We had an issue of a customer whose last-mile media supports bandwidth of 20 Mbps which he should get. But upon actual testing, the client can achieve only an effective throughput of 4 Mbps. Some of engineers are wondering that it has something to do with the MTU on the path.
PE is a cisco 7206 router and lastmile media is a fiber restricted to 20 Mbps. We have also removed any rate-limitting configuration and do no see any bottlenecks.
The CE device is a non-cisco modem able to handle 24 Mbps traffic, and even this device has been replaced.
Whether MTU has anything to do in this scenario. Any clues on this ?
MTU can have an effect. However, it typically does not have a huge impact unless it is unnecessarily low. For example, if it were dropped down to 1492 for PPPoE, that isn't a huge deal. However, what is a big deal is if there is a Path MTU Discover issue alongside a nonstandard MTU (typical is 1500). In that case, you can play with TCP MSS to determine if that is in fact an issue. I always set the MSS to 40 less than the MTU when I have to force down an MTU. For example--ip tcp adjust-mss 1452.
We've been using the tcp adjust-mss cmd where there are applications related issues or web page not loading correctly or applications running terribly slow. However this customer is using the internet and he is getting maximum 498~500 KB/sec download rate which is 4 Mbps, also his speedtest results show only 4.2 Mbps download speed.
Does MTU can make a really big difference ? I shall try tweaking the MSS parameter
A lower MTU adds overhead in the form of headers and protocol interaction with TCP. I seriously doubt the issue you are describing is MTU related (or MSS for that matter). Usually, it is usually only marginally slower (or completely broken with a PMTU-D issue).