# Auto-configuration for nested subnets on a tree topology

This example demonstrates IPv6 subnet auto-configuration for networks on
a tree topology.

This allows to connect multiple links with individual subnets and route
between them.
Each link can have an arbitrary number of hosts and routers.
Routers can have multiple interfaces to connect different downlinks.

![](../../doc/doxygen/src/gnrc_ipv6_auto_subnets.svg)

If you can ensure there is only a single router on each link, you can
skip the coordination protocol and save some resources by enabling
the `gnrc_ipv6_auto_subnets_simple` module.

![](../../doc/doxygen/src/gnrc_ipv6_auto_subnets_simple.svg)

Routers can still have multiple downstream interfaces but there can be
only a single router in each subnet.

## Setup on native

To simulate such a network on `native` a `setup_taps.sh` script is provided that
will create TAP interfaces and bridges.
Each bridge will be it's own subnet and `native` instances will act as routers
to route between them.
The setup script will also start `radvd` to advertise a large prefix that allows
for sub-division by RIOT.

The interfaces that should be created are specified in the `tapology.txt` file.

Each router will have (at least) two interfaces that should be on different
bridges.

To start the first router, run

    make term PORT="tap_a0 tap_b1"

This will route between the `tap_a0` interface on `br0` and the `tap_b1`
interface on `br1`.
Start more `native` instances to simulate a cascading tree network:

    make term PORT="tap_b0 tap_c1"
    make term PORT="tap_c0 tap_d1"
    …

It is also possible to connect non-routing leaf nodes with a single interface:

    make flash term LEAF=1 PORT=tap_b2


## Setup on hardware

On physical hardware the setup will be the same.
Routing nodes need at least two interfaces between which they can route.
For a simple setup, you can use `ethos` or `slipdev` to turn any UART into
a network interface. (If you need to use DHCPv6 IA_PD for obtaining a prefix,
use `slipdev_l2addr` instead of plain `slipdev`)

### Obtaining the prefix

#### Via router advertisements

Usually routers are configured to advertise a /64 prefix that can not be divided
further when using SLAAC.
If you can configure your router to advertise a larger prefix instead, this
will be the easiest solution.

#### Via DHCPv6

If you can't change the configuration of your router, but your router supports
DHCPv6 IA_PD, you can use this to request a larger prefix.

This requires a gateway node that uses the `gnrc_dhcpv6_client_simple_pd`
module *instead* of `gnrc_ipv6_auto_subnets`.

Make sure to set the `CONFIG_GNRC_DHCPV6_CLIENT_6LBR_UPSTREAM` option to the
ID of the upstream interface on the gateway device, e.g.

    CFLAGS += -DCONFIG_GNRC_DHCPV6_CLIENT_6LBR_UPSTREAM=6

As with GNRC the interface ID is based on the PID of the interface thread, this can
change if you add modules / threads and is a common source of errors.
If the configuration is set wrong, you will not get a prefix for the downstream
interface(s).