From: Daniel_Steyn@Dell.com
Date: Fri Feb 09 2007 - 06:50:44 ART
Stefan,
Are you referring to the DHCP process via the helper-address command or
the built in DHCP service for IOS?
If you are referring the to how the helper-address command works - the
client broadcasts a DHCP discover message to 255.255.255.255 MAC
FF:FF:FF:FF:FF:FF on UDP port 67 sourcing from 0.0.0.0 and the client's
MAC. The default-gateway will take that DHCP packet and will strip off
the source and destination IP addresses - replacing the destination IP
with the IP defined in the helper-address command and with the next
hop's MAC. The source IP address will be the interface's PRIMARY ip
address and the MAC will be the outgoing upstream interfaces' MAC.
When the DHCP server receives the packet it will know what scope to
assign an address from because of the SOURCE ip address (router's
primary ip address on the subnet) in the packet. It will source a reply
from its own IP address and MAC and send the packet to the gateway's IP
and the SERVER's gateway's MAC. Note that this can create difficulties
if you have a secondary subnet configured that also needs DHCP.
HTH,
-Daniel
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Stefan Grey
Sent: Friday, February 09, 2007 1:24 AM
To: ccielab@groupstudy.com
Subject: DHCP question
Hello guys,
Could you please explain me the following?? Which addresses (MAC and IP)
do the host and the DHCP server use in the process of communication.
Could you please confirm and expain the following thigs??
Do I correctly understand that the hosts sends the DHCP request with
destination FF:FF:FF:FF MAC and destination IP 255.255.255.255 to the
local network?? It uses its MAC as the source... which IP address does
it use as the source?? And then the DHCP address answers in unicast as I
remember with its IP and MAC as the source and the MAC of the hosts as
the MAC destination... which IP it uses as the destination IP address??
The question appeared because I can't understand how two layer 7 devices
can communicate without know the IP address exactly. How do they
overcome here this problem. I did a short research on this but the
documents didn't give me the details.
Thank you very much.
Please help...
Regards,
Stefan.
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