Introduction
By default, all Alta Labs equipment is set up to autonegotiate Layer 1 link speeds as this is broadly accepted as best practice.
A common misconception, or perhaps even instinct, is to manually set Ethernet/SFP link speeds in order to achieve the best performance. The reality is that manually setting link speeds is actually more likely to be detrimental to your network.
Auto (Auto Negotiation) will automatically adjust the Layer 1 speeds based on the capability of the link itself. Assuming all components of the link are in good operating order, Auto will always use the maximum speed that is both reliably achievable and supported by all components of the link.
What’s a component? In this context, there are 3 components:
- Ethernet/SFP on one end of the link
- The medium being used (Ethernet cable/Fiber cable)
- Ethernet/SFP on the other end of the link
Alta Labs always recommends using the default (Auto) unless there is a specific need to manually set the speed. An Alta Labs support employee will advise if that is necessary.
Limiting advertised speeds vs. disabling autonegotiation
All switches set speeds in two different ways depending on the speed and type of interface.
When selecting a speed of 1 Gbps or greater, the effect is to limit the speeds the interface advertises in autonegotiation. This is because autonegotiation is mandatory on gigabit and faster Ethernet and cannot be disabled.
When selecting a 10 or 100 Mbps speed, autonegotiation is disabled and the port is set to that specific speed and duplex. At 10 and 100 Mbps full duplex, both ends (the switch and connected device) must be configured identically. If a switch port is set to 100 Mb full duplex, then the device plugged into it must also be set to 100 Mb full duplex. If this is not done, it will result in a duplex mismatch, which causes severe performance degradation. 10 and 100 Mb Ethernet with autonegotiation enabled must assume they are connected to a half duplex hub in the absence of autonegotiation from the link peer, and hence fall back to half duplex. One end on full duplex with the other on half duplex results in late collisions and retransmissions on the half duplex side, which causes serious performance problems. Never set a port to 10 or 100 Mb full duplex without the connected device also being set the same. That is not strictly required when setting a port or device to 10/100 half duplex since the side with autonegotiation enabled will fall back to half duplex automatically.
Examples
Scenario: Let’s say you’re plugged into an S24-PoE “Switch A” directly using an Ethernet cable. Downstream from Switch A is another S24-PoE Switch B and the switches are connected via fiber SFPs (1 Gbps). The S24-PoE supports SFP+ (10 Gbps). Instinctually, you may see the 10 Gbps option and think “yes, I want faster speeds”. You override the speed on Switch A to be 10 Gbps. Switch B now falls offline.
Why?: While the S24-PoE supports 10 Gbps SFP+, they’re only connected via a non-+ SFP, so the maximum achievable speed is 1 Gbps. By manually setting the speed to 10gbps, you’re telling the switch “I only want you to talk at 10 Gbps on this interface”, but it can’t due to the non-+ SFP. This scenario can be resolved by simply changing the interface back to Auto, then Switch B will come back online.
Scenario: The exact same situation as the previous scenario, except you set the speed on Switch B. Now, because Switch B is not online, it’s not as simple as changing the speed back to Auto because the setting cannot propagate to an offline device. To resolve this scenario, you would have to do one of the following in order to restore communication to Switch B:
- Move the uplink cable on Switch B to another interface, assuming you have one available
- Factory Default Switch B
- Change the SFP in Switch A to SFP+, at least temporarily, to regain a link between Switch A and Switch B. Then you can set Switch B back to Auto and go back to the standard SFP
Scenario: You installed an AP6-Pro that has a 1 Gbps Ethernet port. Both the AP6-Pro and the switch it is plugged into (model irrelevant for this scenario) are both set to Auto for the Layer 1 speed. Unfortunately, there was an issue with the crimping of the Ethernet cable and there are a lot of errors across this link. The switch and/or the AP will identify this automatically due to errors during the Auto Negotiation process, remove 1 Gbps from the advertised capable speeds, and then attempt 100 Mbps Full Duplex since gigabit was not reliable. Yes, this is a 90% reduction in speed, but it’s not 0. This way, the AP remains online and provides WiFi, but at a reduced speed as the bottleneck is now the Ethernet negotiation. The same thing happens when using a CAT-5 (not CAT-5e) cable, because CAT-5 is not sufficient for gigabit link.
The good news is you can see this plainly in the controller UI. Note in the screenshot above, we see blue ports, orange ports, and white ports. If we mouse over the tooltip for the Legend, we see what each color means:
If what is plugged into port 11 is 1 Gb capable, but I see the port is orange, that shows the port is only operating at 100 Mbps or 10 Mbps, so I know there’s something wrong with that link and can troubleshoot accordingly.
Conclusion
A note about cable testers:
As you can imagine, our engineers have used a plethora of cable testers throughout our individual careers. Unfortunately, many of them are manufactured to subpar quality specifications and are lacking even a minimal set of features. Many testers will only verify continuity across the span of the cable. While that is useful to determine if 1 or more pins are not making good contact or any contact, they’re quite useless in verifying that you have crimped the correct pinout. Many testers will indicate a passing/good cable even if 4 of the 8 wires are in the wrong position. In our collective experience, we strongly recommend using Fluke-branded cable testers, even with their steep price point. In our opinion, the cost of an unhappy user experience and rolling a truck for even a single incident justifies the cost for a quality cable tester.
If you’ve ever had the pleasure of interacting with our technical support team and mentioned that you verified the cable, you were likely asked what tester you were using. This is exactly why we ask those questions.
One last pro-tip: if you notice that a particular AP/switch commonly links at 100 Mbps, or an AP often switches back and forth between mesh mode and wired mode, it is very likely that either 1) the cable termination is poor and needs to be performed again, or 2) the cable is pinched or cut somewhere along the way, and needs to be replaced. Quality cable testers can also help detect these conditions.
In summary, Alta Labs always recommends Auto in almost every scenario. Manually setting Layer 1 link speeds should only be used with extreme caution when you have a very firm understanding of the network and the implications that come with manually setting the speeds OR if you’re instructed to do so by one of our engineers. Even then, we will likely make it clear that it should only be a temporary change and to plan on moving back to Auto.
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