the question about the dhcp relay deployment

Simon Hobson dhcp1 at
Mon May 9 06:51:51 UTC 2011

Derek Wang wrote:

Can you please maintain the thread subject ? When people come back to 
look through the archives, "dhcp-users Digest, Vol 31, Issue 16" will 
mean nothing at all.

>Simon, you mentioned in last paragraph that we also need to make 
>sure the client can contact the server via unicast after
>got the IP in the client side, it's right because the client need to 
>send the DHCPREQUEST packet to the dhcp server to renew
>the information.
>But If the client can not contact dhcp server via unicast, like 
>without correct route info, then what will happen?
>I think the client will time out after the max-lease-time, then the 
>client will initiate the DHCPDISCOVER again,

The normal process is that after about half the lease time, the 
client should start unicasting renewal requests to the server that 
issued the lease. Towards the end of the lease, but before it's 
expired, is should switch to broadcasting renewal requests - which 
means any server can respond. Finally, if nothing has responded to 
the renewal request (either by unicast, then later by broadcast), the 
client will deconfigure the interface and start from the beginning 
and broadcast discover packets.

I would suggest you track down the book "The DHCP Handbook" by Ralph 
Droms and Ted Lemon". This has a thorough (but very readable) 
explanation of how the protocol works and the different states the 
server & client can go through. It also explains why the protocol 
works the way it does, which is rather a case of "we wouldn't start 
from here" given a clean sheet. This is better than observing a 
specific client & server combination and inferring what's happening - 
it's not unknown (for example) for clients to misbehave.

Simon Hobson

Visit for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
Christmas stocking fillers. Some available as e-books.

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