I have Thinkpad T430 with NVIDIA NVS 5400M and Intel HD Graphics 4000.
I installed the bumblebee and nvidia-current, but Ubuntu still doesn't recognise external monitor connected DisplayPort
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It may be that the digital output (the displayport) is driven directly by the nvidia card. This is the case with my W520 (see the blog post here). Bumblebee is designed primarily to render graphics on the nvidia card and output them on the laptop display through the intel card, not to drive external monitors with the nvidia card. Probably the most simple work around is described here (not sure if it will still work on the T430 and the intel driver included with the latest version of ubuntu though).
You may be able to use the VGA port (that was the case with the 420), or you may be able to switch to using the nvidia card for both the external and thinkpad displays in the BIOS menu. This will impact battery life though.