In preparation for the deployment, what statement is correct from the below?

The most reliable and least disruptive method for network migration is a phased approach. This involves migrating Layer 2 functionality first to establish and validate basic network connectivity and VLAN trunking between the existing and new infrastructure. Once Layer 2 is confirmed to be stable and operating as expected, the Layer 3 functions, such as the default gateways (e.g., Switched Virtual Interfaces - SVIs), are migrated. This sequential process isolates potential faults to a specific layer, significantly simplifying troubleshooting and reducing the overall risk and impact of the cutover.

A. Migrating L2 and L3 simultaneously is a high-risk "flash cut" approach that complicates troubleshooting by making it difficult to isolate the source of a problem.
B. A rollback plan is a mandatory component of any professional network deployment, as unforeseen issues can always arise, regardless of design quality.
C. Building a new network in parallel is a common best practice, as it allows for thorough testing and validation without impacting the live production environment.

1. HPE Aruba, "Migrating to an Aruba Mobility-First Campus Network (MRV-1) Validated Reference Guide" (April 2017). Chapter 4: Migration Strategies, section "Phased Migration," pp. 31-32. The guide states, "The first step is to establish Layer-2 connectivity between the old and new core... Once Layer-2 connectivity is established, the Layer-3 gateways (SVIs) can be migrated from the old core to the new core one at a time." This directly validates the L2-then-L3 approach.

2. HPE Aruba, "Aruba Campus for Midsize Networks Validated Solution Guide" (February 2021). Chapter 6: Deployment, section "Migration," p. 103. This guide outlines a controlled migration process, which involves building the new core and then migrating services. This aligns with the principle of a phased, low-risk cutover, which is the foundation of migrating L2 and L3 functions separately.