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New VLan can't talk to DHCP server

We have just created a new vlan (172.18.1.0/24 vlan 3) on our network and we are trying to use an exsisting DHCP server (192.168.0.1 vlan 8) But the clients on the 172.18.1.0 network can't connect to this DHCP server on vlan 8. My questions are.

1. Do we need to specify vlan 3 on the switchport that the DHCP server is plugged into?

2. Is there something @ L3 we need to enable to pass these DHCP server messages too across these vlans?

Has any used DHCP servers on different subnets?

Any info would be great.

1 REPLY
Cisco Employee

Re: New VLan can't talk to DHCP server

You need to configure an IP helper on the appropriate VLAN interface(s). Routers, by default, will not forward broadcast packets. Since DHCP client messages use the destination IP address of 255.255.255.255 (all Nets Broadcast), DHCP clients will not be able to send requests to a DHCP server on a different subnet unless the DHCP/BootP Relay Agent is configured on the router. The DHCP/BootP Relay Agent will forward DHCP requests on behalf of a DHCP client to the DHCP server. The DHCP/BootP Relay Agent will append its own IP address to the source IP address of the DHCP frames going to the DHCP server. This allows the DHCP server to respond via unicast to the DHCP/BootP Relay Agent. The DHCP/BootP Relay Agent will also populate the Gateway IP address field with the IP address of the interface on which the DHCP message is received from the client. The DHCP server uses the Gateway ip address field to determine the subnet from which the DHCPDISCOVER, DHCPREQUEST, or DHCPINFORM message originates.

See more at the following document, with configuration steps and examples:

http://www.cisco.com/warp/public/473/100.html#configdhcpbootpciscoios

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