When upgrading and renovating the network, all aspects must be taken into account, and how to implement DHCP services in a multi-VLAN environment is one of them.
principle
Using DHCP to obtain IP addresses requires the use of broadcast packets. According to normal circumstances, DHCP services can only be implemented in the same broadcast domain. The establishment of VLAN is to isolate broadcast packets. Why can DHCP cross-network segments be implemented on a layer three switch? This requires us to convert the broadcast packet requested by DHCP into a unicast request, so that the request will be forwarded to the VLAN where the DHCP server is located through layer three routing, and then the DHCP cross-VLAN service is realized.
Implementation method
In order to implement DHCP services across VLANs, we need to start from two aspects. On the one hand, we need to specify the IP address of the DHCP server on the switch, and on the other hand, we need to create a new scope on the DHCP server.
The following is a detailed operation step using Cisco's Catlyst 4506 as an example:
1. Configure the DHCP server on the switch:
ip dhcp-server 192.168.0.69
2. Set the same DHCP server IP address for each VLAN in the switch:
interface Vlan11
ip address 192.168.1.254 255.255.255.0
ip helper-address 192.168.0.69 DHCP Server IP
interface Vlan12
ip address 192.168.2.254 255.255.255.0
ip helper-address 192.168.0.69 DHCP Server IP
3. Set the scope of network addresses 192.168.1.0 and 192.168.2.0 on the DHCP server, and set the "router" option of these scopes to the interface IP address of the corresponding VLAN.