When both ends of an Ethernet cable connect to the same networking device (switch/router), it creates a Layer 2 loop. This causes:

• Broadcast storms as packets endlessly circulate
• MAC address table instability (flapping)
• Exponential traffic multiplication
• Potential switch CPU overload (visible via SNMP)

Here's how network engineers can programmatically detect such loops:

import snmp_helper

SWITCH_OID = '1.3.6.1.2.1.17.4.3.1.2' # dot1dTpFdbPort

def check_mac_flapping(switch_ip):
    mac_table = snmp_helper.snmp_get_bulk(switch_ip, SWITCH_OID)
    port_changes = {}
    
    # Analyze MAC port movements
    for entry in mac_table:
        mac, port = entry.split('|')
        if mac in port_changes:
            port_changes[mac] += 1
            if port_changes[mac] > 5:  # Threshold
                return True
        else:
            port_changes[mac] = 0
    return False

Modern switches implement loop prevention protocols:

• Spanning Tree Protocol (STP): Blocks redundant paths
• BPDU Guard: Disables ports receiving bridge protocol data units
• Loop Guard: Detects unidirectional links

Example Cisco IOS configuration:

spanning-tree mode rapid-pvst
spanning-tree portfast bpduguard default
spanning-tree loopguard default

When troubleshooting, check these CLI indicators:

show interface counters errors
show spanning-tree inconsistentports
show mac address-table count

Typical output during a loop event shows rapidly incrementing packet counts and error statistics across multiple ports.

Best practices include:

• Color-coding patch cables
• Implementing port security
• Using managed switches exclusively
• Regular switch port audits with tools like RANCID

When you connect both ends of an Ethernet cable to the same networking device (switch/router), you create what's known as a "network loop." This isn't just ignored - it actively disrupts network operations through broadcast storms and MAC table corruption.

A healthy switch maintains a MAC address table that maps physical ports to device addresses. Here's simplified logic:

// Pseudocode of switch learning mechanism
function handlePacket(incomingPort, sourceMAC, destMAC) {
    macTable[sourceMAC] = incomingPort; // Learn source
    
    if (destMAC in macTable) {
        forwardToPort(macTable[destMAC]); // Unicast
    } else {
        floodAllPorts(); // Broadcast
    }
}

With both ends connected:

1. Broadcast packets infinitely circulate
2. MAC table constantly overwrites with conflicting port info
3. CPU utilization spikes to 100%
4. Legitimate traffic gets starved

You might see these symptoms in Linux:

# Monitor switch CPU (if accessible)
ssh switchadmin "show process cpu" | grep "CPU utilization"

# Check for packet storms
tcpdump -i eth0 -n -c 100 "ether[0] & 1 = 1" # Capture broadcasts

# Alternative detection via syslog
grep -i "storm" /var/log/syslog

Modern networks implement STP (Spanning Tree Protocol):

// Conceptual STP operation
function stpLogic() {
    if (receivedSuperiorBPDU) {
        blockPort();
        startForwardDelayTimer();
    }
    if (timerExpired && noBetterBPDU) {
        unblockPort();
    }
}

Even with STP, temporary loops during topology changes can cause issues. Proper cable management is crucial:

# Cable labeling best practice
for port in $(seq 1 48); do
    echo "Port $port -> $(lldpctl | grep -A1 "Port $port")" >> cabling.txt
done

Cisco switches offer additional safeguards:

switch(config)# storm-control broadcast level 1.00
switch(config)# storm-control action trap
switch(config)# errdisable detect cause loopback