vlan dhcp ip helper-address problem with dhcp client

Unanswered Question
May 14th, 2009

Here is the 3560 conf file:

interface Vlan25

description Test

ip address

ip helper-address


no ip forward-protocol udp tftp

no ip forward-protocol udp tacacs

sw_core#sh ip interface vlan25


Vlan65 is up, line protocol is up

Internet address is

Broadcast address is

Address determined by non-volatile

MTU is 1500 bytes

Helper address is

Directed broadcast forwarding is disabled




I have dhcp server on Microsoft win server 2003(address It works fine for non vlan network. I have created vlan 25. My clients are XPs. Dynamics DHCP allocation works fine for laptops, but it does not work for some of my computers. I have tried everything to reset TCP/IP stack etc.... with no avail. Then I have tried to trace dcp server with ethreal:

When I type the command:

ipconfig /renew

I have on ethreal screen monitoring dhcp server:

1. DHCO discover

but when I use xp support command dhcplock and type d (for discover) I have : DHCP discover DHCP offer

So where is the problem? I do not have any idea what to do next! It is normaly no capable to locate dhcp server, but with dhcplock it can do.

I have this problem too.
0 votes
  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
pieterddejong Sun, 05/17/2009 - 15:21

The ip address is used on 2 vlans 25 & 65 and looks like its being offered from the DHCP server?

hoffenheim Sun, 05/17/2009 - 22:32


That is response from dhcplock. Forget about whether is vlan25 or vlan65. It is 25. It is configured to be, while DHCP is configured to exclude dhcploc is in fact an xp support utility. I used it to get an answer whether that client is recognised by DHCP. The output of that utility is questionable.

What I did is the following I use ethereal to observe DHCP to see communication between client and DHCP server.

I see that client is sending from dhcp discoveries right to the MS 2003 dhcp. I see that client send right netbios and MAC address, but I can not see any response like DHCP offer. There was DHCP offer if I use dhcploc, but I believe that support is not designed for VLAN DHCP helper like relay.

Than I use my laptop, it works and and another XP PC... BUt it seems to me that I could establish DHCP interaction for some computers but I could not for others... The DHCP server is not sending DHCP OFFER for some reason.

I have tried to reset TCP, than I netstat -R and RR. It is still the same, for some computers, but it works for others?

hoffenheim Sun, 05/17/2009 - 22:42

What I want to know is how to debug the vlan helper link in question.

The computers XP SP2 works fine on vlan1. Everyone...! 3 laptops works on vlan! 2 PC XP SP2 as well. However, 2 PC XP SP2 simply could not pass the phase of DHCP discovery.


This Discussion