In our corporate network moving to Big Sur took much troubleshooting and developing a matrix to resolve.
With Cisco AnyConnect installed with Sophos Endpoint protection essentially broke DNS on the machine, our domain was no longer reachable.
The system extensions on the same machine clashed causing the issue, after much troubleshooting removing either of the system extensions resolved the DNS issue.
https://www.cisco.com/c/en/us/td/docs/security/vpn_client/anyconnect/anyconnect49/upgrade/AnyConnect_macOS_BigSur_Advisory.html
https://community.sophos.com/intercept-x-endpoint/f/recommended-reads/116397/sophos-mac-endpoint-how-to-configure-jamf-privacy-preferences-for-10-15-compatibility
I logged support requests with all vendors, we decided to move away from Sophos Endpoint and use Jamf Protect with no system extensions / DNS issues.
jjack
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