Value,Meaning,Reference,Notes
0,Any IGP Protocol,[RFC8287],
1,OSPF,[RFC8287],To be used for OSPFv2 only.
2,ISIS,[RFC8287],
3,OSPFv3,[RFC9214],
4-250,Unassigned,,
251-254,Reserved for Experimental Use,[RFC8287],
255,Reserved,[RFC8287],