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SPAN WARS: Dropping packets in a network near you

May 2, 2019

Span Wars Image

You’ve been tasked with implementing a new performance monitoring tool in your network for advanced application layer visibility. After extensive research you’ve decided a Flowmon Probe is the best solution for your needs, allowing the monitoring of network performance, enhancing user experience, and optimizing MS SQL database performance.  Beyond that it will also deliver detailed information about who communications with whom, when, how long, how often, how much data has been sent, and much more information from L2 - L4.

Implementation seems simple enough; you’ll just plug the Probe into an open SPAN port (mirror port) on your switch and start capturing data. Right? Well not necessarily. Let’s see who wins the battle for best access method for packet capture solutions.

The Opponents: TAP & SPAN


yoda TAP - Network Test Access Point

A TAP is a hardware tool that allows you to access and monitor your network. TAPs transmit both the send and receive data streams simultaneously on separate dedicated channels, ensuring all data arrives at the Flowmon Probe in real time.

vader graySPAN - Switch Port for Analysis

SPAN, aka port mirroring, sends a copy of all network packets seen on one port to another port, where the packets can be analyzed.

 


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mil falcBattle #1 - Passing all packets

TAPs do not distort or drop packets, regardless of bandwidth. SPAN ports on the other hand are likely to drop packets when ports are oversubscribed.

Looks like TAPs are the clear winner here.

xfighterBattle  #2 - Ease of Configuration

SPAN ports on a switch require configuration. They can easily be misconfiguration or turned off due to human error. Misconfigured SPAN ports not only fail to make complete copies of your network traffic, but they can hide critical data within that traffic, such as evidence of breach. TAPs just need to be connected to the network via the correct copper or fiber cable.  Once they are turned on, they provide 24/7 access to all of the data at that link in network. Certain types of TAPs can be configured via dip switches or a GUI/CLI for filtering rules.

Close call, but TAPs still manage to beat SPAN ports.

r2d2Battle #3 - Can they be hacked?

TAPs do not have an IP or MAC address. Thus they are 100% secure and invisible to outside threats. As a port on the switch, a SPAN port has an IP address, so it can be hacked.

TAPs win again.

While TAPs defeated SPAN ports in all three battles, the most important one to focus on is that TAPs won’t drop any packets. By using TAPs as they foundation of your visibility fabric, you  ensure that critical packets get to the Flowmon Probe to provide actionable insight into network performance.

[Interested in learning more? Read our whitepaper: TAP vs SPAN: Real Network Visualization Considerations for Professionals Today.]

 

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Heartbeats Packets Inside the Bypass TAP

If the inline security tool goes off-line, the TAP will bypass the tool and automatically keep the link flowing. The Bypass TAP does this by sending heartbeat packets to the inline security tool. As long as the inline security tool is on-line, the heartbeat packets will be returned to the TAP, and the link traffic will continue to flow through the inline security tool.

If the heartbeat packets are not returned to the TAP (indicating that the inline security tool has gone off-line), the TAP will automatically 'bypass' the inline security tool and keep the link traffic flowing. The TAP also removes the heartbeat packets before sending the network traffic back onto the critical link.

While the TAP is in bypass mode, it continues to send heartbeat packets out to the inline security tool so that once the tool is back on-line, it will begin returning the heartbeat packets back to the TAP indicating that the tool is ready to go back to work. The TAP will then direct the network traffic back through the inline security tool along with the heartbeat packets placing the tool back inline.

Some of you may have noticed a flaw in the logic behind this solution!  You say, “What if the TAP should fail because it is also in-line? Then the link will also fail!” The TAP would now be considered a point of failure. That is a good catch – but in our blog on Bypass vs. Failsafe, I explained that if a TAP were to fail or lose power, it must provide failsafe protection to the link it is attached to. So our network TAP will go into Failsafe mode keeping the link flowing.

Glossary

  1. Single point of failure: a risk to an IT network if one part of the system brings down a larger part of the entire system.

  2. Heartbeat packet: a soft detection technology that monitors the health of inline appliances. Read the heartbeat packet blog here.

  3. Critical link: the connection between two or more network devices or appliances that if the connection fails then the network is disrupted.

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