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When A Simple SPAN Port Is Enough

September 11, 2018

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The two most common ways to access and replicate data within your network are TAP and SPAN technology. A Test Access Point (TAP) is a hardware device that copies all of your network data. SPAN or Switch Port Analyzer are mirroring ports within a switch that copies specific data.

Network TAPs are always an industry best practice but in a few specific situations when a SPAN port suffices. When monitoring products are looking for low bandwidth application layer events like “conversation or connection analysis,” “application flows,” and applications where real time and knowing real delta times are not important. SPAN could also be used in a remote location that doesn’t justify a permanent deployment, offering temporary access for troubleshooting. 

In these low throughput specific situations when a SPAN port suffices, you likely need a way to aggregate a few SPAN lines together and send that combined network traffic out to one or more sets of tools or appliances. When these situations arise, think simplicity.

Think about using one set of network tools rather than having a unique set of tools for each SPAN. Or worse, having to rotate the tools to each SPAN port.

 

 

 

When we are working with a SPAN input, we do not need to worry about failure of the device attached to the SPAN port because SPAN is simply a copy of the network traffic. If there is a failure at the end of the SPAN line, the link is not affected. Knowing this, we can take SPAN inputs directly to our Advanced Aggregators without introducing a point of failure to the network traffic, and then filter, aggregate and load balance, prior to distributing the traffic out one or more tools.

 

 
Six SPANs connected to an Advanced Aggregator feeding two Appliances
 

You can aggregate all the traffic and send it out to the network tools, or you can send some of the traffic out to one monitor port and a different set of the traffic out to the other monitor port.

 

Traffic distribution
 
Two SPANs Connected to 6 Tools or Appliances

 

Now we're changing the scenario to two SPANs that we want to aggregate together. That leaves enough ports available for monitoring up to six tools or appliances.

 

Download Now: TAP vs SPAN - Network Visualization Considerations for Professionals [Free whitepaper]

 

Below we see a scenario where we have 6 SPANs (or more if necessary). We are still connecting to an Advanced Aggregator, so we can distribute the traffic from the 6 SPANs to more monitoring devices. The Monitoring devices can be 1G devices and the Network ports can be 10G.

 

 
Connecting to an Advanced Aggregator allows more flexibility

 

The appliances can be set up to share the traffic load by "load balancing" and even filtering the data. This way, only the traffic of interest is sent out to the appliances and you can minimize the possibility of oversubscribing the 1G monitor ports. All the ports on the Advanced Aggregator are configurable as a Network Port or a Monitor Port, and the speed can be 1G or 10G. So, to use a cliché, the possibilities are endless.

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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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