If you have configured two or more IP addresses on a single interface, the wrong server ID might be delivered to DHCP clients. Unless a custom server ID is configured as a DHCP service option, the DNS/DHCP Server will send clients the server ID matching the first IP address configured on the service interface. This behavior forces DHCP clients to learn the DHCP server IP address that may not match the IP subnet of address lease they obtained. As such, the client will not be able to send DHCP unicast packets back to DHCP server.
A DNS/DHCP Server has the service interface eth0.150 that communicates with VLAN 150. Three IP addresses are assigned to interface eth0.150 as follows: 10.10.150.1/24, 172.30.150.1/24, and 192.0.2.1/24. The address 10.10.150.1/24 is the first IP address on the interface. If you deploy DHCP range 172.30.150.10-172.30.150.20, then the DNS/DHCP Server will properly serve DHCP from the interface eth0.150. However, DHCP clients will receive 10.10.150.1 as their Server ID, which does not match subnet 172.30.150.0/24. Therefore the client-obtained dynamic IP address 172.30.150.14 will believe its DHCP server is 10.10.150.1 (when it should be 172.30.150.1). This client will not be able to send unicast DHCP packets back to the DHCP server; it will not be able to send a DHCP renew request for an existing address lease or inform the DHCP server of release events for that lease.
You must deploy a Server Identifier DHCP service option at the network level to ensure that the IP address sent to clients properly indicates the address of the interface that handled the request.
To set the Server Identifier DHCP service option:
- From Address Manager, navigate to the necessary DHCP range and click the Deployment Options tab.
- Under Deployment Options, click New and select DHCP Service Option.
- Under General, select Server Identifier from the Option drop-down menu.
- Enter one of the IP addresses assigned to the server in the Address field.
Servers, select the servers to which the option
- All Servers—applies the deployment option to all servers in the configuration.
- Server Group—applies the deployment option to a specific server group in the configuration. Select a server group from the drop-down menu.
- Specific Server—applies the deployment option to a specific server in the configuration. Select a server from the drop-down menu.
- Under Change Control, add comments, if required.
- Click Add or Add Next to add another option.
- Deploy DHCP to apply the changes.