Why do I receive network packets with a VLAN tag addressed to a non-tagged network interface?

Issue

• Why do I receive network packets with a VLAN tag addressed to a non-tagged network interface on a RHEL 6.5 host configured with network interfaces for tagged and non-tagged VLAN networks?

• For example: a host with a non-tagged interface bond0 which also has a VLAN interface bond0.10 for VLAN ID 10:

+----------------+
|     host1      |
|                |
|  +----------+  |
|  | bond0.10 |  |
|  |1.2.10.100|  |
|  +----+-----+  |
|       |        |
| +-----+------+ |
| |   bond0    | |
| |  1.2.0.100 | |
| | non-tagged | |
| +------------+ |
+----------------+

• Packets addressed to bond0 interface (with IP address 1.2.0.100) are randomly tagged with VLAN ID 10:

  • Example of correct packet addressed to non-tagged interface bond0 with IP address 1.2.0.100 which does not have a 802.1q header:

  10:27:49.668580 08:d0:9f:4c:78:49 (oui Unknown) > d8:9d:67:61:d5:58 (oui Unknown), 
  ethertype IPv4 (0x0800), length 103: 1.2.0.200.35910 > 1.2.0.100.54321: Flags [P.], 
  seq 2475:2512, ack 77330, win 501, options [nop,nop,TS val 431286140 ecr 98426], length 37
  

  • Example of incorrect packet addressed to non-tagged interface bond0 with IP address 1.2.0.100 which does have incorrect 802.1q header with VLAN ID 10:

  10:27:49.668610 08:d0:9f:4c:78:49 (oui Unknown) > d8:9d:67:61:d5:58 (oui Unknown), 
  ethertype 802.1Q (0x8100), length 395: vlan 10, p 0, ethertype IPv4, 1.2.0.200.35910 > 1.2.0.100.54321: Flags [P.], 
  seq 2512:2837, ack 77330, win 501, options [nop,nop,TS val 431286140 ecr 98426], length 325