ARP after DHCP discover

Question

Hey All,

I was working on DHCP lab As seen in the below figure when I take a pcap between e2 and R1 [in a situation where i shutdown the f0/0 interface on R1 and startup again] I see an ARP happening after DHCP Discover.

However when I remove the R1 machine from network and setup a fresh machine and once the machine comes up for the very first I see DORA happening without any ARP.

Screenshots as below

However if I bring the machine up for the first time in network I see below pcap

I want tpo know why an ARP is happening after DHCP discover and that ARp is initiated from the DHCP Server ..which makes me confirm that it is not ARP probe, as per rfc2131

Answer 1

What you're seeing is a gratuitous ARP (as a reply) originating from the DHCP client being broadcast to the whole subnet. It informs anyone interested of the (possibly new) MAC for 192.168.1.2.

[edit]

The ARP between DHCP Discover (client) and DHCP Offer (server) is the server checking whether the IP address is in use before OFFERing, as per RFC 2131 3.1:

When allocating a new address, servers SHOULD check that the offered network address is not already in use;