[c-nsp] MPLS interface continuity and OSPF configuration ME-3600X

[Eric Louie]

I had a strange anomaly happen yesterday

We have 2 ME-3600's as part of an MPLS network.  There are MPLS interfaces
on both sides of them and between them.  so, like this

R1 ---- R2 ---- R3 ---- R4

and all the links between are MPLS IP

My NOC enabled a new circuit between R2 and R3, and "forgot" to use mpls ip
on both interfaces.  When they decreased the OSPF cost on the two
interfaces (as the preferred route), traffic from R1 to R4 stopped at R2.
 (I didn't get the corresponding result R4 to R1, but our monitoring server
is behind R4, and could not reach any devices connected to R1)  The old
MPLS circuit was still connected and enabled, just costed out via OSPF.

So, it "appears" that when the new link was costed in, because it was not
an MPLS link, the traffic didn't know how to get through the new R2-R3
link, since the MPLS link was now costed out by MPLS.

making the new link an MPLS link solved the problem.

My questions are:
Is this "by design"?  In other words, if we have MPLS links on both sides
of a pair of routers, does that MPLS configuration need to be contiguous
throughout?

Is this a condition of configuring MPLS, that all intermediate paths need
to be either tunnelled or configured?  (something that I might have missed
in my MPLS learning)

Did we run into a bug?  IOS 15.2(4)S

Is this a TAC question?