Hi,

On Thu, 31 Aug 2017, Marcin Siodelski wrote:

Hello Kea Users,

We are currently working on implementation of a "shared networks"
mechanism in Kea, to provide ability for grouping multiple subnets
belonging to the same link.

This is useful to extend address pools for clients on the same physical
link, i.e. clients located on this link may get addresses from different
subnets. In such case, the DHCP server would allocate addresses from
another subnet (and its pools) if there are no more addresses available
in the first subnet.

It is also useful in cable networks, when a cable modem and a router are
on the same physical link but they should get addresses from different
subnets. Client classification is used in such case.

The ISC engineering team has been working on a design for this feature.
One of the contentious points is how to organize shared networks
configuration within the Kea config file.

We have discussed three different options. Let's call them A, B, C,
which are briefly discussed below. The ISC engineering team is leaning
towards A, but we'd also like to get some input from our Users what they
think might be more convenient.

Proposal A

Sample configuration link:
http://kea.isc.org/wiki/SharedSubnetsDesign#ConfigurationFormat

In this case, the shared-networks list contains a full specification of
each subnet that belongs to shared networks. It is still possible to
define subnets outside of the shared-networks scope. Such subnets will
not be associated with any shared network.

Pros:
- Make use of hierarchical nature of JSON - subnets enclosed within
shared-networks structure belong to shared-networks. Other subnets do
not. No need to refer to subnets from another structure by name or id etc.
- Avoid configuration error whereby a single subnet may belong to
different shared networks.
- Avoid configuration error caused by manual matching of subnets with
networks.
- Is compatible with autogenerated subnet identifiers.
- JSON viewing tools can be used to visualize which subnets belong to
shared network by simply looking at the JSON hierarchy.
- Is similar to other configuration structures we use (except option
definitions).
- Is similar to how it is organized in ISC DHCP.

Cons:
- Moving subnets between shared networks requires copy pasting large
portions of configuration (entire subnet specification has to be
copied), possibly between distant locations in the configuration file.
- Makes it hard to see how many subnets are specified within a shared
network without JSON processing tools that can hide portions of the
configuration.

</snipp>

I tend to agree that option A is the most straight forward and I recommend 
keeping it simple.



Greetings
Christian


--
Christian Kratzer                   CK Software GmbH
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