IP churn, maybe due to shared-network?

Simon Hobson dhcp1 at thehobsons.co.uk
Wed Oct 31 16:16:04 UTC 2012

Norman Elton wrote:

>So here's the question ... when there are two subnets defined inside 
>a shared-network, does dhcpd always prefer using A, even if the 
>client has an existing lease from B? That is, does it only use B if 
>A is completely filled with active clients? This would certainly 
>explain the behavior we're observing.

No, the ISC server will give a client the last address it had if it 
is still valid and not reallocated. This doesn't change when using a 
shared network.

However, I think your numbers are out. You have only 2,500 available 
leases per shared network - which means you have around 5 times as 
many devices as addresses in a single network. How many of those 
devices will have moved around between sites/zones ?

If the devices do move around between networks, then you have a very 
large over-subscription and you will get a lot of churn.

Simon Hobson

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