When the hold-down timer is active, all traffic coming
from this mesh sdp will be blocked. This timer will be
activated for any of the following cases:
1. when a mesh SDP becomes operational;
2. when a 'disputed' BPDU is received over this mesh sdp;
This is typically a symptom of one way communication
(the peer at the other side of the mesh sdp does not
receive our BPDUs).
3. when a MSTP BPDU from outside the region is received
over this mesh SDP.