I came across this article on Cisco’s site:
It’s a great read, and covers a lot of information that will help you minimize fragmentation issues across your network. It’s great for understanding when routers fragment packets and how to configure Path-MTU-Discovery. It’s also helpful if you have GRE or IPsec tunnels in your network.
One thing that stands out is to remember that Path-MTU-Discovery relies heavily on certain ICMP messages. Routers reply with “fragmentation needed and DF set” and you won’t hear about it if your firewall is blocking ICMP.