An identifier used to distinguish between multiple
tunnels of the same encapsulation method, with the
same endpoints. If the encapsulation protocol only
allows one tunnel per set of endpoint addresses (such
as for GRE or IP-in-IP), the value of this object is
1. For encapsulation methods (such as L2F) which
allow multiple parallel tunnels, the manager is
responsible for choosing any ID which does not
conflict with an existing row, such as choosing a
random number.