From: Stefan Grey (examplebrain@hotmail.com)
Date: Fri Feb 09 2007 - 07:18:21 ART
Whel in that case the DHCP relay would probably receive the DHCP reply
packet from the DHCP server with the destination of it's IP and MAC. OK. But
how would it know that it should forward it somewhere else and whrere?? so
that this message can be delivered to the Client which originally requested
fro DHCP service??
>From: <Daniel_Steyn@Dell.com>
>Reply-To: <Daniel_Steyn@Dell.com>
>To: <examplebrain@hotmail.com>, <ccielab@groupstudy.com>
>Subject: RE: DHCP question
>Date: Fri, 9 Feb 2007 03:50:44 -0600
>
>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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