the scope or subsequent scopes wherever it is free.

This causes a problem, For example when a user from 10.0.1.0 would try to get an IP Address and if there are no IP Address available in 10.0.1.0, it would get any free IP Address that is available in 192.168.0.0.  But, since the port would in VLAN 10.0.1.0, enduser will not be able to communicate at all with anyone.

I am looking to resolve it that like when user from Network 10.0.1.0 sends request for IP Address, he should get the IP Address from 10.0.1.0 network. and same for 10.2.0.0 Network users getting 10.2.0.0 IP Address.  How can this be achieved?

• Edited by Technet Junkie Monday, July 27, 2015 7:06 AM

Hi Junkie,

When using DHCP relay agent, DHCP server would check IP address of the request packet. (The packet is sent by unicast)

Request from network 10.0.1.0 would get IP address from the corresponding scope. If there is no available IP addresses in the scope, it would not respond to the request.

Best Regards,

Leo

Thanks for the reply.

I am not using DHCP Relay Agent at the moment, I have just IP Helper defined at the Global level on my Multilayer Switch.

Request from network 10.0.1.0 would get IP address from the corresponding scope. If there is no available IP addresses in the scope, it would not respond to the request.

This is not what is happening with me.  Clients are getting IP Address from other scopes as well when scope runs out of IP Addresses, which is very strange.

Hi Junkie,

I understand your situation. And as I mentioned before, configure different DHCP relay agents(IP helper in your environment) for different VLANs. And DHCP server would offer IP addresses from different scopes.

Best Regards,

Leo

Thanks for the reply.

If I have understood you correctly, you are basically saying that I should configure IP Helper on respective VLANs rather than Globally?  Please, correct me.

Hi Junkie,

Yes, that's right.

Best Regards,

Leo

Thanks for the reply.

Actually, the Network Admin says that it is configured the same way.  I will test again and get back to you.