10 Linux brctl Command Examples for Ethernet Network Bridge

brctl stands for Bridge Control.

In Linux, this command is used to create and manipulate ethernet bridge.

This is typically used when you have multiple ethernet networks on your servers, and you want to combine them and present it as one logical network.

For example, if you have eth0 and eth1, you can combine them and present it as just br0, which will inturn use both eth0 and eth1 for network traffic.

In this tutorial, we’ve covered the following:

1. Create New Ethernet Bridge using addbr
2. Display Available Ethernet Bridge using show
3. Delete Existing Ethernet Bridge using delbr
4. Add an Interface to Existing Bridge
5. Add Multiple Interfaces to Existing Bridge
6. Tracking MAC address of a Bridge
7. Set Ageing Time for Mac Address on a Bridge
8. Setup Spanning Tree on Ethernet Bridge
9. Display STP Parameter Values of a Bridge
10. Change Bridge Parameters Values

1. Create New Ethernet Bridge using addbr

Using brctl addbr, we can create a new ethernet bridge.

In the following example, we have created three ethernet bridges on this server: dev, stage and prod.

brctl addbr dev

brctl addbr stage

brctl addbr prod

Please note that at this stage, this is just an empty ethernet bridge which doesn’t have any other ethernet network on it.

Please note that this brctl is bit different than the Linux EtherChannel NIC bonding that we discussed earlier.

2. Display Available Ethernet Bridge using show

Using brctl show, we can view all available ethernet bridges on your server.

The following example will display all the current instances of the three bridges that we just created.

# brctl show
bridge name     bridge id               STP enabled     interfaces
dev             8000.000000000000       no
prod            8000.000000000000       no
stage           8000.000000000000       no

If you notices the last column “interfaces” has nothing on it. This means that none of this bridge currently has any ethernet device associated with it yet.

On a related note: 9 Linux ethtool Examples to Manipulate Ethernet Card

3. Delete Existing Ethernet Bridge using delbr

Using brctl delbr, we can delete an existing ethernet bridge.

The following example will delete the “stage” instance of the ethernet bridge.

brctl delbr stage

As you see from the show command, we don’t see the “stage” ethernet bridge anymore.

# brctl show
bridge name     bridge id               STP enabled     interfaces
dev             8000.000000000000       no
prod            8000.000000000000       no

Please note that when a bridge is active (i.e up), you can remove it. You need to first bring down the bridge and then delete it.

Also, please note that earlier brctl command was called as brcfg.

4. Add an Interface to Existing Bridge

Using brctl addif, we can add an interface to an existing ethernet bridge.

The following example will add the ethernet network eth0 to the bridge “dev” that we just created.

Warning: Don’t try this on any critical machines, as you might loose network connection to that machine when something goes wrong. Try this only on a test instance where you have console access.

brctl addif dev eth0

In the above:

• This will really make the eth0 as a port of of the bridge “dev”.
• So, all the frames that arrives on eth0 will be processed as if it is really arriving at the bridge.
• Also, when the frames are sent-out on “dev” bridge, it will use eth0. When multiple interfaces are part of “dev” bridge, then eth0 will be a potential candidate to sent-out the outgoing frames from the bridge.

After adding the eth0 to “dev” bridge, the brctl show command will display the following.

# brctl show
bridge name     bridge id               STP enabled     interfaces
dev             8000.0050568954bc       no              eth0
prod            8000.000000000000       no

As you see from the above output, the last column “interfaces” how has eth0 for “dev” bridge.

Note: If something goes wrong while adding eth0 to the “dev” bridge, execute the following command from the console to delete the dev bridge.

# brctl delbr dev

Also, note that if you try to add the loopback interface to the bridge, you’ll get the following invalid argument command.# brctl addif dev lo
can’t add lo to bridge dev: Invalid argument

As you can imagine, you can’t add an interface that doesn’t exist on the system to the bridge.

# brctl addif dev eth2
interface eth2 does not exist!

5. Add Multiple Interfaces to Existing Bridge

In the above example, we added only one ethernet device (eth0) to the bridge.

But, the whole idea of using a bridge is to add more interfaces to the bridge.

In the following example, we are adding both eth0 and eth1 to the bridge “dev”.

# brctl addif dev eth0 eth1

As you see from the following output, for the dev bridge we see two lines. In the last column “interfaces”, we see both eth0 and eth1 for “dev” ethernet bridge.

# brctl show
bridge name     bridge id               STP enabled     interfaces
dev             8000.0050568954bc       no              eth0
                                                        eth1
prod            8000.000000000000       no

Note: If a particular ethernet interface is already part of a bridge, then you can’t add it to another bridge.

Basically, one network can be part of only one bridge. If not, we’ll get the following error message when trying to add it to another bridge.

# brctl addif prod eth1
device eth1 is already a member of a bridge; can't enslave it to bridge prod

6. Tracking MAC address of a Bridge

Using brctl showmacs, we can see all the learned MAC addresses of a bridge.

In the following example, as of now, the following are the two mac addresses that connected to the ethernet bridge “prod”. This data will keep changing depending on the current status of what is connected to the bridge.

# brctl showmacs prod
port no mac addr                is local?       ageing timer
1       00:50:56:89:54:bc       yes                0.00
2       00:60:50:46:bc:40       no                 0.00

7. Set Ageing Time for Mac Address on a Bridge

In the following example, we are setting the mac address ageing time to 120 seconds on “dev” ethernet bridge.

brctl setaging dev 120

In the above:

• dev is the name of the bridge where we are setting this value
• 120 is seconds
• So, if a frame is not seen for “dev” bridge in 120 seconds, then “dev” bridge will delete the mac address from the forwarding database.

8. Setup Spanning Tree on Ethernet Bridge

Using brctl stp, we can setup spanning tree on ethernet bridge.

But default, spanning tree is not enabled when we create a bridge.

Spanning tree is helpful when you have multiple bridges on your network, and they can all collaborate to find the shortest path between two ethernets.

In the following example, we are turning on the spanning tree on “dev” ethernet bridge.

We can use either on or yes to enable spanning tree. So, both of the following command will do exactly the same.

brctl stp dev on

brctl stp dev yes

As you see from the following show output, the values in the “STP enabled” column for “dev” bridge is now “yes”.

# brctl show
bridge name     bridge id               STP enabled     interfaces
dev             8000.000000000000       yes
prod            8000.000000000000       no

To turn off spanning tree on your ethernet bridge, do the following:

# brctl stp dev off

# brctl show
bridge name     bridge id               STP enabled     interfaces
dev             8000.000000000000       no
prod            8000.000000000000       no

9. Display STP Parameter Values of a Bridge

STP stands for Spanning Tree Protocol.

Once you’ve enabled stp on your bridge, you can use showstp as shown below to view all the stp parameter values of your bridge.

The following will display the stp parameter and its current value for “dev” bridge.

# brctl showstp dev
dev
 bridge id              000a.000000000000
 designated root        000a.000000000000
 root port                 0       path cost                  0
 max age                  19.99    bridge max age            19.99
 hello time                1.99    bridge hello time          1.99
 forward delay            14.99    bridge forward delay      14.99
 ageing time             299.95
 hello timer               0.00    tcn timer                  0.00
 topology change timer     0.00    gc timer                   0.00
 hash elasticity           4       hash max                 512
 mc last member count      2       mc init query count        2
 mc router                 1       mc snooping                1
 mc last member timer      0.99    mc membership timer      259.96
 mc querier timer        254.96    mc query interval        124.98
 mc response interval      9.99    mc init query interval    31.24
 flags

10. Change Bridge Parameters Values

The spanning tree parameter default values for a specific bridge can be changed. You can also change other parameter values of the bridge that you created.

All of the following set commands mentioned in the table below starts with brctl. For example:

brctl setageing dev 100

The following table displays available set commands for brctl. The time value is in seconds.

.