Indicates, per address family, for which protocols this circuit wants Bidirectional Forwarding Detection (BFD) sessions established to neighbors: clearing a bit for an address type means no BFD session is attempted for it, setting it causes a BFD session to be established to each peer that also supports BFD for that type, with resulting status reported in isisISAdjProtSuppBfdStatus; unlike many other circuit objects, this one can be modified at any time, not just while disabled. An admin sets the ipv4 (and/or ipv6) bit here to get fast sub-second failure detection on a circuit where IS-IS's own hello timers alone would be too slow to notice a link failure.