How to change the currently active slave of a bonded interface

RedHat / CentOS :

Interface Bonding as we all know is very useful in providing the fault tolerance and increased bandwidth. We can change the active slave interface of bonding without interrupting the production work. In the example below we have the interface bonding bond0 with 2 slaves em0 and em1 (em1 being the active slave). We will be replacing slave em0 with new slave em2.

# cat /proc/net/bonding/bond0
Ethernet Channel Bonding Driver: v3.7.1 (April 27, 2011)

Bonding Mode: fault-tolerance (active-backup)
Primary Slave: None
Currently Active Slave: em0
MII Status: up
MII Polling Interval (ms): 100
Up Delay (ms): 5000
Down Delay (ms): 5000

Slave Interface: em0
MII Status: up
Speed: 1000 Mbps
Duplex: full
Link Failure Count: 0
Permanent HW addr: 00:21:28:b2:65:26
Slave queue ID: 0

Slave Interface: em1
MII Status: up
Speed: 1000 Mbps
Duplex: full
Link Failure Count: 0
Permanent HW addr: 00:21:28:b2:65:27
Slave queue ID: 0

1. Change the active slave to em1

ifenslave command can be used to attach or detach or change the currently active slave interface from the bonding. Now, Change the active slave interface to em1.

# ifenslave -c bond0 em1

Check the bonding status again to ensure that em1 is the new active slave :

# cat /proc/net/bonding/bond0
Ethernet Channel Bonding Driver: v3.7.1 (April 27, 2011)

Bonding Mode: fault-tolerance (active-backup)
Primary Slave: None
Currently Active Slave: em1
MII Status: up
MII Polling Interval (ms): 100
Up Delay (ms): 5000
Down Delay (ms): 5000

Slave Interface: em0
MII Status: up
Speed: 1000 Mbps
Duplex: full
Link Failure Count: 0
Permanent HW addr: 00:3b:26:b2:68:26
Slave queue ID: 0

Slave Interface: em1
MII Status: up
Speed: 1000 Mbps
Duplex: full
Link Failure Count: 0
Permanent HW addr: 00:3b:26:b2:68:27
Slave queue ID: 0
The switch of active slave should get effective immediately, but on critical production systems, please schedule maintenance window or make some test in an identical test environment first.

2. Attach the new slave interface

We can now attach the new slave interface em2 to the bonding.

# ifenslave bond0 em2

3. Unattach the old slave interface

Once we have attached a new slave interface, we can unattach the old slave and remove it from the bonding.

# ifenslave -d bond0 em0

4. Verify

Confirm that the new slave is now the standby interface in the bonding.

# cat /proc/net/bonding/bond0
Ethernet Channel Bonding Driver: v3.7.1 (April 27, 2011)

Bonding Mode: fault-tolerance (active-backup)
Primary Slave: None
Currently Active Slave: em1
MII Status: up
MII Polling Interval (ms): 100
Up Delay (ms): 5000
Down Delay (ms): 5000

Slave Interface: em1
MII Status: up
Speed: 1000 Mbps
Duplex: full
Link Failure Count: 0
Permanent HW addr: 00:21:29:bf:55:30
Slave queue ID: 0

Slave Interface: em2
MII Status: up
Speed: 1000 Mbps
Duplex: full
Link Failure Count: 0
Permanent HW addr: 00:19:1a:d1:43:61
Slave queue ID: 0

If you want to make the changes more permanent

The changes we just made, are temporary and will be cleared after a reboot of the server. To make these changes permanent we will have to make few changes.

Make sure you delete the file /etc/sysconfig/network-scripts/ifcfg-em0 as we are no longer are using this interface in bonding. Create a new file for the new slave interface in the bonding :

# rm /etc/sysconfig/network-scripts/ifcfg-em0
# vi /etc/sysconfig/network-scripts/ifcfg-em2
DEVICE=em2
BOOTPROTO=none
ONBOOT=yes
MASTER=bond0
SLAVE=yes

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