Private IPs cannot be configured on interconnects. Interconnect IPs must be globally routable and reachable. Secondary IPs aren’t supported on an interconnect. We only support 1 IPv4 and 1 IPv6 on a single interconnect.
The following subnet sizes for interconnects are supported:
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/31 or larger for IPv4
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/127 or larger for IPv6
It’s up to the ISP to decide which IP from /31 (/127) goes on which end of the interconnect. Google doesn't have any rules or preferences.