No Free Addresses?

Simon Hobson dhcp1 at
Tue Dec 14 08:03:12 UTC 2010

Bob Proulx wrote:

>  > When the communication between them breaks they do not automatically
>>  assume the other server is down,
>Right.  It could be a network split.  Each server might continue to be
>up and online on an isolated segment of the network.

More seriously, there are people running failover pairs with out of 
band communication - ie the server-server traffic follows a different 
route to server-client traffic. In this situation (and one or two 
other corner cases such as routing errors) it's possible to have both 
servers still able to serve clients ON THE SAME SUBNET while unable 
to communicate with each other. You wouldn't want either of them 
going into partner down state in that situation, so it's always been 
left to the admin to decide.

If you wanted it to be automatic, you could always script a "is 
partner there ? If not put myself into partner down state" process - 
external to the DHCP service itself.

>  > it is your responsibility to tell the remaining server that the
>>  other is really dead, not just unable to communicate.
>It died today.  But it will be back tomorrow.  I am provisioning a new
>server for it right now.  It isn't permanently gone.  It isn't even
>going to be gone long term.

Doesn't matter - just put the remaining one into partner down state. 
When the other recovers (or is replaced) they will sort themselves 
out automatically.

>  > Until the server gets the information that its partner really is dead,
>>  it will not hand out leases belonging to the partner -> effectively you
>>  are missing half your address space until then.
>Correct.  That is why you need twice the available address pool.

You don't have to, it's a design decision you've made that imposes 
that. Running a network is always a matter of being pragmatic about 
these things - at least with RFC1918 addresses there are plenty to go 

The fun thing is when you come to try and explain to (supposedly) 
technical peers that x.x.1.1/23 really is not a ".1" address (but 
rather think of it as .257 relative to the network address) which in 
their addressing plan is reserved for the primary router :-/

Simon Hobson

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

More information about the dhcp-users mailing list