Noticed some websites were not working over the weekend from my newly installed Cisco 2801 Router on BT Infinity. Checked and confirmed all DNS and cabling was correct and run some basic tests. I could not access websites such as microsoft.com or speedtest.net.
The websites did resolve so I knew it was not a DNS issue so started to look at the Cisco Router. The issue was resolved by adding this to the Dialer Interface:
Router# ip tcp adjust-mss 1452
If you would like to know what the above command does please read: