diff options
Diffstat (limited to 'Documentation/networking/bridge.rst')
| -rw-r--r-- | Documentation/networking/bridge.rst | 101 |
1 files changed, 101 insertions, 0 deletions
diff --git a/Documentation/networking/bridge.rst b/Documentation/networking/bridge.rst index a717563eaa15..c14410008ddf 100644 --- a/Documentation/networking/bridge.rst +++ b/Documentation/networking/bridge.rst @@ -47,6 +47,107 @@ Bridge sysfs The sysfs interface is deprecated and should not be extended if new options are added. +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/>`_. + +The 802.1D-2004 removed the original Spanning Tree Protocol, instead +incorporating the Rapid Spanning Tree Protocol (RSTP). By 2014, all the +functionality defined by IEEE 802.1D has been incorporated into either +IEEE 802.1Q (Bridges and Bridged Networks) or IEEE 802.1AC (MAC Service +Definition). 802.1D has been officially withdrawn in 2022. + +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 === |