DHCP transactions per second

Simon Hobson dhcp1 at thehobsons.co.uk
Mon Jun 18 08:35:56 UTC 2007

Ian Anderson wrote:
>With further investigation it appears that we are getting multiple
>requests for each client.  We have several ip helper  addresses (foundry
>switchs) in our network, and I believe we may have a misconfiguration
>For example, if a client has never powered up on our network before it
>performs the typical DISCOVER/OFFER/REQUEST/ACK  Because all of those
>are broadcast the dhcp server is receiving the client messages 4 times. 
>(and responding 4 times as well) 4 DISCOVERS / 4 REQUESTS.  I know dhcp
>can cope with this, and the server will ACK the first REQUEST it
>receives from any given client.   Even though dhcp can handle this, is
>this a significant source of additional load on the server?

More or less, each request adds one unit of extra work.

>Are duplicate broadcasts treated any differently by dhcp than receiving just
>one broadcast?


>Or in other words, does dhcp know the difference between a normal 
>broadcast and one that is a duplicate?

How can it ? They are simply relayed packets and will be identical 
other than the GIAddr - there is nothing to say "this is a duplicate".

>I would imagine in order to get an accurate representation of your dhcp
>servers transactions per second you would need to fix whatever is
>causing the duplicate messages.

Yes - it would help !

However, as you say, these extra packets don't cause any problems 
other than the extra load - except ...

IIRC there is or was a problem related to multiple relayed requests 
that come in closely spaced. I can't remember what it was now !

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