After a subordinate switch requests a domain ID form a principal switch, the requested domain ID becomes the runtime domain ID, and all interfaces on the affected VSAN are isolated. What is a solution to the issue?

When a subordinate switch joins a fabric, the principal switch assigns it a domain ID. If this dynamically assigned domain ID is not present in the subordinate switch's configured "allowed domain ID list" for that specific VSAN, the switch intentionally isolates the VSAN to prevent potential fabric instability. This results in all interfaces on that VSAN entering an isolated state. The direct solution is to modify the configuration on the subordinate switch to permit the assigned ID by adding it to the allowed list using the fcdomain allowed [domain-id] vsan [vsan-id] command. This allows the switch to accept the ID and join the fabric correctly.

B. Configuring a static domain ID is a different assignment method; it does not resolve the root cause, which is the allowed list filtering the dynamically assigned ID.
C. FICON is a specific protocol for mainframe connectivity and is irrelevant to resolving a standard Fibre Channel domain ID assignment and isolation issue.
D. IVR NAT is a feature used for routing traffic between different VSANs and does not address domain ID assignment problems within a single VSAN.

1. Cisco MDS 9000 Series Fabric Configuration Guide, Release 8.x, Chapter: "Configuring Fibre Channel Domains", Section: "Configuring the Allowed Domain ID List".

The document states, "By default, the allowed list includes all domain IDs. You can configure the allowed list of domain IDs for a VSAN to restrict the assignment of domain IDs to a switch. If the assigned domain ID is not in the allowed list, the VSAN on that switch is suspended." This directly supports that an unallowed ID causes isolation and that adding it to the list is the solution.

2. Cisco MDS 9000 Series System Messages Guide, Chapter: "FCSP Messages".

The explanation for the FCSP-3-VSANISOLATED system message often includes the reason "Domain id %d is not in the allowed list." The recommended action is to "Check the allowed domain ID list on the switch and add the assigned domain ID to the list if required." This confirms the problem and the corrective action.

3. Cisco Nexus 9000 Series NX-OS SAN Switching Configuration Guide, Release 10.4(x), Chapter: "Configuring Fibre Channel Domains", Section: "Allowed Domain ID List".

This guide provides a similar explanation: "If the assigned domain ID is not in the allowed list, the VSAN on that switch is isolated." This demonstrates the consistency of this behavior across Cisco SAN platforms.