.. SPDX-License-Identifier: GPL-2.0 ================= Ethernet Bridging ================= Introduction ============ The IEEE 802.1Q-2022 (Bridges and Bridged Networks) standard defines the operation of bridges in computer networks. A bridge, in the context of this standard, is a device that connects two or more network segments and operates at the data link layer (Layer 2) of the OSI (Open Systems Interconnection) model. The purpose of a bridge is to filter and forward frames between different segments based on the destination MAC (Media Access Control) address. Bridge kAPI =========== Here are some core structures of bridge code. Note that the kAPI is *unstable*, and can be changed at any time. .. kernel-doc:: net/bridge/br_private.h :identifiers: net_bridge_vlan Bridge uAPI =========== Modern Linux bridge uAPI is accessed via Netlink interface. You can find below files where the bridge and bridge port netlink attributes are defined. Bridge netlink attributes ------------------------- .. kernel-doc:: include/uapi/linux/if_link.h :doc: Bridge enum definition Bridge port netlink attributes ------------------------------ .. kernel-doc:: include/uapi/linux/if_link.h :doc: Bridge port enum definition 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) `_. 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either IEEE 802.1Q (Bridges and Bridged Networks) or IEEE 802.1AC (MAC Service Definition). 802.1D has been officially withdrawn in 2022.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 `` is called by the kernel when STP is enabled/disabled on a bridge (via ``brctl stp `` or ``ip link set 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 === What does a bridge do? ---------------------- A bridge transparently forwards traffic between multiple network interfaces. In plain English this means that a bridge connects two or more physical Ethernet networks, to form one larger (logical) Ethernet network. Is it L3 protocol independent? ------------------------------ Yes. The bridge sees all frames, but it *uses* only L2 headers/information. As such, the bridging functionality is protocol independent, and there should be no trouble forwarding IPX, NetBEUI, IP, IPv6, etc. Contact Info ============ The code is currently maintained by Roopa Prabhu and Nikolay Aleksandrov . Bridge bugs and enhancements are discussed on the linux-netdev mailing list netdev@vger.kernel.org and bridge@lists.linux-foundation.org. The list is open to anyone interested: http://vger.kernel.org/vger-lists.html#netdev External Links ============== The old Documentation for Linux bridging is on: https://wiki.linuxfoundation.org/networking/bridge