SPAN (Switched Port Analyzer)
- Overview
SPAN (Switched Port Analyzer), also known as port mirroring or port monitoring, is a critical feature in network switches that allows the replication of traffic from specified ports (source ports) to a designated monitoring port (destination port). This enables real-time traffic analysis, troubleshooting, and security monitoring without disrupting normal network operations. SPAN provides a non-intrusive method for administrators to gain visibility into network traffic.
- Key Functions of SPAN
- Traffic Capture: Replicates traffic from specific ports or VLANs for analysis by tools like Wireshark or IDS/IPS.
- Fault Diagnosis: Helps identify network congestion, protocol errors, or abnormal behavior.
- Security Auditing: Monitors sensitive areas (e.g., server farms) to detect attacks or data breaches.
- Performance Analysis: Measures bandwidth utilization and application traffic distribution (e.g., HTTP vs. VoIP).
- How SPAN Works
- Core Concepts
- Source Port: The port or VLAN being monitored; traffic is copied from this port.
- Destination Port: The port receiving the mirrored traffic, typically connected to an analysis tool.
- Traffic Direction:
- Rx (Receive): Captures traffic entering the source port.
- Tx (Transmit): Captures traffic leaving the source port.
- Both: Captures bidirectional traffic.
- Types of SPAN
- Local SPAN:
- Source and destination ports are on the same switch.
- Supports ports, VLANs, or MAC addresses as sources.
- Remote SPAN (RSPAN):
- Source and destination ports are on different switches.
- Uses a dedicated RSPAN VLAN to transport mirrored traffic across switches.
- ERSPAN (Encapsulated RSPAN):
- Encapsulates mirrored traffic in GRE tunnels for transmission over Layer 3 networks, enabling long-distance monitoring (e.g., across data centers).
- Traffic Replication Logic
- Hardware-Based: Traffic is replicated by the switch's ASIC, minimizing performance impact.
- Many-to-One: Multiple source ports can be mirrored to a single destination port (requires careful bandwidth planning).
- Use Cases for SPAN
- Security Monitoring: Mirrors traffic from firewalls or core switches to IDS/IPS for threat detection.
- Application Performance Management (APM): Analyzes QoS markings and latency for VoIP or video conferencing traffic.
- Compliance Auditing: Records communication logs for specific users or servers to meet regulatory requirements.
- Network Forensics: Investigates attack paths by analyzing historical mirrored traffic.
- Limitations and Considerations
- Bandwidth Constraints: The destination port must have sufficient bandwidth to handle the mirrored traffic; otherwise, packets may be dropped.
- Unmirrored Traffic: Control plane traffic (e.g., CDP, STP BPDUs) and encrypted traffic (e.g., IPsec) may not be mirrored.
- Performance Impact: Heavy mirroring can strain CPU or memory resources, especially on software-based switches.
- Security Risks: The destination port exposes sensitive data; access should be restricted using ACLs.
- Best Practices for SPAN
- Isolate Monitoring Networks: Connect destination ports to dedicated security appliances to prevent data leakage.
- Filter Redundant Traffic: Use ACLs or SPAN filters to mirror only critical traffic (e.g., HTTP/HTTPS).
- Prioritize RSPAN Traffic: Assign high priority to RSPAN VLANs to avoid packet drops.
- Verify Configuration: Regularly check SPAN status using commands like show monitor session all.
