This value sets, in microseconds, the minimum interval between received BFD Echo packets the local system can support, and per the description it must be zero for a multihop BFD session. Admins tune it when using BFD echo mode on a directly connected session to control how frequently echo packets are expected back. Setting this to 0 on a session tells the admin's configuration this must be a multihop BFD session, since echo mode requires a nonzero value only on single-hop sessions.