path mtu discovery---route.metrics.mtu

Last week we had a problem with one of our virtual server's sessions. When i checked tcpdump at virtual servers side, i saw "destination unreachable-fragmentation needed" icmp packets. At the begining of tcp session mms was 1300 after this icmp packets mms was reduced to 576. At real server side despite the configured mtu was 1460 mms reduced to 576. Regarding to belowing articles i displayed the cmetrics with "tmsh show net cmetrics" command, the mtu was 576 for real server's ip address which has problem. Than i deleted the cmetrics with "tmsh delete net cmetrics dest-addr 10.x.x.x" and disable this parameter with "tmsh modify sys db route.metrics.mtu value disable". After disabling this parameter problem was solved. But i could not understand the problem's exact reason and should i disable this parameter on other bigip devices? I could not find detailed technical documents about path mtu recovery and route.metrics.mtu on f5 side. Have you ever come across this kind of problem?

 

https://support.f5.com/kb/en-us/solutions/public/14000/500/sol14532.html?sr=43515503 https://support.f5.com/kb/en-us/solutions/public/13000/900/sol13948.html