DHCP Relay across VLAN problem

[HEATH FREEL]

I have a client on a swtich plugged into an access port vlan 98

I have a trunk on the same switch that carries two VLAN's 98 and 50 - where 98 is the native vlan.

This trunk connects to a router/fw. The router that has a helper address configured on each vlan to the DHCP Server configured with two scopes.

I run a debug on the router and see the following:

When the client requests a DHCP address, I can see two requests - one from VLAN 50 and one from VLAN 98. I can also see the reply from the DHCP Server giving two IP's one from the 50 scope and one from the 98 scope to the same mac address.

If I turn off the helper address for the 50 subinterface of the router the client gets the correct 98 address, the problenm is I need DHCP to work on both of these VLANS.

Any ideas?

[Wilson Samuel]

Hi,

May I request you to paste some of your config please, as this really helps us to figure out the things pretty soon.

Secondly on the other hand, I was wondering if you have already used, IP Helper command in order to get the DHCP Relay Agent working across the VLANs.

Kindly rate if it helps.

Regards,

Wilson Samuel

[HEATH FREEL]

interface FastEthernet0/29

switchport access vlan 98

switchport mode access

spanning-tree portfast

interface FastEthernet0/47

switchport trunk allowed vlan 50,98

switchport mode trunk

spanning-tree portfast

Not much more to it. The other device that the trunk connects to is not a Cisco, but the debugs clearly show two requests comming in - one from VLAN 98 and one from VLAN 50 - at this point only one client is connected to the switch.

## 2006-11-10 15:52:38 : DHCP: got packet from if .

## 2006-11-10 15:52:38 : DHCP: got packet from if .

[relsethagen]

It sounds like you have these two VLANs bridged together at some point in your network.

[HEATH FREEL]

I don't think so - I just built these two networks (98 and 50) new and am integrating them into my existing network.

[relsethagen]

You mentioned that vlan 98 was your native VLAN (is that how the router is configured) Your switch doesn't have a nativel VLAN configured making it VLAN 1 by default

[HEATH FREEL]

Here is the native VLAN info....The router it connects to is not a Cisco, so the native VLAN concept does not exist.

interface Vlan1

no ip address

no ip route-cache

shutdown

!

interface Vlan98

ip address 10.161.98.2 255.255.255.0

no ip route-cache

[glen.grant]

Where is the dhcp server ? Is it on one of the 2 vlans ? If so technically you would only need 1 helper address on the vlan that the dhcp server is not . This shouldn't be the problem , really can't imagine how you could get 2 requests . You have a user on 98 it gets passed to the trunk interface which then tags the packet with a 98 tag and trunks it across , my guess would the router is doing something strange with it or is not configured quite right . Possibly check the router docs on how to trunk the vlan for any config mistakes ,the switch end looks fine though . Sure the dhcp server is correct ?

[HEATH FREEL]

DHCP Server is on a separate segment. If I turn off helper address for the 50 VLAN the 98 gets his address via DHCP. and if I turn off helper on the 98 the 50 gets his address. I am leaningtowards a probelm with the router running Helper - although I am not sure why I am seeing two request come from the 2950 - one for each VLAN.

[pkrohn]

Hi -

Can you provide a sh interface trunk from the Cisco device. The Native VLAN concept is a part of the 802.1Q standard, and must be identical in both ends of trunk (according to Cisco to prevent Spanning Tree loops).