MPLS TE FA requires bidirectional LSPs but I havent been able to get sufficient reasoning as to why it needs bidir LSPs
(I read that CSPF does some bidir checks before installing it but I need some more insight into this)
Anyone has more detailed knowledge on why FA requires bidir LSPs ?
You should consider two scenarios:
(A) The tail-end LSR advertises point-to-point adjacency to head-end LSR without having an actual transport path. This is plain stupid and it's not hard to build a scenario resulting in a routing loop.
(B) The head-end LSR adverises point-to-point link to the tail-end LSR in its LSA, the tail-end LSR doesn't advertise the adjacency, but other routers still consider the unidirectional adjacency in their SPF algorithm. In OSPF case, this is a clear violation of step 2.b of algorithm described in Section 16.1 of RFC 2328 (page 163 of the RFC).
There are very good reasons why the bidirectional check is in mandated in the SPF algorithm and even if a vendor X might decide it's time to break the RFC rules (and maybe even call that "innovation") the third-party routers (from vendors still believing in following the RFCs) in a mixed-vendor network would ignore such "adjacencies".
Fixed an obvious stupidity: I write about "LSRs" not "LSPs"