uboot/doc/README.VLAN
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   1U-Boot has networking support for VLANs (802.1q), and CDP (Cisco
   2Discovery Protocol).
   3
   4You control the sending/receiving of VLAN tagged packets with the
   5"vlan" environmental variable. When not present no tagging is
   6performed.
   7
   8CDP is used mainly to discover your device VLAN(s) when connected to
   9a Cisco switch.
  10
  11Note: In order to enable CDP support a small change is needed in the
  12networking driver. You have to enable reception of the
  1301:00:0c:cc:cc:cc MAC address which is a multicast address.
  14
  15Various defines control CDP; see the README section.
  16