Cisco IOS DHCP Relay Agent

[lagapidis (Lazarus Agapidis)]

Hello Marit

All of the communication that takes place between the DHCP relay and the DHCP host occur based on the regular DHCP process as described in this lesson:

Introduction to DHCP

Based on this lesson, and on how DHCP functions, both the OFFER and the ACK messages of DHCP are broadcast using the 255.255.255.255 IP address. However, it is true that at least the MAC address of the requesting host is known by both the server and the relay agent after the initial DISCOVERY packet is sent. The reasons behind why both the OFFER and the ACK are broadcast rather than unicast is further explained in this post:

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…

I hope this has been helpful!

Laz