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
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.