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The 101 Series: Link Failure Propagation and High Availability

August 29, 2017

Copper cable network visibility garland technology

Many customers that we talk to have more than one Internet connection at their facility. Referred to as a high availability (HA) scenario, this active/active or active/passive link setup ensures customers that when the primary link goes down (because it will fail at some point), traffic can automatically be forced to the secondary link.

The following graphic is a basic illustration of how link failure propagation works.

Modes-LFP2-1

You have two options. Under traditional network designs, you would tell the CIO that you need to work with IT to build a new network access point in the problem area—which could take days. If you design your network with visibility in mind, you could have a network analyzer hooked up in an hour. When it comes to designing architectures for efficient network monitoring, it all comes down to visibility with network TAPs.

Link Failure Propagation in Copper Gigabit Environments

Link failure propagation (LFP) is essential for ensuring 100% network uptime when network elements fail. This feature is imperative in high availability situations with 1G copper network TAPs.

Copper gigabit networks differ because copper gigabit requires that each network port negotiate with the network TAP individually (i.e. the switch to the tap, the tap to the router).

Without LFP in a copper gigabit environment, if a network element link goes down, there’s nothing to tell the corresponding network element that there’s an issue. The functional network element continues to send packets and you start to lose visibility as a result.

Companies can’t afford to suffer downtime on critical links that support in-line appliances. Link failure propagation ensures an instant switch to the secondary link to maintain 100% uptime while networking professionals go through troubleshooting processes.

Download Now: Network TAPs 101 - The Networking User Guide [Free eBook]

LFP is Essential to All Network Designs 

Some network TAPs are sold for copper gigabit environments without LFP. Without LFP, you run the risk of introducing costly points of failure in your network.

Link failure is inevitable and the LFP feature of Garland network TAPs gives you peace of mind when these issues occur. The network will keep running but the critical link will not. LFP is designed to shut down the link attached to the TAP if one side or the other fails. In a non-HA environment, the link is turned off by the TAP and it is up to diagnostic tools to alert the maintenance folks that there is a problem.

In a HA environment, shutting down the link causes the network (not the TAP) to force a failover to the backup network so the critical link can keep flowing. This is when you can troubleshoot on the primary network to figure out what went wrong. Bear in mind that it is the critical link that we want to keep flowing. The LFP function of a TAP shuts down the critical link. That is why the LFP feature is generally used in a HA environment.

Copper networks continue to play a vital role in today’s networks and data centers and is the backbone of many existing enterprises. When selecting a TAP for a copper network, it’s important to partner with a vendor that is investing R&D to support copper in the future.

Looking to add copper TAPs to your next deployment, but not sure where to start? Join us for a brief network Design-IT consultation or demo. No obligation - it’s what we love to do!

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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