Value,Name,Reference
0,"Reserved for advertising the metric associated    
with the default topology (see Section 4.2)",[RFC4915]
1,"Reserved for advertising the metric associated    
with the default multicast topology",[RFC4915]
2,Reserved for IPv4 in-band management purposes,[RFC4915]
3-31,Unassigned,[RFC4915]
32-127,"Reserved for development, experimental and        
proprietary features",[RFC4915][RFC3692]
128-255,Invalid and SHOULD be ignored,[RFC4915]