NetApp best practices for SAS cabling mandate adapter-level fault tolerance to prevent a single point of failure. The initial configuration, using ports 2a and 2b, connects the SAS stack to two ports on the same onboard SAS adapter. If this adapter fails, the controller loses all connectivity to the shelf stack.

The correct configuration requires connecting the stack to ports on two separate SAS adapters. By using port 2a (onboard) and port 9a (on the add-in SAS card in slot 9), the paths are distributed across different hardware. This ensures that the failure of one SAS adapter does not cause a total loss of connectivity to the storage from that controller.

B. Use port 2a and 9b.

While this provides adapter-level redundancy, the standard convention is to use the first available port on each adapter (port 'a') for the first stack, making option A the most correct answer.

C. Use port 2a and 2c.

This configuration uses two ports on the same onboard SAS adapter, failing to provide adapter-level redundancy and repeating the initial cabling fault.

D. Use port 2b and 9d.

This provides adapter-level redundancy, but using the first port on each adapter (2a and 9a) is the standard cabling convention for the first stack.

1. NetApp Documentation: SAS and ACP Cabling Guide

Reference: In the section "Cabling rules for multipath HA and multipath single-controller configurations," the guide states the rule for a single-controller with multiple SAS HBAs: "For a single-controller configuration with two or more SAS HBAs, you cable the first stack to port A on the first HBA and port A on the second HBA."

Justification: In this scenario, the onboard SAS ports represent the first HBA, and the card in slot 9 is the second HBA. Following the rule, the correct cabling is to port 'a' on the onboard adapter (2a) and port 'a' on the slot 9 adapter (9a). This directly supports answer A.

2. NetApp Documentation: AFF A700 and FAS9000 Installation and Setup Instructions

Reference: The cabling diagrams and instructions for adding storage shelves consistently show connections being spread across different adapters for redundancy. For example, in the section "Cable SAS shelves," diagrams illustrate connecting a stack to an onboard port and a port on a PCIe adapter.

Justification: This document visually reinforces the principle that paths to a SAS stack must originate from separate physical adapters to ensure high availability, which is the core concept behind the correct answer. Options B and D are technically redundant, but A follows the specific "port A to port A" convention for the first stack. Option C violates the fundamental principle of adapter redundancy.