Any optimization of the MoFRR primary LSP should be performed by the Make Before Break (MBB) mechanism. For example, if the primary LSP fails, a switch to the backup LSP occurs and the primary LSP is signaled. After the primary LSP is successfully re-established, MoFRR switches from the backup LSP to the primary LSP.
MBB is performed from the leaf node to the root node, and therefore it is not performed per autonomous system (AS); the MBB signaling must be successful from the leaf PE to the root PE, including all ASBRs and P routers in between.
The conditions of MBB for mLDP LSPs are:
re-calculation of the SFP
failure of the primary ASBR
If the primary ASBR fails and a switch is made to the backup ASBR, and the backup ASBR is the only other ASBR available, the MBB mechanism does not signal any new LSP and uses this backup LSP as the primary.