Hi,
Each DFS Namespace has a referral cache, by default the cache is set to 300 seconds (5 minutes), so if a DFS Namespace member server goes offline it may take between 0 - 300 seconds (depending on how much cache time was left when the DFS Namespace member server went offline) before the clients get the new DFS configuration and can connect to the next available DFS Namespace member server.
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Best regards,
Leon