Thanks for your replay, Reco.
Yes, in Stretch networking works well both - with and/or without directive "auto".
In contrast Buster with:
auto bond0.20
iface bond0.20 inet manual
auto bond0.21
iface bond0.21 inet manual
gateway is unreachable by ping.
So I don't know why yet.
Ales
From: Reco <recoverym4n@enotuniq.net>
Date: Fri, 4 Jun 2021 16:51:05 +0300
>It's just a guess, but you have "auto" for "bond0":
>> auto bond0
>> iface bond0 inet manual
>But what you do not have is "auto" for VLAN interfaces you build on top of bond0:
>> iface bond0.20 inet manual
>> iface bond0.21 inet manual