We do not support downgrading from 4.3.0+ to pre-4.3.0 due to architectural changes in the images. While the system allows you to do this and its entirely possible it will have no visible effect sometimes it does, and other times an issue may lurk in the background and appear later on; so really its best to just not do this.
There are two ways to downgrade in this case:
1. We can turboboot to 4.2.1
2. After the 'install activate' (assuming you were upgrading 4.2.1 to 5.1.2) and router reload wait until all services are confirmed and then issue 'install commit', if a rollback is needed simply reboot and the router will go back to the old image as the 5.1.2 image is not committed yet.
As for the bootloop can you paste the console logs?
okay, actually this is the requirement of our customer. but, its that a different architecture? ive read the minimum release on my LC[ASR9K-MOD80-TR and RSP-440-TR] are supported by those release. but, i dont see the LC goes UP.
Hopefully this will help to explain it a bit more: In 4.2.0 we introduced the PX image for the RSP440 (to support x86) and still had the old P image for the PPC based RSP2 (RSP4G and RSP8G). In 4.3.0 we merged P and PX into the PX image (confusing I know). So downgrading to an old pre-4.3.0 PX or P image is a different architecture, but upgrading is fine as it has the old architectures plus more.
I don't see the reload reason in here, just the CBC PRE RESET notification. Please open a TAC case to investigate the logs further and see why the card is not booting.