It is about a IPSEC/GRE over WAN...
Would you please confirm or comment the following in terms of MTU:
1. On GRE tunnel interfaces "ip mtu" and "ip tcp adjust-mss" is mandatory. "tunnel path-mtu-discovery" is good to have and will allow DF bit to be set in the outer header. If "tunnel path-mtu-discovery" is to be applied, ICMP should not be blocked between routers.
2. On inside router interfaces "ip tcp adjust-mss" is mandatory and will be the same value as on the tunnel interfaces. This will make sure TCP traffic from inside hosts is OK.
3. It is mandatory that ICMP messages are not blocked between inside hosts and WAN routers in order for PMTUD for hosts to be working.
Thanks in advance,