.. SPDX-License-Identifier: GPL-2.0

=======================================
DSA switch configuration from userspace
=======================================

The DSA switch configuration is not integrated into the main userspace
network configuration suites by now and has to be performed manually.

.. _dsa-config-showcases:

Configuration showcases
-----------------------

To configure a DSA switch a couple of commands need to be executed. In this
documentation some common configuration scenarios are handled as showcases:

*single port*
  Every switch port acts as a different configurable Ethernet port

*bridge*
  Every switch port is part of one configurable Ethernet bridge

*gateway*
  Every switch port except one upstream port is part of a configurable
  Ethernet bridge.
  The upstream port acts as different configurable Ethernet port.

All configurations are performed with tools from iproute2, which is available
at https://www.kernel.org/pub/linux/utils/net/iproute2/

Through DSA every port of a switch is handled like a normal linux Ethernet
interface. The CPU port is the switch port connected to an Ethernet MAC chip.
The corresponding linux Ethernet interface is called the conduit interface.
All other corresponding linux interfaces are called user interfaces.

The user interfaces depend on the conduit interface being up in order for them
to send or receive traffic. Prior to kernel v5.12, the state of the conduit
interface had to be managed explicitly by the user. Starting with kernel v5.12,
the behavior is as follows:

- when a DSA user interface is brought up, the conduit interface is
  automatically brought up.
- when the conduit interface is brought down, all DSA user interfaces are
  automatically brought down.

In this documentation the following Ethernet interfaces are used:

*eth0*
  the conduit interface

*eth1*
  another conduit interface

*lan1*
  a user interface

*lan2*
  another user interface

*lan3*
  a third user interface

*wan*
  A user interface dedicated for upstream traffic

Further Ethernet interfaces can be configured similar.
The configured IPs and networks are:

*single port*
  * lan1: 192.0.2.1/30 (192.0.2.0 - 192.0.2.3)
  * lan2: 192.0.2.5/30 (192.0.2.4 - 192.0.2.7)
  * lan3: 192.0.2.9/30 (192.0.2.8 - 192.0.2.11)

*bridge*
  * br0: 192.0.2.129/25 (192.0.2.128 - 192.0.2.255)

*gateway*
  * br0: 192.0.2.129/25 (192.0.2.128 - 192.0.2.255)
  * wan: 192.0.2.1/30 (192.0.2.0 - 192.0.2.3)

.. _dsa-tagged-configuration:

Configuration with tagging support
----------------------------------

The tagging based configuration is desired and supported by the majority of
DSA switches. These switches are capable to tag incoming and outgoing traffic
without using a VLAN based configuration.

*single port*
  .. code-block:: sh

    # configure each interface
    ip addr add 192.0.2.1/30 dev lan1
    ip addr add 192.0.2.5/30 dev lan2
    ip addr add 192.0.2.9/30 dev lan3

    # For kernels earlier than v5.12, the conduit interface needs to be
    # brought up manually before the user ports.
    ip link set eth0 up

    # bring up the user interfaces
    ip link set lan1 up
    ip link set lan2 up
    ip link set lan3 up

*bridge*
  .. code-block:: sh

    # For kernels earlier than v5.12, the conduit interface needs to be
    # brought up manually before the user ports.
    ip link set eth0 up

    # bring up the user interfaces
    ip link set lan1 up
    ip link set lan2 up
    ip link set lan3 up

    # create bridge
    ip link add name br0 type bridge

    # add ports to bridge
    ip link set dev lan1 master br0
    ip link set dev lan2 master br0
    ip link set dev lan3 master br0

    # configure the bridge
    ip addr add 192.0.2.129/25 dev br0

    # bring up the bridge
    ip link set dev br0 up

*gateway*
  .. code-block:: sh

    # For kernels earlier than v5.12, the conduit interface needs to be
    # brought up manually before the user ports.
    ip link set eth0 up

    # bring up the user interfaces
    ip link set wan up
    ip link set lan1 up
    ip link set lan2 up

    # configure the upstream port
    ip addr add 192.0.2.1/30 dev wan

    # create bridge
    ip link add name br0 type bridge

    # add ports to bridge
    ip link set dev lan1 master br0
    ip link set dev lan2 master br0

    # configure the bridge
    ip addr add 192.0.2.129/25 dev br0

    # bring up the bridge
    ip link set dev br0 up

.. _dsa-vlan-configuration:

Configuration without tagging support
-------------------------------------

A minority of switches are not capable to use a taging protocol
(DSA_TAG_PROTO_NONE). These switches can be configured by a VLAN based
configuration.

*single port*
  The configuration can only be set up via VLAN tagging and bridge setup.

  .. code-block:: sh

    # tag traffic on CPU port
    ip link add link eth0 name eth0.1 type vlan id 1
    ip link add link eth0 name eth0.2 type vlan id 2
    ip link add link eth0 name eth0.3 type vlan id 3

    # For kernels earlier than v5.12, the conduit interface needs to be
    # brought up manually before the user ports.
    ip link set eth0 up
    ip link set eth0.1 up
    ip link set eth0.2 up
    ip link set eth0.3 up

    # bring up the user interfaces
    ip link set lan1 up
    ip link set lan2 up
    ip link set lan3 up

    # create bridge
    ip link add name br0 type bridge

    # activate VLAN filtering
    ip link set dev br0 type bridge vlan_filtering 1

    # add ports to bridges
    ip link set dev lan1 master br0
    ip link set dev lan2 master br0
    ip link set dev lan3 master br0

    # tag traffic on ports
    bridge vlan add dev lan1 vid 1 pvid untagged
    bridge vlan add dev lan2 vid 2 pvid untagged
    bridge vlan add dev lan3 vid 3 pvid untagged

    # configure the VLANs
    ip addr add 192.0.2.1/30 dev eth0.1
    ip addr add 192.0.2.5/30 dev eth0.2
    ip addr add 192.0.2.9/30 dev eth0.3

    # bring up the bridge devices
    ip link set br0 up


*bridge*
  .. code-block:: sh

    # tag traffic on CPU port
    ip link add link eth0 name eth0.1 type vlan id 1

    # For kernels earlier than v5.12, the conduit interface needs to be
    # brought up manually before the user ports.
    ip link set eth0 up
    ip link set eth0.1 up

    # bring up the user interfaces
    ip link set lan1 up
    ip link set lan2 up
    ip link set lan3 up

    # create bridge
    ip link add name br0 type bridge

    # activate VLAN filtering
    ip link set dev br0 type bridge vlan_filtering 1

    # add ports to bridge
    ip link set dev lan1 master br0
    ip link set dev lan2 master br0
    ip link set dev lan3 master br0
    ip link set eth0.1 master br0

    # tag traffic on ports
    bridge vlan add dev lan1 vid 1 pvid untagged
    bridge vlan add dev lan2 vid 1 pvid untagged
    bridge vlan add dev lan3 vid 1 pvid untagged

    # configure the bridge
    ip addr add 192.0.2.129/25 dev br0

    # bring up the bridge
    ip link set dev br0 up

*gateway*
  .. code-block:: sh

    # tag traffic on CPU port
    ip link add link eth0 name eth0.1 type vlan id 1
    ip link add link eth0 name eth0.2 type vlan id 2

    # For kernels earlier than v5.12, the conduit interface needs to be
    # brought up manually before the user ports.
    ip link set eth0 up
    ip link set eth0.1 up
    ip link set eth0.2 up

    # bring up the user interfaces
    ip link set wan up
    ip link set lan1 up
    ip link set lan2 up

    # create bridge
    ip link add name br0 type bridge

    # activate VLAN filtering
    ip link set dev br0 type bridge vlan_filtering 1

    # add ports to bridges
    ip link set dev wan master br0
    ip link set eth0.1 master br0
    ip link set dev lan1 master br0
    ip link set dev lan2 master br0

    # tag traffic on ports
    bridge vlan add dev lan1 vid 1 pvid untagged
    bridge vlan add dev lan2 vid 1 pvid untagged
    bridge vlan add dev wan vid 2 pvid untagged

    # configure the VLANs
    ip addr add 192.0.2.1/30 dev eth0.2
    ip addr add 192.0.2.129/25 dev br0

    # bring up the bridge devices
    ip link set br0 up

Forwarding database (FDB) management
------------------------------------

The existing DSA switches do not have the necessary hardware support to keep
the software FDB of the bridge in sync with the hardware tables, so the two
tables are managed separately (``bridge fdb show`` queries both, and depending
on whether the ``self`` or ``master`` flags are being used, a ``bridge fdb
add`` or ``bridge fdb del`` command acts upon entries from one or both tables).

Up until kernel v4.14, DSA only supported user space management of bridge FDB
entries using the bridge bypass operations (which do not update the software
FDB, just the hardware one) using the ``self`` flag (which is optional and can
be omitted).

  .. code-block:: sh

    bridge fdb add dev swp0 00:01:02:03:04:05 self static
    # or shorthand
    bridge fdb add dev swp0 00:01:02:03:04:05 static

Due to a bug, the bridge bypass FDB implementation provided by DSA did not
distinguish between ``static`` and ``local`` FDB entries (``static`` are meant
to be forwarded, while ``local`` are meant to be locally terminated, i.e. sent
to the host port). Instead, all FDB entries with the ``self`` flag (implicit or
explicit) are treated by DSA as ``static`` even if they are ``local``.

  .. code-block:: sh

    # This command:
    bridge fdb add dev swp0 00:01:02:03:04:05 static
    # behaves the same for DSA as this command:
    bridge fdb add dev swp0 00:01:02:03:04:05 local
    # or shorthand, because the 'local' flag is implicit if 'static' is not
    # specified, it also behaves the same as:
    bridge fdb add dev swp0 00:01:02:03:04:05

The last command is an incorrect way of adding a static bridge FDB entry to a
DSA switch using the bridge bypass operations, and works by mistake. Other
drivers will treat an FDB entry added by the same command as ``local`` and as
such, will not forward it, as opposed to DSA.

Between kernel v4.14 and v5.14, DSA has supported in parallel two modes of
adding a bridge FDB entry to the switch: the bridge bypass discussed above, as
well as a new mode using the ``master`` flag which installs FDB entries in the
software bridge too.

  .. code-block:: sh

    bridge fdb add dev swp0 00:01:02:03:04:05 master static

Since kernel v5.14, DSA has gained stronger integration with the bridge's
software FDB, and the support for its bridge bypass FDB implementation (using
the ``self`` flag) has been removed. This results in the following changes:

  .. code-block:: sh

    # This is the only valid way of adding an FDB entry that is supported,
    # compatible with v4.14 kernels and later:
    bridge fdb add dev swp0 00:01:02:03:04:05 master static
    # This command is no longer buggy and the entry is properly treated as
    # 'local' instead of being forwarded:
    bridge fdb add dev swp0 00:01:02:03:04:05
    # This command no longer installs a static FDB entry to hardware:
    bridge fdb add dev swp0 00:01:02:03:04:05 static

Script writers are therefore encouraged to use the ``master static`` set of
flags when working with bridge FDB entries on DSA switch interfaces.

Affinity of user ports to CPU ports
-----------------------------------

Typically, DSA switches are attached to the host via a single Ethernet
interface, but in cases where the switch chip is discrete, the hardware design
may permit the use of 2 or more ports connected to the host, for an increase in
termination throughput.

DSA can make use of multiple CPU ports in two ways. First, it is possible to
statically assign the termination traffic associated with a certain user port
to be processed by a certain CPU port. This way, user space can implement
custom policies of static load balancing between user ports, by spreading the
affinities according to the available CPU ports.

Secondly, it is possible to perform load balancing between CPU ports on a per
packet basis, rather than statically assigning user ports to CPU ports.
This can be achieved by placing the DSA conduits under a LAG interface (bonding
or team). DSA monitors this operation and creates a mirror of this software LAG
on the CPU ports facing the physical DSA conduits that constitute the LAG slave
devices.

To make use of multiple CPU ports, the firmware (device tree) description of
the switch must mark all the links between CPU ports and their DSA conduits
using the ``ethernet`` reference/phandle. At startup, only a single CPU port
and DSA conduit will be used - the numerically first port from the firmware
description which has an ``ethernet`` property. It is up to the user to
configure the system for the switch to use other conduits.

DSA uses the ``rtnl_link_ops`` mechanism (with a "dsa" ``kind``) to allow
changing the DSA conduit of a user port. The ``IFLA_DSA_CONDUIT`` u32 netlink
attribute contains the ifindex of the conduit device that handles each user
device. The DSA conduit must be a valid candidate based on firmware node
information, or a LAG interface which contains only slaves which are valid
candidates.

Using iproute2, the following manipulations are possible:

  .. code-block:: sh

    # See the DSA conduit in current use
    ip -d link show dev swp0
        (...)
        dsa master eth0

    # Static CPU port distribution
    ip link set swp0 type dsa master eth1
    ip link set swp1 type dsa master eth0
    ip link set swp2 type dsa master eth1
    ip link set swp3 type dsa master eth0

    # CPU ports in LAG, using explicit assignment of the DSA conduit
    ip link add bond0 type bond mode balance-xor && ip link set bond0 up
    ip link set eth1 down && ip link set eth1 master bond0
    ip link set swp0 type dsa master bond0
    ip link set swp1 type dsa master bond0
    ip link set swp2 type dsa master bond0
    ip link set swp3 type dsa master bond0
    ip link set eth0 down && ip link set eth0 master bond0
    ip -d link show dev swp0
        (...)
        dsa master bond0

    # CPU ports in LAG, relying on implicit migration of the DSA conduit
    ip link add bond0 type bond mode balance-xor && ip link set bond0 up
    ip link set eth0 down && ip link set eth0 master bond0
    ip link set eth1 down && ip link set eth1 master bond0
    ip -d link show dev swp0
        (...)
        dsa master bond0

Notice that in the case of CPU ports under a LAG, the use of the
``IFLA_DSA_CONDUIT`` netlink attribute is not strictly needed, but rather, DSA
reacts to the ``IFLA_MASTER`` attribute change of its present conduit (``eth0``)
and migrates all user ports to the new upper of ``eth0``, ``bond0``. Similarly,
when ``bond0`` is destroyed using ``RTM_DELLINK``, DSA migrates the user ports
that were assigned to this interface to the first physical DSA conduit which is
eligible, based on the firmware description (it effectively reverts to the
startup configuration).

In a setup with more than 2 physical CPU ports, it is therefore possible to mix
static user to CPU port assignment with LAG between DSA conduits. It is not
possible to statically assign a user port towards a DSA conduit that has any
upper interfaces (this includes LAG devices - the conduit must always be the LAG
in this case).

Live changing of the DSA conduit (and thus CPU port) affinity of a user port is
permitted, in order to allow dynamic redistribution in response to traffic.

Physical DSA conduits are allowed to join and leave at any time a LAG interface
used as a DSA conduit; however, DSA will reject a LAG interface as a valid
candidate for being a DSA conduit unless it has at least one physical DSA conduit
as a slave device.