HI, Nilay
Thank you for your question.
APIC SSL certificate is used for communications within the fabric.
When the certificate is expired, connections will be insecure, but won't lose access to APIC GUI and there won't be any impact. Self-signed certificate (SSC) gets renewed automatically once it has expired, however, it will take some period to get updated automatically. If the expiration was for more than 2 weeks, further investigation is needed.
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Best Regards
Lucy
