Question

Suppose some repeaters (hubs), rather than bridges, are connected into a loop. (a) What will happen when somebody transmits? (b) Why would the spanning tree mechanism be difficult or impossible to implement for repeaters? (c) Propose a mechanism by which repeaters might detect loops and shut down some ports to break the loop. Your solution is not required to work \(100 \%\) of the time.

Short answer

a) The transmission will cause a broadcast storm. b) Repeaters lack the intelligence to implement the spanning tree protocol. c) Use unique ID packets to detect loops and shut down ports.

Step-by-step solution

- Analyze the Transmission in a Loop

When somebody transmits in a network with repeaters connected in a loop, the transmitted signal will be amplified and sent around the loop continuously. This will result in a broadcast storm, where the transmission keeps circling the loop indefinitely, leading to network congestion and a potential network collapse.

- Explain Spanning Tree Mechanism for Repeaters

The spanning tree algorithm is used to prevent loops in a network by logically disabling some pathways. However, repeaters operate at the physical layer and lack the intelligence required to participate in or execute the spanning tree protocol, which works at the data link layer using bridge protocol data units (BPDUs). Without understanding these protocols, repeaters cannot effectively determine which port to shut down to prevent loops.

- Propose a Loop Detection Mechanism

One possible mechanism for repeaters to detect loops could involve sending out unique ID packets periodically. Each repeater would keep track of these packets and their source. If a repeater receives the same unique ID packet on a different port within a specific time frame, it can infer a loop presence. Upon detecting a loop, the repeater can shut down one of its ports to break the loop. This method might not be reliable all the time but could reduce the likelihood of persistent loops.

Key concepts

Broadcast Storm

When a network contains repeaters connected in a loop, transmitting data creates an endless cycle. The original signal is amplified and retransmitted continuously, leading to a phenomenon called a broadcast storm. This is like a feedback loop on a microphone, where sound keeps getting louder.

Broadcast storms can cause serious problems:

• Network Congestion: The excessive traffic can clog the network, preventing legitimate data from being transmitted.
• Device Overload: Network devices get overwhelmed by the constant flow of duplicated packets, potentially crashing the system.
• Performance Degradation: The network becomes extremely slow, making it nearly unusable for regular activities.

Avoiding broadcast storms is crucial to maintain network efficiency and stability. Understanding how they occur prepares you to implement strategies to prevent them.

Spanning Tree Protocol

The Spanning Tree Protocol (STP) is a networking protocol designed to prevent loops in Ethernet networks. It ensures a loop-free topology by selectively blocking some network paths.

• How It Works: STP uses Bridge Protocol Data Units (BPDUs) to detect network topologies and determine the best paths. When a loop is detected, STP disables the redundant paths, creating a 'spanning tree' that spans the entire network without looping.
• Limitations with Repeaters: Repeaters operate at the physical layer and do not understand data link layer protocols like BPDUs. This makes them incapable of executing STP, as they lack the necessary 'intelligence' to determine which paths to block.

Therefore, while STP is highly effective in networks with bridges and switches, it is not applicable in networks solely comprised of repeaters. The absence of STP capabilities in repeaters necessitates alternative loop prevention mechanisms.

Loop Detection

Detecting and breaking loops is essential to prevent broadcast storms in networks with repeaters. While STP is not suitable, alternative methods can be used to identify and mitigate loops.
One such mechanism is periodically sending unique ID packets:

• Sending Out Unique IDs: Repeaters can periodically send unique identifier packets.
• Tracking IDs: Each repeater tracks the IDs and their source ports. If the same ID packet appears on different ports within a certain timeframe, a loop is likely present.
• Breaking the Loop: Upon detecting a loop, the repeater can shut down one of its ports to break the cycle and restore normal network operations.

This method, while not foolproof, helps in reducing the likelihood of persistent loops. Implementing loop detection mechanisms in your network infrastructure enhances stability and performance by preventing broadcast storms caused by loops.