bitcoind syncing backwards

Question

I recently upgraded an old bitcoind install (core 0.11) to 0.16.2. After starting it up, it seems that it's syncing backwards: going from higher block numbers to lower ones:

2018-09-12 06:35:20 UpdateTip: new best=00000000000000000085ff97d49dff0d3749732f88b4cf2ff32a8f7b74697e76 height=486700 version=0x20000000 log2_work=87.159802 tx=256643645 date='2017-09-23 23:59:03' progress=0.703573 cache=325.2MiB(2108184txo)
2018-09-12 06:35:20 UpdateTip: new best=000000000000000000ebdd4cb84ef4c6f2d00bdc029838beafdd860da347ae16 height=486699 version=0x20000000 log2_work=87.159763 tx=256642866 date='2017-09-23 23:52:21' progress=0.703571 cache=325.5MiB(2110896txo)
2018-09-12 06:35:21 UpdateTip: new best=000000000000000000ba68cee791bfd516a4cb44cee245e607c1f43cb5259277 height=486698 version=0x20000000 log2_work=87.159723 tx=256642079 date='2017-09-23 23:45:27' progress=0.703569 cache=325.9MiB(2114087txo)
2018-09-12 06:35:21 UpdateTip: new best=000000000000000000533029bfa46ff56c4e3309f346218ed5c58d1fe48ed38b height=486697 version=0x20000000 log2_work=87.159684 tx=256640873 date='2017-09-23 23:35:16' progress=0.703565 cache=326.2MiB(2116306txo)
2018-09-12 06:35:21 UpdateTip: new best=0000000000000000002b8aea6df40767e1f429ce3b822034c32bc15f88400a2d height=486696 version=0x20000000 log2_work=87.159644 tx=256638869

This behavior doesn't make much sense to me. I haven't found anything online regarding this. Why is it doing this?

Answer 1

This will happen because you are upgrading from a non-segwit node to a segwit node. This behavior happens because Bitcoin Core will rewind the blockchain to the point where segwit was activated so that it can download the missing segwit data and verify that the blocks are valid.