DHCP layer 2 broadcast

[lagapidis (Lazarus Agapidis)]

Hello Sumant

@alex.fritzsche had an excellent answer for you and clarifies most of the issues involved. Just one note:

@alex.fritzsche

All traffic from DHPC-Server to Clients is Unicast to the MAC Address of the Client on Layer 2 and to the IP Address offered to the Client on Layer 3.

Typically, a DHCP DISCOVERY message is indeed multicast with a destination MAC address of FF:FF:FF:FF:FF:FF:FF. However, there are cases where DHCP OFFER which is the response to the DISCOVERY is also a broadcast. Take a look at the following post concerning this:

Introduction to DHCP

Hello Samit This is an excellent question and it shows that you’re thinking deeply about the subject. It is true that the DHCPOFFER when sent can technically be sent using a unicast MAC address since the MAC address of the host making the request, and thus the destination of the DHCPOFFER frame, is known. However, some operating systems and NIC drivers don’t always use this logic when operating DHCP. Some client implementations are unable to receive such unicast frames until the implementati…

Even in such a case, if two clients ask for an IP address at the same time, they will be able to distinguish which DHCP response is for them and which is not from the “Client Identifier” and chaddr (Client Hardware address) fields.

I hope this has been helpful!

Laz