This post has been created in an attempt to broaden my knowledge, and also because I thought someone might find this interesting.
I have messed a little with DHCP in the attached network. All has been done in packet tracer so I could follow everything in the nice simulator 
What happens is:
1. Client sends DHCP Discover
2. IP helpers on both L3 switches sends it to DHCP server which is R0
3. DHCP server sends DHCP Offer
4. it comes to L3 switch 1 (the one on the right)
5. L3 switch 1 sends it to L3 switch 0 ( dest. ip is 255.255.255.255 / dest. port. is UDP 68 / fa0/10 on L3 switch 1 is blocked by stp)
6. L3 switch 0 sends the Offer to the switch with client, but it also triggers the ip helper-address and sends it back to the server as a unicast
The client receives the offer, but so does the server that sent it.
Packet tracer behaves a little bit different in Linux and Windows version, but it happens on both of them in the end.
In RFC 1542 is mentioned the following -
A relay agent MUST silently discard any received UDP messages whose
UDP destination port number is BOOTPC (68).
DISCUSSION:
There should be no need for a relay agent to process messages
addressed to the BOOTPC port. Careful reading of the original
BOOTP specification [1] will show this. Nevertheless, some
relay agent implementations incorrectly relay such messages.
And so it seems that cisco implemented ip helper that it would relay both 67 and 68 dest. ports -
http://www.ciscopress.com/articles/article.asp?p=330807&seqNum=9
So i guess it works as designed. But why was it implemented in this way?
Is it not unwanted traffic that is coming back to the DHCP server?
Or is it bad implementation on my part ?
Thanks very much for any response.
Cheers
