yes - DHCP requests are broadcasts, and a DHCP helper takes the
broadcast, and converts it to a unicast packet destined for the helper
address. Since broadcasts don’t cross routers (imagine the possibility
for feedback loops!) the help must be on the same segment
On 04.09.2012 08:19, Shaun Pond wrote:[color=blue]
Scott,
[color=green]
will this fail?[/color]
yes - DHCP requests are broadcasts, and a DHCP helper takes the
broadcast, and converts it to a unicast packet[/color]
So why would it fail when - as your perfectly correct explanation says -
the relay turns it into an unicast?
[color=blue]
destined for the helper
address. Since broadcasts don’t cross routers (imagine the possibility
for feedback loops!) the help must be on the same segment[/color]
Correct, but that wasn’t the question. The helper must be on the same
subnet as the client, certainly. But the DHCP server can be on a
remote subnet from the dhcp helper (or relay).
“Relay agents receive Dynamic Host Configuration Protocol (DHCP)
messages and then generate a new DHCP message to send out on another
interface.”[/color]
But that is exactly what Massimo says. Ie it must be able to listen on
the same segment as the client. It can then forward the packet on any
other interface.
–
Anders Gustafsson (NKP)
The Aaland Islands (N60 E20)
On 05.09.2012 06:59, Anders Gustafsson wrote:[color=blue]
Scott A. Campbell,[color=green]
“Relay agents receive Dynamic Host Configuration Protocol (DHCP)
messages and then generate a new DHCP message to send out on another
interface.”[/color]
But that is exactly what Massimo says. Ie it must be able to listen on
the same segment as the client. It can then forward the packet on any
other interface.
[/color]
Well, not quite exactly what was said. Yes, it forwards the request, but not as Broadcast, which the original request was, but instead it sends
a regular UDP packet with the proper target IP. That’s because (and why)
a DHCP helper needs to know the IP address of the DHCP server. If it
would just forward/recreate another broadcast, then it would only work
when the server too is directly connected to it. And in that case, the
DHCP helper wouldn’t need to know the IP of the server.
Yes, it forwards the request, but not as Broadcast, which the original request was, but instead it sends
a regular UDP packet with the proper target IP. That’s because (and why)
a DHCP helper needs to know the IP address of the DHCP server.[/color]
Yes. In that context, the sentence:
“Relay agents receive Dynamic Host Configuration Protocol (DHCP)
messages and then generate a new DHCP message to send out on another
interface.”
Is wrong or at least incomplete as it leads the reader to think that it is
“just” a new DHCP message.
–
Anders Gustafsson (NKP)
The Aaland Islands (N60 E20)
