When using `--external-beacon-api-url`, one has to accompany it with
either `--trusted-block-root` or `--trusted-state-root`. If neither is
specified, we can fallback to a deeply finalized noncontroversial block
root. For networks that started post Altair, e.g., Holesky, the genesis
block root fulfills that requirement, as in, it is implicitly trusted.
Therefore, if only `--external-beacon-api-url` is provided without any
`--trusted-block-root` or `--trusted-state-root`, use genesis block root
if it is a viable starting point (post-Altair).
```
build/nimbus_beacon_node \
--network=holesky \
--data-dir="$HOME/Downloads/nimbus/data/holesky" \
"--external-beacon-api-url=http://unstable.holesky.beacon-api.nimbus.team" \
--tcp-port=9010 --udp-port=9010 \
--rest --log-level=DEBUG \
--no-el
```
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