IP helper-address issues with DHCP

Unanswered Question
Oct 9th, 2008

An issue just crept up and I hoping to post here for some clarification.

We have multiple vlans but only two are being effected.

vlan 26 - 10.10.26.x

vlan 27 - 10.10.27.x

both configured with ip helper-address 10.10.16.45

Nobody currently can recieve dhcp addresses on the 26 but if I turn off the helper address on the 26 vlan they recieve 10.10.27.x addresses.

If I turn the helper back on the 26 and turn it off on the 27, 27 users get a 26 address.

Would this be an issue on the 6500 or is this a problem with a possible rogue dhcp server? Or for that matter a problem with our DHCP server? (10.10.16.45 is the only authorized dhcp server on 2k3 box)

I have this problem too.
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John Blakley Thu, 10/09/2008 - 11:19

Has anything changed in your network? Are these new vlans? Have DHCP servers changed? Do you run individual subnets for each?

--John

jmilay Thu, 10/09/2008 - 11:23

Nothing has changed and these are not new vlans. DHCP server has not changed either. Each are individual subnets.

-Jeff

Edison Ortiz Thu, 10/09/2008 - 11:22

If you turn off ip helper-address on Vlan 26, devices on that subnet shouldn't get any address as the initial DHCP request is a broadcast which remains within the same Vlan.

If they are getting an address, your assumption is correct, there is a DHCP server in the subnet. You can verify the DHCP server in Windows by using IPCONFIG /ALL. There is a DHCP Server entry listed under the interface that obtained the dynamic address.

HTH,

__

Edison.

Rick Morris Thu, 10/09/2008 - 11:25

are you running DHCP locally?

Is there another place that has the helper address?

When you turn the ip helper off on Vlan 26 is the command still on vlan 27 and do the people attached to that vlan get their IP's? Are both affected at the same time, and if you turn off the helper on vlan 27 do you get the IP address on Vlan 27?

jmilay Thu, 10/09/2008 - 11:44

Nowhere do I see ip helper addresses anywhere else. No other boxes using 10.10.16.45 (dhcp) out there. (When I disconnect the DHCP box I get no (ping) responses)

vlan 26 user>vlan 26 port>ip helper-address on = no address

vlan 27 user>vlan 27 port>ip helper-address on = 10.10.27.x address

vlan 26 user>vlan 26 port>ip helper-address off = 10.10.27.x address

vlan 27 user>vlan 27 port>ip helper-address on = 10.10.27.x address

vlan 26 user>vlan 26 port>ip helper-address on = 10.10.26.x address

vlan 27 user>vlan 27 port>ip helper-address off = 10.10.26.x address

jmilay Thu, 10/09/2008 - 11:49

Here is the config on the 6500 for the vlan interfaces:

interface Vlan27

ip address 10.10.27.1 255.255.255.0

ip helper-address 10.10.16.45

no ip unreachables

appletalk cable-range 27-27 27.1

appletalk zone Business

appletalk getzonelist-filter 602

mls ip directed-broadcast exclude-router

interface Vlan26

ip address 10.10.26.1 255.255.255.0

ip helper-address 10.10.16.45

no ip unreachables

appletalk cable-range 26-26 26.1

appletalk zone Business

appletalk getzonelist-filter 602

mls ip directed-broadcast exclude-router

(Yes we still use appletalk!) :)

Edison Ortiz Thu, 10/09/2008 - 11:57

Why do you need to forward directed-broadcasts?

mls ip directed-broadcast exclude-router

It's due to the AppleTalk network?

Remove that command and see if the DHCP issue get resolved.

HTH,

__

Edison.

jmilay Thu, 10/09/2008 - 12:04

Removed the command but still no go.

We have around 60 vlans all using the 10.10.16.45 as the helper with no problems. All are using appletalk zones as well.

These 2 vlans are the only ones with this problem.

These vlans are not new either. It just started yesterday with these vlans being well over three yrs old.

jmilay Thu, 10/09/2008 - 12:10

vlan 26 user>vlan 26 port>ip helper-address off = 10.10.27.x address

vlan 27 user>vlan 27 port>ip helper-address on = 10.10.27.x address

vlan 26 user>vlan 26 port>ip helper-address on = 10.10.26.x address

vlan 27 user>vlan 27 port>ip helper-address off = 10.10.26.x address

Both scenarios - ipconfig/all state the dhcp server as 10.10.16.45

Edison Ortiz Thu, 10/09/2008 - 12:54

The configs seems fine and I have no explanation other than a switch failure. Try reloading the switch with ip helper-address configured under each interface and report back with results.

HTH,

__

Edison.

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