How is Mac Flooding carried out if attacker is outside of the network?

Recently I'm studying Comptia Security+ related subjects and when it came to Mac Flooding I couldn't find an answer corresponding it. I know that layer 2 switches forward data based on the Mac addressess and since Mac Flooding requires an attacker to tamper with switch's address book, is it an insider attack? if it's not limited to the network how do they get access to the switch? I mean what information should they gather? sorry for my noobie question and …

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