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Home > Archive > 70-218 > April 2002 > DHCP Relay Agent
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| Sh0tgun 2002-04-03, 10:47 am |
| I was studying DHCP relay agents in class yesterday (doing MOC 2126) and I have a question about it.
We were discussing the boot threshold timeout value. The agent will wait X number of seconds before forwarding client requests, so that if there is a DHCP server on the local segment it will be able to respond to the client first.
My question is: How does the DHCP relay agent know that a DHCP server responded to the client? That is, before the threshold is exceeded, let's say the client gets info from a local network DHCP server. How does the agent know this happened so it can stop "worrying" about that client? Is it because it stops hearing further requests from that client? Is it because it's checking all broadcasts and it sees the offer from the DHCP server? Or what? The only thing in my book is "...the DHCP relay agent contacts a remote DHCP server only if the local DHCP server is not available." Gee, thanks -- but how does it know if it's available or not?
My instructor also didn't give me a firm reason on how the relay agent figures this out. If anyone knows, I'd like to find out why. Thanks | |
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| Sh0tgun 2002-04-03, 12:49 pm |
| Doesn't help  Sorry! | |
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| Sh0tgun 2002-04-03, 4:14 pm |
| Actually I did just think it was a spartan page. Those navigation arrows are mighty small. Now I've looked over it, but I still don't think it's telling me what I want to know. | |
| wbafrank 2002-04-03, 5:08 pm |
| Before I send you all over the world ....
When a DHCP client requests information, the DHCP Relay Agent forwards the request to the list of DHCP servers specified when the DHCP Relay Agent is started. When a DHCP server returns a reply, the reply is broadcast or unicast on the network that sent the original request. | |
| Sh0tgun 2002-04-03, 6:14 pm |
| I talked this over with my instructor again. I believe what she had to say concurs with what you said.
Apparently, no matter what, a relay agent does what it's supposed to do & sends the discover broadcast on as a directed packet/unicast to what DHCP servers it's been configured to use. The boot threshold time value (default of 4 seconds) simply delays the offer from a remote segment from beating out an offer from a local DHCP server (if one is indeed running).
It makes sense, I just thought there would be a way for a relay agent (whether rfc 1542-compliant router or Win2k Server -- or whatever) to tell whether or not it needed to bother providing it's service. Whether or not there was a DHCP server on the same network that would be answering the request, before it began its work. Oh well. |
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