Transiting a 802.1q or 802.1ad (q-in-q) vlans for a third party. What sort of L2 protections to use?

Hello,

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If I were to transit VLANs (be it 802.1Q or Q-in-Q) for a third party through my L2 links/switches, what sort of L2 protection features to use?

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Assuming that I do not know what is going to be used/how many devices visible on either end will there be by the third party.

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What are the best practices in such situation from preventing potential bad things to happen?

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Thanks.