Using a bottom-to-top approach means starting from the physical layer and moving up the OSI model to troubleshoot a network problem. Checking for a link light is a physical layer check that verifies the connectivity of the network cable and device. Reference: https://www.professormesser.com/network-plus/n10-007/troubleshooting-methodologies-2/

A virtual IP (VIP) address must be configured to ensure connectivity during a failover. A VIP address is a single IP address that is assigned to a group of servers or network devices. When one device fails, traffic is automatically rerouted to the remaining devices, and the VIP address is reassigned to the backup device, allowing clients to continue to access the service without interruption.
Reference:
CompTIA Network+ Certification Study Guide, Exam N10-007, Fourth Edition, Chapter 6: Network Servers, p. 300

The network administrator can use the Simple Network Management Protocol (SNMP) to monitor the temperatures of all the devices. SNMP is a widely-used protocol for managing and monitoring network devices, such as routers, switches, servers, and other networking equipment. SNMP allows network administrators to gather information about the performance and status of devices on the network, including temperature readings.
To retrieve the temperature information, the administrator will have to configure SNMP on the devices and configure SNMP manager software on their computer. Once the SNMP manager software is configured, it will be able to send SNMP requests to the devices and retrieve information such as temperature, voltage, fan speeds, etc. Many network devices have built-in SNMP support, and the administrator may also need to install SNMP agent software on the devices to enable SNMP monitoring.
The administrator can also use some specific command or tool like IPMI (Intelligent Platform Management Interface) or DCIM (Data Center Infrastructure Management) tools for monitoring the temperatures of all the devices.

Firmware is a type of software that controls the low-level functions of a hardware device, such as a router, switch, printer, or camera. Firmware updates are patches or upgrades that fix bugs, improve performance, add features, or address security vulnerabilities in firmware. Firmware updates are commonly used to address CVEs (Common Vulnerabilities and Exposures) on network equipment and operating systems, as CVEs are publicly known flaws that can be exploited by attackers. Reference: https://www.comptia.org/blog/what-is-firmware