An older matrix shows that the 4800 models can't be a BCB in an SPBM environment. Is that the case for the ERS4900? From what I can tell, it does work as a BCB.
No, the ERS4800 never had NNI-NNI forwarding support. So it always activates ISIS Overload as well, which basically prevent the SPB node from even being considered as transit on paths to other destinations.
