[PATCH net-next 05/10] docs: bridge: add STP doc

[Hangbin Liu <liuhangbin@gmail.com>]

Add STP part for bridge document.

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
---
 Documentation/networking/bridge.rst | 85 +++++++++++++++++++++++++++++
 1 file changed, 85 insertions(+)

diff --git a/Documentation/networking/bridge.rst b/Documentation/networking/bridge.rst
index 84aae94f6598..1fd339e48129 100644
--- a/Documentation/networking/bridge.rst
+++ b/Documentation/networking/bridge.rst
@@ -51,6 +51,91 @@ options are added.
 .. kernel-doc:: net/bridge/br_sysfs_br.c
    :doc: Bridge sysfs attributes
 
+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 which bridge becomes the Root Bridge. Careful configuration can
+optimize network performance and path selection.
+
 FAQ
 ===
 
-- 
2.41.0