Why does ifconfig for bonded devices show different MAC address than the physical devices?

Solution Verified - Updated

Environment

  • Red Hat Enterprise Linux 5
  • Red Hat Enterprise Linux 6
  • Bonding

Issue

  • Why do we not have same mac address for interface eth8 in the output of ifconfig and /proc/net/bonding/bond0?
[root@example ~]# cat /proc/net/bonding/bond0
Ethernet Channel Bonding Driver: v3.4.0-1 (October 7, 2008)

Bonding Mode: load balancing (round-robin)
MII Status: up
MII Polling Interval (ms): 0
Up Delay (ms): 0
Down Delay (ms): 0

Slave Interface: eth0
MII Status: up
Speed: 1000 Mbps
Duplex: full
Link Failure Count: 0
Permanent HW addr: 60:eb:69:82:2a:c8

Slave Interface: eth8
MII Status: up
Speed: 1000 Mbps
Duplex: full
Link Failure Count: 0
Permanent HW addr: 00:19:99:96:cd:69

[root@example ~]# ifconfig eth0
eth0      Link encap:Ethernet  HWaddr 60:EB:69:82:2A:C8  
          UP BROADCAST RUNNING SLAVE MULTICAST  MTU:1500  Metric:1
          RX packets:618065993 errors:0 dropped:0 overruns:1 frame:0
          TX packets:569953145 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:88552706929 (82.4 GiB)  TX bytes:517841087824 (482.2 GiB)
          Memory:92120000-92140000 

[root@example ~]# ifconfig eth8
eth8      Link encap:Ethernet  HWaddr 60:EB:69:82:2A:C8  
          UP BROADCAST RUNNING SLAVE MULTICAST  MTU:1500  Metric:1
          RX packets:592106164 errors:0 dropped:0 overruns:1 frame:0
          TX packets:569423011 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:86620296239 (80.6 GiB)  TX bytes:517075103956 (481.5 GiB)
          Memory:91a80000-91b00000
  • How to check MAC address of network interfaces when configured with bonding?

Resolution

  • If we use interfaces eth0 and eth8 in the bonding configuration of bond0, then bond0 is called the master (MASTER) while eth0 and eth8 are called slaves (SLAVE).

  • In bonding configuration the MASTER and SLAVE interfaces will both assume the MAC address of one of the slave interfaces if the MAC address is not explicitly configured (with ifconfig or the ip link command). By default the MAC address of a bonding device is taken from its first slave device (e.g. eth0), and other slave interfaces assume the same MAC address.

  • The reason for using a single MAC address for a bonding interface as well as the slave interfaces is to avoid packet drops during fail over between slaves.

  • It is expected behavior to see duplicate MAC address in case bonding is configured. In the following example ifconfig output, all slaves of bond0 have the same MAC address (HWaddr) as slave interface eth0 (60:EB:69:82:2A:C8). 

[root@example ~]# ifconfig eth0
eth0      Link encap:Ethernet  HWaddr 60:EB:69:82:2A:C8                 <===
          UP BROADCAST RUNNING SLAVE MULTICAST  MTU:1500  Metric:1
          RX packets:618065993 errors:0 dropped:0 overruns:1 frame:0
          TX packets:569953145 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:88552706929 (82.4 GiB)  TX bytes:517841087824 (482.2 GiB)
          Memory:92120000-92140000 


[root@example ~]# ifconfig eth8
eth8      Link encap:Ethernet  HWaddr 60:EB:69:82:2A:C8                  <===  
          UP BROADCAST RUNNING SLAVE MULTICAST  MTU:1500  Metric:1
          RX packets:592106164 errors:0 dropped:0 overruns:1 frame:0
          TX packets:569423011 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:86620296239 (80.6 GiB)  TX bytes:517075103956 (481.5 GiB)
          Memory:91a80000-91b00000
  • The actual hardware MAC addresses can be found in /proc/net/bonding/bond0. 
[root@example ~]# cat /proc/net/bonding/bond0
Ethernet Channel Bonding Driver: v3.4.0-1 (October 7, 2008)

Bonding Mode: load balancing (round-robin)
MII Status: up
MII Polling Interval (ms): 0
Up Delay (ms): 0
Down Delay (ms): 0

Slave Interface: eth0
MII Status: up
Speed: 1000 Mbps
Duplex: full
Link Failure Count: 0
Permanent HW addr: 60:eb:69:82:2a:c8                                    <=== Permanent MAC address of interface eth0

Slave Interface: eth8
MII Status: up
Speed: 1000 Mbps
Duplex: full
Link Failure Count: 0
Permanent HW addr: 00:19:99:96:cd:69                                    <=== Permanent MAC address of interface eth8
  • For detailed information refer to the kernel documentation:
# less /usr/share/doc/kernel-doc-*/Documentation/networking/bonding.txt
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