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DHCP Server & IP Helper-Address

We have setup a DHCP server in our network. VLAN A and VLAN B are connected to our 3560 switch. The DHCP server is on VLAN A and the clients attempting to pull an IP are on VLAN B. Within VLAN B I have input the ip helper-address command with the IP address of the DHCP server. I am unable to pull a valid IP within VLAN B. Is there something that I need to do to tell my 3560 switch to relay the IP to the client? I am trying to determine whether my server is having a problem or the switch.

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Accepted Solutions
Hall of Fame Super Gold

Re: DHCP Server & IP Helper-Address

Tom

Thanks for posting the partial config information. It does confirm that routing is enabled on the 3560 and the VLAN interfaces look ok, and the helper address looks ok. So it does not look like a configuration issue at this point.

So I would wonder about the other suggestions in my previous response: does the DHCP server have a correct scope for 220.100.100.0? And can a PC with hard coded address info ping the DHCP server?

[edit] I now see your response that you can ping the server. That is helpful to know. At this point I would believe that there is some issue on the DHCP server. Can you provide any information about how the DHCP server is configured?

HTH

Rick

7 REPLIES
Hall of Fame Super Gold

Re: DHCP Server & IP Helper-Address

Tom

If you have correctly configured the helper address in the VLAN B interface and pointed to the address of the DHCP server in VLAN A then there is not anything else that you should do on the switch to get it to relay the information.

Are you sure that the DHCP server has a correct scope configured for VLAN B?

Is routing enabled on the 3560 switch? If you hard code an IP address, subnet mask, and default gateway on a PC in VLAN B can it then ping to the DHCP server?

It might be helpful if you would post the configuration of the switch (or at least the VLAN interfaces) so we can verify it.

HTH

Rick

Community Member

Re: DHCP Server & IP Helper-Address

Attached are my VLAN configs.

Community Member

Re: DHCP Server & IP Helper-Address

I can ping the DHCP server from a workstation that I have given a static address to.

Re: DHCP Server & IP Helper-Address

Things to check

check the SVI interface status for VLAN B on your L3 device, make sure it is up/up, also check vlans passing through the trunk if any to ensure vlan B is passed through that trunk.

Also make sure the host that is connected to switchport in VLANB is actually in proper vlan membership for that vlan.

make sure the DHCP scope of VLANB in your server is active .

On your L3 device

show ip interface brief

show interface trunk

on your L2 switch

show vlan ( to show dynamic vlan database)

show running-config interface

Rgds

Jorge

Hall of Fame Super Gold

Re: DHCP Server & IP Helper-Address

Tom

Thanks for posting the partial config information. It does confirm that routing is enabled on the 3560 and the VLAN interfaces look ok, and the helper address looks ok. So it does not look like a configuration issue at this point.

So I would wonder about the other suggestions in my previous response: does the DHCP server have a correct scope for 220.100.100.0? And can a PC with hard coded address info ping the DHCP server?

[edit] I now see your response that you can ping the server. That is helpful to know. At this point I would believe that there is some issue on the DHCP server. Can you provide any information about how the DHCP server is configured?

HTH

Rick

Community Member

Re: DHCP Server & IP Helper-Address

Thanks for the help. I will examine my server settings.

Hall of Fame Super Gold

Re: DHCP Server & IP Helper-Address

Tom

I am glad that our answers were helpful. Thank you for using the rating system to indicate that your issue was resolved (and thanks for the rating). It makes the forum more useful when people can read about an issue and can know that there were answers that helped resolve the issue.

The forum is an excellent place to learn about Cisco networking. I encourage you to continue your participation in the forum.

HTH

Rick

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