[Kea-users] Host Reservations and Subnets

Tomek Mrugalski tomasz at isc.org
Tue Oct 4 11:14:14 UTC 2016

W dniu 04.10.2016 o 09:22, Rui Pedro Caldeira pisze:
> Hello Kea users list, I once seek your guidence.
> I'm currently building a system that does not rely in the notion of
> subnet. Instead relies purely on host reservations via relays that can
> assign any IP possible. I can see that version 1.1.0 brought very nice
> features like library parameterization, reservations via DCHP option 82
> relay data (section 7.3 of Kea Administrator Reference Manual) and the
> possibility to insert options (such as default gateway) in the
> reservation itself (section 7.3 of Kea Administrator Reference Manual).
> My question is if I can configure Kea only with reservations (without
> subnets)?
You can't. Subnets represent topology of the network. If your network is
completely flat, you could define a single subnet, make sure that this
single subnet is always selected (either use interface parameter or
specify relay ip address) and then define all reservations in that
subnet. If you don't want to have any dynamic allocations, you can skip
the pools definition. This would give you completely static setup - only
clients with reservations would get an address.

If you're interested in rationale behind this approach, here's a bit of
background information. In the old ISC DHCP implementation all
reservations were global. In deployments that have many subnets with
many reservations in each, the code has to search in the whole list of
reservations, which has performance implication. Even worse, if there
are networks with mobile clients that have reservations in multiple
subnets, the search could result in multiple reservations, and one of
them has to be chosen for a particular request being replied to.

To avoid those problems, we made the reservation subnet-specific. It's
more efficient and seems more natural this way. If host X visits subnet
Y, it should get this set of parameters and addresses. If you have
mobile client, you can define independent set of parameters in each
subnet it is expected to visit.

Hope that helps,

Tomek Mrugalski

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