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101 Series: SPAN Mode On A Network TAP – What Is The Point?

November 6, 2015

question 101

Today I'm going to review another way that a network TAP can be used when trying to get access to network traffic. The SPAN mode is a unique mode for a TAP that takes the SPAN or mirrored output from a network switch or router into the A port and can replicate all the traffic it receives and send it out ports B, and/or C and/or D of the TAP.

When would you use the SPAN port on a TAP?


Figure 1: Output to TAP port

Figure 1: SPAN output of Link 1 to TAP port A
 

Figure 1: SPAN Port Sends a Mirrored Output

The SPAN/Mirrored output of the switch is the aggregation of the “send” and “receive” traffic of the link as illustrated above.

An important observation shows us that, unlike the Breakout or Aggregation TAP modes, there is no need to worry about the network TAP being a point of failure. The link itself does not run through the TAP; only the copy of the link’s traffic is sent to the TAP.

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There are many situations when there are not enough SPAN/Mirrored ports available on a router or switch to allow access to all of the monitoring tools that need to see the traffic of the Link, so introducing a TAP set to SPAN/Regeneration mode provides a way to distribute a Link’s traffic to up to three network tools as illustrated in Figure 2.

Figure 2: Span/Replication Mode on an Aggregration TAP
Figure 2: SPAN output of Link 1 to TAP port A. TAP replicated and passes the SPAN traffic out to Ports B, C and D of the TAP 

Figure 2: SPAN/Mirrored Output Regenerates to Send to Appliances

When planning network access using SPAN/Mirrored traffic from a router or switch, keep in mind that the router or switch’s job is to be a router or switch first, and if they start to get really busy, they will forego servicing the SPAN ports so that they can keep up with their primary function.

So, if you need to monitor all of the network traffic without exception, then you need to use a Breakout or Aggregation TAP to provide access rather than a SPAN/Mirror switch.

Well, that’s about all that is pertinent about a SPAN/Regeneration TAP. There are times when they are simple solution.

Garland Technology's, The 101 Series is an educational series on how network TAPs work and the different functions they provide to the overall network design for access and visibility.

Next time, we will review the TAPs that can provide Media Conversion. They provide a way to convert Network Fiber links to Copper on the monitor side of the TAP. Until then read this great white paper on the differences of TAP vs. SPAN.

 

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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.

NETWORK MANAGEMENT | THE 101 SERIES