@@ -38,6 +38,100 @@ Bridge port netlink attributes
.. kernel-doc:: include/uapi/linux/if_link.h
:doc: Bridge port enum definition
+STP
+===
+
+The STP (Spanning Tree Protocol) implementation in the Linux bridge driver
+is a critical feature that helps prevent loops and broadcast storms in
+Ethernet networks by identifying and disabling redundant links. In a Linux
+bridge context, STP is crucial for network stability and availability.
+
+STP is a Layer 2 protocol that operates at the Data Link Layer of the OSI
+model. It was originally developed as IEEE 802.1D and has since evolved into
+multiple versions, including Rapid Spanning Tree Protocol (RSTP) and
+`Multiple Spanning Tree Protocol (MSTP)
+<https://lore.kernel.org/netdev/20220316150857.2442916-1-tobias@waldekranz.com/>`_.
+
+Bridge Ports and STP States
+---------------------------
+
+In the context of STP, bridge ports can be in one of the following states:
+ * Blocking: The port is disabled for data traffic and only listens for
+ BPDUs (Bridge Protocol Data Units) from other devices to determine the
+ network topology.
+ * Listening: The port begins to participate in the STP process and listens
+ for BPDUs.
+ * Learning: The port continues to listen for BPDUs and begins to learn MAC
+ addresses from incoming frames but does not forward data frames.
+ * Forwarding: The port is fully operational and forwards both BPDUs and
+ data frames.
+ * Disabled: The port is administratively disabled and does not participate
+ in the STP process. The data frames forwarding are also disabled.
+
+Root Bridge and Convergence
+---------------------------
+
+In the context of networking and Ethernet bridging in Linux, the root bridge
+is a designated switch in a bridged network that serves as a reference point
+for the spanning tree algorithm to create a loop-free topology.
+
+Here's how the STP works and root bridge is chosen:
+ 1. Bridge Priority: Each bridge running a spanning tree protocol, has a
+ configurable Bridge Priority value. The lower the value, the higher the
+ priority. By default, the Bridge Priority is set to a standard value
+ (e.g., 32768).
+ 2. Bridge ID: The Bridge ID is composed of two components: Bridge Priority
+ and the MAC address of the bridge. It uniquely identifies each bridge
+ in the network. The Bridge ID is used to compare the priorities of
+ different bridges.
+ 3. Bridge Election: When the network starts, all bridges initially assume
+ that they are the root bridge. They start advertising Bridge Protocol
+ Data Units (BPDU) to their neighbors, containing their Bridge ID and
+ other information.
+ 4. BPDU Comparison: Bridges exchange BPDUs to determine the root bridge.
+ Each bridge examines the received BPDUs, including the Bridge Priority
+ and Bridge ID, to determine if it should adjust its own priorities.
+ The bridge with the lowest Bridge ID will become the root bridge.
+ 5. Root Bridge Announcement: Once the root bridge is determined, it sends
+ BPDUs with information about the root bridge to all other bridges in the
+ network. This information is used by other bridges to calculate the
+ shortest path to the root bridge and, in doing so, create a loop-free
+ topology.
+ 6. Forwarding Ports: After the root bridge is selected and the spanning tree
+ topology is established, each bridge determines which of its ports should
+ be in the forwarding state (used for data traffic) and which should be in
+ the blocking state (used to prevent loops). The root bridge's ports are
+ all in the forwarding state. while other bridges have some ports in the
+ blocking state to avoid loops.
+ 7. Root Ports: After the root bridge is selected and the spanning tree
+ topology is established, each non-root bridge processes incoming
+ BPDUs and determines which of its ports provides the shortest path to the
+ root bridge based on the information in the received BPDUs. This port is
+ designated as the root port. And it is in the Forwarding state, allowing
+ it to actively forward network traffic.
+ 8. Designated ports: A designated port is the port through which the non-root
+ bridge will forward traffic towards the designated segment. Designated ports
+ are placed in the Forwarding state. All other ports on the non-root
+ bridge that are not designated for specific segments are placed in the
+ Blocking state to prevent network loops.
+
+STP ensures network convergence by calculating the shortest path and disabling
+redundant links. When network topology changes occur (e.g., a link failure),
+STP recalculates the network topology to restore connectivity while avoiding loops.
+
+Proper configuration of STP parameters, such as the bridge priority, can
+influence network performance, path selection and which bridge becomes the
+Root Bridge.
+
+User space STP helper
+---------------------
+The user space STP helper *bridge-stp* is a program to control whether to use
+user mode spanning tree. The ``/sbin/bridge-stp <bridge> <start|stop>`` is
+called by the kernel when STP is enabled/disabled on a bridge
+(via ``brctl stp <bridge> <on|off>`` or ``ip link set <bridge> type bridge
+stp_state <0|1>``). The kernel enables user_stp mode if that command returns
+0, or enables kernel_stp mode if that command returns any other value.
+
FAQ
===
Add STP part for bridge document. Signed-off-by: Hangbin Liu <liuhangbin@gmail.com> --- Documentation/networking/bridge.rst | 94 +++++++++++++++++++++++++++++ 1 file changed, 94 insertions(+)