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Webcast-Catalyst9k
New Member

Using DHCP server behind 2 different Switches on different VLANs

Dear All;

i need help; I have three switches, 2960(2) 3750 (1) having 6 VLANs. My PC is attached with 2960 switch on VLAN 2. this switch is attached with 3750 switch via trunk. i have DHCP server attached on VLAN 6 on 3750 switch. but my PC is not able to contact with DHCP server. My configurations are:

On Switch 2960;

VLANs = 2 & 3

service dhcp

ip dhcp relay information policy replace

VTP domain = ABC

VTP mode = Cient

TRUNK = trunk on

ON Switch 3760;

VLANs = 4,5&6

service dhcp

VTP domain = ABC

VTP mode = server

TRUNK = switchport mode trunk

when i attached my PC in default VLAN on other side DHCP server on default VLAN (Both are in same VLANs) then it works fine and get IP address from DHCP server. but when i Attached my PC on VLAN2 in first switch (2960) and DHCP server on another VLAN 6 (on 3750 Switch) then it doen't work. i need help.

5 REPLIES
Cisco Employee

Re: Using DHCP server behind 2 different Switches on different V

Hi Friend,

Can you please confirm if 3750 doing intervlan routing between your all vlans? If not that which router/switch is doing an inter vlan routing between your vlan 2 and vlan 6. I do not see vlan 2 configured on your 3750 switch? Can you please confirm.

Also the router/switch who is doing routing between your vlan 2 and vlan 6 on that router/switch you need to add "ip helper-address " under interface which is doing routing for your vlan2, it will be interface vlan 2 if your switch is doing routing or subinterface for vlan 2 if router is doing routing for your vlans.

Please revert back if you have any doubts.

HTH

Ankur

*Please rate all helpfull post

New Member

Re: Using DHCP server behind 2 different Switches on different V

Hi dear friend;

Thank you very much for your kind reply;

you are right. 3750 switch is my core switch but i didn't configure it for interVLAN routing. nodoubt TRUNK port which i configured,is part of all VLANs. this port should route source VLAN traffic to destination VLAN traffic. if not then it means i have to configure 3750 for Inter VLAN routing.

on the other hand it means i have to configure the IP helper-address command in all VLAN interfaces on both switches? so could you please giude me further.

i will wait for your kind reply

Cisco Employee

Re: Using DHCP server behind 2 different Switches on different V

Hi Friend,

Yes you have to configure inter vlan routing on your 3750 switch so that vlan 2 can talk to vlan 6 and all vlans can talk to each other. Trunk allow to pass all vlans but will not route packets.

Trunk will make your vlan 2 pc on 2960 talk to vlan 2 pc on 3750 but not any pc in vlan 6.

So just configure 3750 for intervlan routing like this

Switch(config)#ip routing

!

Switch(config)#interface vlan 2

Switch(config-if)#ip address

Switch(config-if)#ip helper-address

Switch(config-if)#no shut

!

Switch(config)#interface vlan 4

Switch(config-if)#

Switch(config-if)#ip helper-address

Switch(config-if)#no shut

!

Switch(config)#interface vlan 6

Switch(config-if)#ip address

Switch(config-if)#no shut

You need not configure "ip helper-command".

Once you are done with the same, everything should work fine.

Also you can have a look at this link which will guide with inter vlan routing config

http://www.cisco.com/en/US/tech/tk389/tk815/technologies_configuration_example09186a008015f17a.shtml

HTH

Ankur

*Pls rate all helpfull post

Hall of Fame Super Blue

Re: Using DHCP server behind 2 different Switches on different V

Is your 3750 switch acting as the layer 3 switch ie. do you have the routed interfaces for the vlans on this switch.

if so you need to add the following under the vlan 2 layer 3 configuration on the 3750 switch

ip helper-address "address of DHCP server"

HTH

New Member

Re: Using DHCP server behind 2 different Switches on different V

you are right. 3750 switch is my core switch but i didn't configure it for interVLAN routing. nodoubt TRUNK port which i configured,is part of all VLANs. this port should route source VLAN traffic to destination VLAN traffic. if not then it means i have to configure 3750 for Inter VLAN routing.

on the other hand it means i have to configure the IP helper-address command in all VLAN interfaces on both switches? so could you please giude me further.

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