Bandwidth Consumed by DHCP Message

Simon Hobson dhcp1 at
Thu Mar 27 11:43:17 UTC 2008

Aggarwal Vivek-Q4997C wrote:

>Can you please help me with actual figures?
>As per the RFC I guess DHCP Message size is 576 bytes. And lets take the
>worst case scenario that all 150,000 clients require IP at the same
>time. Then according to you how much bandwidth will be required.

Sorry, that's a nonsensical question !

If taken literally, and all 150,000 clients transmitted a 576 byte 
packet at the very same microsecond in time, then you could be 
looking at filling a gigabit link for as long as it takes to transmit 
150,000 x 576 byte packets !

It won't happen, and the server won't deal with it<period>.

You need to work out what are your normal scenarios (including actual 
packet sizes), and what abnormal scenarios are reasonably likely to 
happen. We cannot do that based on the limited information you've 
provided which does not go beyond "150,000 clients" !

What sort of clients ? Desktop PCs (what OS ?) Clients router 
appliances ? Something else altogether ?

What sort of network ? All devices in one building ? One corporate 
WAN across one or two timezones ? A corporate WAN across many 
timezones ? Random customers of an ISP ?

What sort of startup strategy can you **REASONABLY** expect as a 
'normal worst case' ? OK, if all the clients are in one building then 
it's feasible to see them all suffering a common power outage - but 
if they are ISPs customers spread across half of the North American 
continent then a common power outage is going to be VERY rare indeed 
(but yes I know they do happen - just VERY rarely)

What sort of startup spread would be sensible to expect after a 
widespread power outage ? Even if you switched off the whole of the 
USA and turned it back on at once (doesn't happen !), then the 
clients will have a spread of startup times. ADSL lines will take 
different times to sync, different makes/models of hardware router 
will take different times to boot, users using a directly connected 
PC will take different times to turn them on, and then the PCs will 
take differing times to boot.

In a really worst case scenario, what is a realistic target recovery 
time - 'instant' is not realistic and you'll simply spends lots of 
unneccessary cash not achieving it !

If you cannot answer ALL of those questions for yourself, then no-one 
else can do it for you - we don't know what your network is, and what 
your requirements are.

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