Master Linux Network Bridges: Concepts, Commands, and Configuration Guides

Bridge Concept

In everyday life a bridge connects two places; in networking a bridge is a Layer‑2 device that links multiple LAN segments using MAC addresses to forward or filter frames, effectively dividing broadcast domains.

On Linux, a bridge is a logical interface (e.g., br0) that aggregates two or more network interfaces (physical like eth0 or virtual like vnetX) so they behave as a single interface.

How a Bridge Works

Network interface role

udev creates and manages device nodes; interface names (e.g., eth0, ens33) are logical identifiers linked to kernel network structures.

Tools such as ifconfig or nmcli use these names to interact with the kernel networking stack.

When a physical NIC and a virtual NIC are added to a bridge, traffic between them is handled by the bridge device, not directly by the kernel stack. The bridge acts like a virtual switch, and all member interfaces share the same Layer‑2 subnet.

Bridge Data Flow and IP Assignment

Data flow : After adding eth0 to br0, the NIC no longer communicates directly with the IP stack; all Layer‑2 frames are processed by the bridge, which forwards them between member ports.

IP address : The bridge interface itself must be assigned an IP address because member interfaces lose their IP configuration once bridged. The bridge’s IP is used for communication with external networks.

Managing Bridges with Tools

1. Using brctl (bridge‑utils)

# Create bridge
sudo brctl addbr br0

# Add member interfaces
brctl addif br0 eth0
brctl addif br0 vnetX

# Bring bridge up
ifconfig br0 up   # or: ip link set br0 up

# Delete bridge
brctl delbr br0

# Enable STP
brctl stp br0 on

2. Using NetworkManager ( nmcli )

# Create bridge
nmcli con add type bridge con-name br0 ifname br0

# Add member interface
nmcli con add type bridge-slave con-name br0-eth1 ifname eth1 master br0

# Bring bridge up/down
nmcli con up br0
nmcli con down br0

# Delete bridge
nmcli con delete br0

# Enable STP
nmcli con modify br0 bridge.stp yes

3. Using ip (iproute2)

# Create bridge
ip link add name br0 type bridge

# Add member interface
ip link set dev eth1 master br0

# Bring bridge up/down
ip link set dev br0 up
ip link set dev br0 down

# Delete bridge
ip link delete dev br0 type bridge

# Enable STP
sudo ip link set br0 type bridge stp_state 1

Configuration Files

CentOS 7

Ensure the bridge module is loaded and persistent:

lsmod | grep bridge
modprobe bridge
# make persistent by adding to /etc/modules

Create /etc/sysconfig/network-scripts/ifcfg-br0:

DEVICE=br0
TYPE=Bridge
ONBOOT=yes
BOOTPROTO=static
IPADDR=10.0.0.20
NETMASK=255.255.255.0
GATEWAY=10.0.0.2
STP=on

Modify physical NIC files ( ifcfg-eth0, ifcfg-eth1) to include BRIDGE=br0, then restart NetworkManager:

systemctl restart NetworkManager

Ubuntu 16.04

Load the bridge module and install bridge-utils:

sudo modprobe bridge
sudo apt install bridge-utils

Edit /etc/network/interfaces:

auto lo
iface lo inet loopback

auto br0
iface br0 inet static
    address 192.168.14.108
    netmask 255.255.248.0
    gateway 192.168.12.1
    dns-nameserver 180.76.76.76
    bridge_ports ens33 ens37
    bridge_stp on

auto ens33
iface ens33 inet manual
    up ip link set $IFACE up
    down ip link set $IFACE down

auto ens37
iface ens37 inet manual
    up ip link set $IFACE up
    down ip link set $IFACE down

Apply changes:

sudo systemctl restart networking

Ubuntu 20.04 (netplan)

network:
  version: 2
  renderer: networkd
  ethernets:
    ens33:
      dhcp4: no
    ens34:
      dhcp4: no
  bridges:
    br0:
      interfaces: [ens33, ens34]
      dhcp4: yes
      parameters:
        stp: true
        forward-delay: 4

Apply the configuration: sudo netplan apply Verify bridge status:

bridge link show br0

Spanning Tree Protocol (STP)

STP prevents Layer‑2 loops by placing ports through three states: Blocking, Listening, and Learning, before reaching Forwarding. Enabling STP on a bridge is essential when member NICs connect to the same physical switch, otherwise broadcast storms can occur.

Summary

The article provides a comprehensive overview of Linux network bridges, covering their definition, internal operation, and the role of member interfaces. It details how bridges handle data flow and IP configuration, and presents practical management methods using brctl, nmcli, and ip commands, as well as persistent configuration via distribution‑specific files for CentOS 7, Ubuntu 16.04, and Ubuntu 20.04. Finally, it explains the importance of enabling STP to avoid network loops.