Blog

Which statement describes the function of the SPAN tool used in a Cisco switch?

• It supports the SNMP trap operation on a switch.
• It provides interconnection between VLANs over multiple switches.
• It is a secure channel for a switch to send logging to a syslog server.
• It copies the traffic from one switch port and sends it to another switch port that is connected to a monitoring device.

The correct answer is:

“It copies the traffic from one switch port and sends it to another switch port that is connected to a monitoring device.”

Detailed Explanation:

Introduction to SPAN (Switched Port Analyzer)

The Switched Port Analyzer (SPAN) is a powerful tool used in Cisco switches for network monitoring and troubleshooting. SPAN is a feature that allows the network administrator to copy network traffic from one or more ports, known as source ports, to another port, known as a destination port. The destination port is typically connected to a monitoring device, such as a network analyzer or an intrusion detection system (IDS), which can then inspect the copied traffic for various purposes.

Understanding SPAN Operation

In a typical network setup, network traffic flows between devices connected to a switch. However, in most cases, the traffic between two devices on different ports of a switch is only visible to those devices. This means that a network administrator or a monitoring device cannot easily capture and analyze this traffic. This is where SPAN comes in.

SPAN enables the copying of traffic from designated source ports or VLANs to a destination port. This allows an administrator to monitor the traffic without interrupting or altering the network flow. The destination port then forwards the copied traffic to a connected monitoring device, which can analyze it for troubleshooting, performance monitoring, or security purposes.

Types of SPAN

There are several variations of the SPAN feature in Cisco switches:

1. Local SPAN:
   • Local SPAN is the most basic form of SPAN. It mirrors traffic from source ports or VLANs within the same switch to a destination port on that switch.
   • Example Use Case: An administrator wants to monitor the traffic from a specific server connected to port 5. By configuring SPAN, the traffic from port 5 can be copied to port 10, where a monitoring device is connected.
2. RSPAN (Remote SPAN):
   • RSPAN is an extension of SPAN that allows the mirroring of traffic across different switches within the same Layer 2 network. This is useful when the monitoring device is not directly connected to the same switch as the source traffic.
   • RSPAN uses a special VLAN, known as the RSPAN VLAN, to transport the mirrored traffic between switches. The source traffic is encapsulated and forwarded over this VLAN to the destination switch, where it is then sent to the monitoring device.
   • Example Use Case: If the traffic to be monitored is on Switch A and the monitoring device is on Switch B, RSPAN can mirror the traffic from Switch A to Switch B.
3. ERSPAN (Encapsulated Remote SPAN):
   • ERSPAN extends RSPAN by allowing the mirroring of traffic across Layer 3 boundaries, meaning the mirrored traffic can be sent over IP networks. ERSPAN encapsulates the mirrored traffic in GRE (Generic Routing Encapsulation) packets and sends it to a destination IP address, where it is decapsulated and forwarded to the monitoring device.
   • Example Use Case: ERSPAN is useful in large networks where the monitoring device is in a different location or even across a different network.

Configuring SPAN on a Cisco Switch

The configuration of SPAN involves defining the source and destination ports. Below is an example of how to configure SPAN on a Cisco switch:

1. Define the source port:

   Switch(config)# monitor session 1 source interface fastethernet0/1
   

   This command specifies that the traffic from interface FastEthernet 0/1 will be mirrored.

2. Define the destination port:

   Switch(config)# monitor session 1 destination interface fastethernet0/2
   

   This command sets FastEthernet 0/2 as the destination port, where the copied traffic will be sent.

3. Verify the configuration:

   Switch# show monitor session 1
   

   This command allows you to verify that the SPAN session has been correctly configured.

Use Cases for SPAN

SPAN is used in various scenarios within a network, including:

1. Network Troubleshooting:
   • Administrators can monitor specific traffic flows to identify network issues such as congestion, packet loss, or latency. By analyzing the traffic on a monitoring device, the root cause of the problem can be determined more efficiently.
2. Security Monitoring:
   • SPAN is often used to monitor for malicious activity. For example, traffic can be mirrored to an IDS, which will analyze the traffic for potential threats. If abnormal or suspicious activity is detected, the IDS can alert the network administrator to take action.
3. Performance Monitoring:
   • SPAN can be used to monitor the performance of certain applications or devices by capturing traffic patterns, bandwidth usage, and packet flow characteristics. This information is vital for capacity planning and ensuring that network resources are optimally utilized.
4. Compliance and Auditing:
   • Organizations may need to monitor and log network traffic for compliance with industry regulations. SPAN allows for non-intrusive monitoring of data flows, ensuring that sensitive information is handled according to policy.

Limitations and Considerations

While SPAN is a valuable tool, it does have some limitations and considerations:

1. Impact on Performance:
   • SPAN can consume significant resources on a switch, particularly when monitoring high-traffic ports or multiple VLANs. This can impact the performance of the switch, so it is essential to consider the capacity of the switch when configuring SPAN.
2. Potential for Dropped Packets:
   • If the destination port is oversubscribed (i.e., receiving more traffic than it can handle), packets may be dropped. This could lead to incomplete monitoring data, which may affect the accuracy of the analysis.
3. Lack of Encryption:
   • SPAN does not encrypt the mirrored traffic. If the traffic is sensitive, this lack of encryption could be a security risk, especially in scenarios where the monitoring device is not under the direct control of the network administrator.
4. Monitoring Multiple VLANs:
   • While SPAN can mirror traffic from multiple VLANs, this can increase the complexity of the configuration and the volume of traffic on the destination port. RSPAN or ERSPAN may be better suited for such scenarios.

Conclusion

The SPAN tool is an essential feature in Cisco switches, providing administrators with the ability to monitor and analyze network traffic in real time. By copying traffic from one or more ports to a designated monitoring port, SPAN enables a wide range of network troubleshooting, performance monitoring, and security activities. Despite its limitations, when used correctly, SPAN is a powerful tool that can greatly enhance the visibility and manageability of a network. The correct understanding and configuration of SPAN are crucial for maximizing its benefits while minimizing potential drawbacks.

In summary, “It copies the traffic from one switch port and sends it to another switch port that is connected to a monitoring device” is the most accurate description of the SPAN tool’s function, and it plays a vital role in network management and security.