How do I switch a pruned node to "neutrino mode"?

Question

According to Lightning Lab's documentation, Bitcoin Core can be run in "neutrino mode" to serve block data without storing transactions.

I've been running Bitcoin Core in pruning mode and everything is okay. Now I would like to enable this mode to serve data for lnd. The documentation says that adding these parameters will set up Bitcoin Core to run in "neutrino mode":

blockfilterindex=1
peerblockfilters=1

When I add them, Bitcoin Core gives this error:

Error: basic block filter index best block of the index goes beyond pruned data. Please disable the index or reindex (which will download the whole blockchain again)

Of course, it is necessary to re-download the blockchain. So my questions are:

• Will this configuration cause Bitcoin Core to store the entire blockchain, or will it just keep the headers and continue pruning the blocks?
• How can a reindex be triggered?

Answer 1

Are you running a sufficiently recent version? If you are running an older version, you would need to have the full blockchain archive to serve compact client-side block filters.
Originally blockfilterindex and pruning were exclusive features. This was only changed recently when PR#15946 allowed serving compact client-side block filters from pruned nodes. The feature was included in the Bitcoin Core v22.0 release.

You should be able to trigger the reindex by stopping your Bitcoin Core, and then starting Bitcoin Core with the -reindex startup parameter once.

Answer 2

Bitcoin Core doesn't run in Neutrino mode, it can only serve compact filters. Neutrino is a Lightning Lab's implementation of BIP157 and BIP158 basic filter type. It is a standalone backend, and you can run lnd without Bitcoin Core.
Moreover, the only way to compute the missing filters is to download all blocks again, compute the filters and then prune again.