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New Member

Router VLAN's and DHCP with Windows 2000

I have created layer 3 VLAN's on a 3620 router with IOS (tm) 3600 Software (C3620-JS-M), Version 12.2(1), RELEASE SOFTWARE (fc2).

For example:

interface FastEthernet1/0.9

description Interface to LAN Subnet

encapsulation isl 709

ip address

ip broadcast-address

ip helper-address

no ip redirects

This is connected to 2900XL and 3500XL switches as a trunk, IOS (tm) C2900XL Software (C2900XL-H2S-M), Version 12.0(5.1)XP.

For example:

interface FastEthernet0/1

duplex full

speed 100

switchport mode trunk

Some have hubs connected and others are connected directly into a switch port, the problem happens with both. When a users PC that has been connected to the LAN leaves the office and goes to another Office they pickup an IP with DHCP but when they return they are unable to pickup an IP without forcing a renew and sometimes lately this has not worked either and I end up putting them on another VLAN and it works. I have watched the traffic when these requests are made and find that the PC is doing its part but the packet never seems to make it to the Server segment. If we put the PC on the Server VLAN it works great. So it looks like the packet is not being tagged properly between VLAN's or it is unaware of the IP Helper command on the orignal request. We do not have this trouble in our Offices with layer 3 switches only the Offices with a router in a stick. Any help would be appreciated.

Thank You

Curtis Sherwood

New Member

Re: Router VLAN's and DHCP with Windows 2000

do you have the IP directed broadcast command on the router?

As long as the server with DHCP service is then the helper command is accurate.

Other than that, I can think of very little


Re: Router VLAN's and DHCP with Windows 2000

Get rid of the ip broadcast address that you have configured , it doesn't need to be there . Instead of using a regular broadcast of it will use the one you have configured , this screws up the dhcp discover ,offer, request , ack processes . We had the exact same problem and it took me a week with 2 sniffers to figure what was going on. If you do a "show ip int F1/0.9 , you notice that the broadcast address is the one you configured it should not be , it should read . Just do a "no ip broadcast-address " then try it . I'll almost gurantee this will work , this was the exact same problem we ran into . Let me know how you make out .

New Member

Re: Router VLAN's and DHCP with Windows 2000

Hi Glenn,

In my preliminary tests it work perfectly. I was sniffing the traffic and I did see what you were talking about, when we forced a renew it would broadcast twice with the but when it did it on its own it would take the VLAN's broadcast address. I thought it was supposed to take the VLAN's interface broadcast address so I never would have thought to remove it. I can't thank you enough. This may not seem like a big issue but it can be very inconvenient and embarrassing if your boss asks what's wrong and you just can't figure this out.

Thanks Again


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