Designing Ethernet networks with redundancy involves creating multiple physical paths between switches to prevent single points of failure. However, these physical loops are problematic for standard Ethernet (Layer 2) forwarding, which lacks an inherent loop-avoidance mechanism like the IP TTL field. Unmanaged loops cause broadcast storms, MAC address table instability, and duplication of frames, which can quickly render a network unusable. Therefore, a fundamental design requirement is to implement a protocol, such as the Spanning Tree Protocol (STP) or its modern variants (RSTP, MSTP), to logically prune the redundant paths, creating a single, loop-free forwarding topology while maintaining the physical paths for failover.

A. TTL (Time to Live) is a mechanism used in Layer 3 (IP) packets to prevent routing loops, not in Layer 2 Ethernet frames.

B. In a looped environment, the MAC address table becomes unstable (flapping), but the learning rate itself is not doubled.

C. Redundancy does not double the number of devices or MAC addresses; thus, the required FDB (MAC table) size does not double.

1. Nokia 7750 SR OS Services Guide, Release 21.10.R1: In the "Spanning Tree Protocol" section, it states, "The main purpose of STP is to ensure that you do not create loops when you have redundant paths in your network. Loops are deadly to a network." This directly confirms the need for a protocol to manage loops created by redundancy. (Reference found in sections describing Layer 2 services like VPLS and Epipe).

2. Kurose, J. F., & Ross, K. W. (2017). Computer Networking: A Top-Down Approach (7th ed.). Pearson. In Chapter 6, Section 6.4.3 "Link-Layer Switches," the text explains: "To deal with the problem of loops in redundant topologies, switches use the Spanning Tree Protocol (STP)... The protocol's goal is to create a spanning tree from the graph of switches, resulting in a loop-free topology." This is standard university courseware material.

3. IEEE Std 802.1D-2004, IEEE Standard for Local and metropolitan area networks—Media Access Control (MAC) Bridges. Clause 1, "Overview," states that the standard specifies an algorithm for bridges to "provide for the automatic reconfiguration of the active topology... to provide loop-free data paths." This standard is the foundational source for Spanning Tree Protocol.