Having to Change DNS servers due to broadband slowdowns

Robert McMaster
Robert McMaster used Ask the Experts™

I am running a Asus router with Asus WRT-Merlin as the firmware.

Until recently I always used Google's DNS servers of and rather than my ISP's as I thought for international traffic there was a slight speed improvement and that has worked fine for years.

However I recently moved to a new property and as per usual was using the Google DNS servers and everything is fine for a few days until my broadband starts getting really slow and if I go and change the DNS servers to automatically use my ISP's the broadband speed goes back to normal again for a few days until the same thing happens and then I again switch back to Google and keep repeating this process for months now.

I am fairly technical when it comes to networks but I can't figure out why this would be the case that I have to change the DNS servers three times a week to keep my broadband speed consistent. I know that DNS servers could be experiencing a slow down so it makes sense to switch DNS servers for a while but to have to repeat this on such as regular basis seems to apply there is a something else up with either my broadband connection as a whole or my router. It's not such a hardship to make the change but would rather not have to do it if I can avoid it and I am more curious than annoyed with the situation.

If someone has any ideas about what is going on or ever heard of something similar before?

Doubt it helps or makes a difference but previously I have always lived in a city with fibre to the cab in the street and then copper to the house and I was less than a mile from the telephone exchange were as now I live in more rural location and probably around three miles from the exchange and all of that served over copper. Because of the rural location my broadband is half the speed of my previous property but it's still around 40mbp/s so speed isn't a issue until it starts to crawl and then I change the DNS and it's immediately back to full speed.

Anyway would greatly appreciate any help in solving this slightly annoying mystery.
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I would start by getting the free DNS Workbench: https://www.grc.com/dns/benchmark.htm

This will tell you the best response times after you run the benchmark.
Select some that cover a range of companies, responsiveness, etc.
So maybe your ISP, Google and some other.

Now enter those DNS addresses into your NIC settings.  
It almost sounds like you're using but one DNS address at a time....
I always use 3 so there are fallback options.
If a DNS lookup times out then the next address on the list should be used, etc.  I think that's correct at least.


Hi Fred

Thank you for your reply.

Sounds like excellent advice. I will give that a go tomorrow and see how I get on in the coming days  and will update this post once I have some more information.

Thanks again for your expertise / help it's much appreciated.


Out of curiosity do you have to restart if you change the DNS setting?

It  could be that your router is running out of memory, or its CPU is struggling to cope with the load.  If you are using this at home have a look to see what is communicating, if its being used in a business environment I may not be a business grade device

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