How to forward DHCP traffic through a Cisco switch?

After moving your DHCP server and it doesn’t receive client requests but everything is “OK” with the server, then that could cause a problem in your corporate environment:)

In a nutshell, the client sends a DHCPMESSAGE on its local subnet (UDP port 67) but if the DHCP server is on a different subnet then the client uses a BROADCAST address (255.255.255.255) and that represents a problem on the switch.

An overcome of this problem is the “ip helper-address” which we use on the client vlan interface. Here is a simple topology:

 

 

 

 

 

 

 

Here is the configuration needed:

interface vlan2
ip address 172.16.0.1 255.255.255.0

interface vlan3
ip address 172.20.0.1 255.255.255.0
ip helper-address 172.16.0.10 –> DHCP server IP address

 

Now the packet or the broadcast that comes on the interface vlan3 will be forwarded to the DHCP server. It’s a simple command but without it you could encounter a lot of unhappy users.

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