6509 VLAN issue

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Aug 14th, 2007
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I have just installed a 6509 and put all o f the ports in card2 into VLAN 110. this has an ip address of on card three is VLAN1 (Native) and this is on the address. ON VLAN 1 is a DHCP server and on VLAN 110 there isn't but I have configured ip helper to go to a DHCP server on a remote network.

When I fire up a PC on VLAN110 it come back with a 156 address from the DHCP server on VLAN 1,rather than a 10.address from VLAN110. HOW is this happening? surely a DHCP address can't be passed from VLAn 110 to VLAN 1. IF I do a tracert from the PC it shows that it is directly connected to the 156 range. my head hurts....

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Jon Marshall Tue, 08/14/2007 - 08:39
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Hi Tim

Is the DHCP server in the ip helper-address a different DHCP server from the one on vlan 1 ?

Can you confirm that the pc you start is connected to a port that is in vlan 110.

Could you post the relevant parts of your config ?


timdeadman Tue, 08/14/2007 - 09:06
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HI jon,

The DHCP server isn VLAN1 is a different one to the ones I am pointing the IP helper at. We are migrating the users from the 156 address range to a 10 address range. Yes the pC was on a port connected to VLAN 110.

interface GigabitEthernet2/2

description 1st Floor user

no ip address

duplex half

no snmp trap link-status


switchport access vlan 110

switchport mode access

spanning-tree portfast

interface Vlan1

description Legacy

ip address 156.n.n.1

ip ospf cost 3

standby 1 ip 156n.n.n

standby 1 timers 2 6

standby 1 priority 120

standby 1 preempt


interface Vlan110

description First Floor

ip address

ip helper-address

ip helper-address



timdeadman Tue, 08/21/2007 - 05:05
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Okay, we've found the problem. I had left a port that shoul sdhave stayed in VLAN 1 and put it into VLAN 110 creating a bridging loop.



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