U-Boot has networking support for VLANs (802.1q), and CDP (Cisco
  Discovery Protocol).
  
  You control the sending/receiving of VLAN tagged packets with the
  "vlan" environmental variable. When not present no tagging is
  performed.
  
  CDP is used mainly to discover your device VLAN(s) when connected to
  a Cisco switch.
  
  Note: In order to enable CDP support a small change is needed in the
  networking driver. You have to enable reception of the
  01:00:0c:cc:cc:cc MAC address which is a multicast address.
  
  Various defines control CDP; see the README section.