dhcp failover

John Abbott abbottj at stgeorge.com.au
Thu Jul 6 07:57:16 UTC 2006

>>> Simon Hobson <dhcp1 at thehobsons.co.uk> 07/06/06 5:25 PM >>>
John Abbott wrote:

>Hi listers I am new to the list, in my experience this would only be a
>real issue when dhcp must server many hundreds+ of clients as each
>client when starting will get NAK and then start the negotiation from

>>No, that is not what will happen !

>>Clients will not get a NAK unless the address they are currently
>>using has been re-allocated (leased) to another device. If the client
>>has been switched off since before the servers were switched then
>>this won't matter, but if it's been on-line then it will have been
>>running (or rather not) with a duplicate address. 

ok I was commenting on what I have seen in the past, right or wrong
thats what happened.

>It has also all sorts of additional issues such as dups as
>mentioned for networks that don't respond quick enough,

>>It's nothing to do with 'networks that respond quickly enough', it's
>>to do with whether devices respond to pings or not - given the number
>>of systems that now have built-in firewalls that block pings (which I
>>personally think is a stupid idea, but then that fits with my views
>>on certain OSs anyway) there is a fair chance of something using an
>>address but being invisible to the server. 

not what I was trying to say but I am not going to press the point, but
refer to your following comment as part explanation of myabove point.

>>What may happen is a storm of discover-offer-decline sequences as
>>devices are offered a duplicate address and then detect (via ARP)
>>that it's in use and so reject it. The server will then work through
>>it's address space offering addresses until the client is happy.

>on large networks it can cause backoff on the switch port your server
>connected to.

>>Can you elaborate on that ? I can't see why dhcp traffic would cause
>>problems on a switch unless you've set some silly filtering/traffic
>>shaping rules. 

We run 3 day leases, and if someone blows away the lease database and
desktops must neg a new  lease it gets noisy especially  when they all
7500+ want lease between 9-10am the switch port gets busy. 

I am not chalenging here on what should happen with dhcp please dont
misunderstand I am just staing what has worked and hasn't worked for us,
we also provide slp updates in the dhcp answer and this can also get a
little troublesome especially if the lease database had disappeared.


This document should be read only by those persons to whom it is 
addressed and its content is not intended for use by any other 
persons.  If you have received this message in error, please notify 
us immediately.  Please also destroy and delete the message from 
your computer.  Any unauthorised form of reproduction of this message 
is strictly prohibited.

St George Bank Limited AFSL 240997, Advance Asset Management Limited 
AFSL 240902,  St George Life Limited AFSL 240900, ASGARD Capital Management Limited 
AFSL 240695 and Securitor Financial Group Limited AFSL 240687 is not liable for 
the proper and complete transmission of the information contained in 
this communication, nor for any delay in its receipt.

More information about the dhcp-users mailing list