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) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Type 83 for Tunnel-Preference Length Always 6. Tag 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). Value 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.