Em man interfaces :
IFACE OPTIONS
The following "command" options are available for every family and
method. Each of these options can be given multiple times in a single
stanza, in which case the commands are executed in the order in which
they appear in the stanza. (You can ensure a command never fails by
suffixing them with "|| true".)
pre-up command
Run command before bringing the interface up. If this command
fails then ifup aborts, refraining from marking the interface as
configured, prints an error message, and exits with status 0.
This behavior may change in the future.
up command
post-up command
Run command after bringing the interface up. If this command
fails then ifup aborts, refraining from marking the interface as
configured (even though it has really been configured), prints
an error message, and exits with status 0. This behavior may
change in the future.
...
There exists for each of the above mentioned options a directory
/etc/network/if-<option>.d/ the scripts in which are run (with no argu‐
ments) using run-parts(8) after the option itself has been processed.
Please note that as post-up and pre-down are aliases, no files in the
corresponding directories are processed. Please use if-up.d and if-
down.d directories instead.
Portanto, o up no início é usado para indicar que o comando deve ser executado depois que a interface estiver ativa.