Network Diagnostics conundrum

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Network Diagnostics conundrum

I have a 27 inch, (late 2009) iMac with 12 GB or RAM running Yosemite (10.10.3).

The machine is connected via Ethernet to an Gigabit POE ethernet switch, that then goes through a Wireless link back to my office in town ( about 15km line of sight) where it hits a mikrotik router and then an ADSL internet line.

Recently I have noticed some speed degradation when browsing the Internet, and went searching as to why.

After running the Apple Network Diagnostics tool and leaving it up an running for a period of time, I noticed that the ISP, Internet and Server indicator lights ( on the left) would all intermittently cycle to Red or a failed status. This literally lasts for a second and then the status turns green across the board again.

Has anyone else seem this before? and if so, where should I start my next step in fault diagnosis?

iMac, Mac OS X (10.0.x), network diagnostics

Posted on May 5, 2015 5:33 PM

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Answer 1

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Linc Davis

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209,547 points

May 6, 2015 5:53 PM in response to Working Dog

Connect another device to the same Ethernet cable and see whether it does the same thing.

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Answer 2

Working Dog Author

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May 6, 2015 5:53 PM in response to Linc Davis

Thanks for the speedy reply Linc.

Tried the following based on your advice:

Another device (Macbook Pro running 10.10.3) - No Issues

Another patch cable to the problematic machine - Same issue

Also tried

Unplug Ethernet and connect via wifi to an Airport Extreme (A 1408) that services the wifi devices in the house - Same issue.

A Macbook Pro running 10.10.3 connected to the wifi - with Network diagnostics running - No Issue

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Answer 3

Linc Davis

Level 10

209,547 points

May 6, 2015 6:02 PM in response to Working Dog

Start up in Recovery mode. In the OS X Utilities screen, select Get Help Online. A clean copy of Safari will launch. No plugins, such as Flash, will be available. While in Recovery, you'll have no access to your saved bookmarks or passwords, so make a note of those before you begin, if they're needed for the test.

Test. After testing, restart as usual and post the results.

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Answer 4

Working Dog Author

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May 6, 2015 10:23 PM in response to Linc Davis

Issue resolved. Thanks for your help Linc.

One of our ISP's is currently experiencing DNS issues and putting the Google DNS of 8.8.8.8 as the primary DNS instead of the ISP's has resolved the problem.

We run two separate ISP connections with different providers with failover provisions if one link goes down. The iMac in question had a fixed IP address and had all of it's traffic routed through one ISP as it's first preference. In this instance it was the ISP with the DNS issues. The cycling that I identified in the Apple Network Diagnostics would have been the router testing the failed link and then reverting back to its failover position for this iMac's connection.

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