dhcp failover

[John Abbott]

>>> 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
>begining.

>>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
is
>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.

Rgs 
John 

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