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[video] Observing The Impact Multiple Default Gateways With Wireshark

February 16, 2017

I was speaking with a client the other day and they were asking me how Microsoft behaves if a device has two default gateways.

One thing I have to say about my job and clients, I always get interesting scenarios and questions to figure out.

He mentioned that they did some research online for a few hours and found many articles contradicted one another. They became frustrated and thought they would ask me. I searched around as well and understood their point.

I asked if they simply tried it in their lab, capture some packets while rebooting one of the routers to see what happens. I figure that it would take more time to setup the lab, than capturing and reviewing the packets.

In this video, I show you how I tested their question, how I used Wireshark and what I found out.

As I went through this example, I realized that I can go through the process of playing with metric settings but then remembered the client wanted to know how it behaved with default settings.  I also wondered if I left the test computer alone for a longer period, if it would eventually go back to the first router.  All things that I can go back and observe. That’s the nice thing of having a methodology, its easy to go back and try your ‘what if scenarios.’

I always recommend you start by documenting and understanding default behavior. Then you can confirm if any tuning parameters help.

 

Want more? Download the Network TAPs 101 - The Networking User Guide, where we discuss the connectivity strategy of the various TAP modes, as well as environmental considerations such as passive or active, and media conversion.

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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