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33 .Nd "IEEE 802.1Q VLAN network interface"
35 To compile this driver into the kernel,
36 place the following lines in your
37 kernel configuration file:
38 .Bd -ragged -offset indent
42 Alternatively, to load the driver as a
43 module at boot time, place the following line in
45 .Bd -literal -offset indent
51 driver demultiplexes frames tagged according to
52 the IEEE 802.1Q standard into logical
54 network interfaces, which allows routing/bridging between
55 multiple VLANs through a single switch trunk port.
59 interface is created at runtime using interface cloning.
61 most easily done with the
71 interface must be assigned a parent interface and
72 numeric VLAN tag using
74 A single parent can be assigned to multiple
76 interfaces provided they have different tags.
77 The parent interface is likely to be an Ethernet card connected
78 to a properly configured switch port.
79 The VLAN tag should match one of those set up in the switched
84 assumes the same minimum length for tagged and untagged frames.
85 This mode is selected by the
88 .Va net.link.vlan.soft_pad
90 However, there are network devices that fail to adjust frame length,
91 should it fall below the allowed minimum due to untagging.
92 Such devices should be able to interoperate with
94 after changing the value of
95 .Va net.link.vlan.soft_pad
99 will pad short frames before tagging them
100 so that their length stays not less than the minimum value
101 after untagging by the non-compliant devices.
105 driver supports efficient operation over parent interfaces that can provide
106 help in processing VLANs.
107 Such interfaces are automatically recognized by their capabilities.
108 Depending on the level of sophistication found in a physical
109 interface, it may do full VLAN processing or just be able to
110 receive and transmit long frames (up to 1522 bytes including an Ethernet
112 The capabilities may be user-controlled by the respective parameters to
117 However, a physical interface is not obliged to react to them:
118 It may have either capability enabled permanently without
119 a way to turn it off.
120 The whole issue is very specific to a particular device and its driver.
122 By now, the list of physical interfaces able of full VLAN processing
123 in the hardware is limited to the following devices:
144 The rest of the Ethernet interfaces can run
145 VLANs using software emulation in the
148 However, some of them lack the capability
149 of transmitting and receiving long frames.
150 Assigning such an interface as the parent to
152 will result in a reduced MTU on the corresponding
155 In the modern Internet, this is likely to cause
157 connectivity problems due to massive, inadequate
159 filtering that breaks the Path MTU Discovery mechanism.
161 The following interfaces support long frames for
187 driver automatically recognizes devices that natively support long frames
190 use and calculates the appropriate frame MTU based on the
191 capabilities of the parent interface.
192 Some other interfaces not listed above may handle long frames,
193 but they do not advertise this ability of theirs.
196 can be corrected manually if used in conjunction with such a parent interface.
201 No 802.1Q features except VLAN tagging are implemented.