Vlan Configuration/DHCP

[michael.spence]

I have configured multiple vlans at a remote site that connects to the main campus through a T-1 frame relay circuit. As long as the computers are on VLAN 1 it will pick up an ip address from our DHCP server, but if I change the port to a vlan that I created I loose connectivity. Where am I going wrong.

[glen.grant]

You need to add ip helper addresses to each subnet that is connected to these vlans . Somewhere those layer 2 vlans have to be routed and that's where you would apply helper addresses .

[JASON WELCH]

I assume that your DHCP server is located in vlan1. In order to 'relay' DHCP client broadcasts from one vlan to another, you need to enter in the 'ip helper-address

' command at the vlan SVI; i.e. say your dhcp server IP address is 10.1.1.1 and you want vlan2 to get dhcp assigned addresses, you would enter in the command 'ip helper-address 10.1.1.1' in the vlan2 SVI.

However, if you do this, unless you create multiple DHCP scopes all your vlans will recieve IP addreses in the same subnet, which is bad :)

I would consider using local DHCP pools\assignments from your 'core' switch at the remote site using a config similar to this:

!

ip dhcp pool VLAN2

network 10.2.0.0 255.255.0.0

default-router 10.2.0.1

domain-name xyz.corp.com

dns-server 10.2.0.2

!

ip dhcp excluded-address

!

Just duplicate this command for every vlan that you want to have dhcp assigned addresses; changing the network, default router and pool names.

HTH,

Jason

[thisisshanky]

Your layer 3 device that routes between vlans, (most likely its the remote site router), needs to be configured as a dhcp relay agent for relaying DHCP requests from other vlans configured on the switch back to the DHCP server at corporate.

Your router's config should look most likely like this for routing between vlans 1,2,3.

int fa0/0

no ip add

int fa0/0.1

ip add 10.10.10.1 255.255.255.0

description vlan 1

encap dot1q 1

int fa0/0.2

ip add 10.10.20.1 255.255.255.0

description vlan 2

ip helper-address

encap dot1q 2

int fa0/0.1

ip add 10.10.30.1 255.255.255.0

description vlan 3

ip helper-address

encap dot1q 1

HTH