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Recurring MTU related issue.

I want a list of probable causes that I can investigate, any advice appreciated.

The situation is this:

Brand new router.
SOME https sites don't work:
Examples being: Qantas, Westpac, etc.

SOME https sites do work:

The symptoms of loading are the page just sits there trying to open the page without doing anything.
Every month/2 months the issue recurs. Resetting the MTU fixes the issue temporarily.
The issue was occurring on the old router (happened 4 times) and has occurred on the new router (6  as (5 times) well.

NOTE: We have tried changing the MTU to lowest acceptable setting, issue recurs. Have tried with highest possible setting, issue recurs. Have tried dead on in the middle issue recurs, note that it still gets fixed every time (temporarily) and the time it takes to recur does not differ with the MTU value.
1 Solution
It seems the Path MTU Discovery does not work properly.

In general, it is the web server that is trying to send you large packets, and when it tries to figure out how large these can be, the negotiations does not succeed.  Typically what should happen is that the link from your ISP to your router is the "problem" and your ISPs router will signal by sending an ICMP packet back to the web server.

The problem here can be either that your ISPs router does not send the ICMP, or it could be that there is a firewall in-front of the web server that does not allow the ICMP packet through.

If you can fix it by setting a smaller MTU on your LAN, then your router will send the ICMP to the web server - and that seems to work. That could indicate that the web server and its firewall is fine. In that case your ISP should have a look at their MTU settings and whether they send ICMP or not (for packet to large and don't fragment set).

If you do manage to fix it, and it only works for some time, then it seems you do not save the config on the router, and it looses it. On a Cisco the command to save the config is:

Router# copy running-config startup-config
admiralpantsAuthor Commented:
Thanks for the info.
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