RADIUS attribute from rfc2868


If more than one set of tunneling attributes is returned by the
      RADIUS server to the tunnel initiator, this Attribute SHOULD be
      included in each set to indicate the relative preference assigned
      to each tunnel.  For example, suppose that Attributes describing
      two tunnels are returned by the server, one with a Tunnel-Type of
      PPTP and the other with a Tunnel-Type of L2TP.  If the tunnel
      initiator supports only one of the Tunnel-Types returned, it will
      initiate a tunnel of that type.  If, however, it supports both
      tunnel protocols, it SHOULD use the value of the Tunnel-Preference
      Attribute to decide which tunnel should be started.  The tunnel
      having the numerically lowest value in the Value field of this
      Attribute SHOULD be given the highest preference.  The values
      assigned to two or more instances of the Tunnel-PreferenceAttribute within a given Access-Accept packet MAY be identical.
      In this case, the tunnel initiator SHOULD use locally configured
      metrics to decide which set of attributes to use.  This Attribute
      MAY be included (as a hint to the server) in Access-Request
      packets, but the RADIUS server is not required to honor this hint.

   A summary of the Tunnel-Preference Attribute format is shown below.
   The fields are transmitted from left to right.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |     Type      |    Length     |     Tag       |     Value
              Value (cont)         |

      83 for Tunnel-Preference

      Always 6.

      The Tag field is one octet in length and is intended to provide a
      means of grouping attributes in the same packet which refer to the
      same tunnel.  Valid values for this field are 0x01 through 0x1F,
      inclusive.  If the Tag field is unused, it MUST be zero (0x00).

      The Value field is three octets in length and indicates the
      preference to be given to the tunnel to which it refers; higher
      preference is given to lower values, with 0x000000 being most
      preferred and 0xFFFFFF least preferred.