I've got a VPN between two PC, based on OpenVPN (UDP).
Some time ago I've been using Linux-based DSL-router (on usual TI7300 MIPS processor). There was an check-box "Force MTU" and 1492 value for PPPoE. Everything have worked fine.
Now I've installed Cisco 1841 as router. I made following changes in addition to standart config:
ip policy route-map clear-df
route-map clear-df permit 10
match ip address 1
set ip df 0
access-list 1 permit any
But large packets can't pass through VPN until I set mtu 1400 on OpenVPN tunnel interfaces on both sides.
ICMP is open.
Since everything was fine on old BusyBox, I decide that Cisco1841 does not fragment packets even though DF->0 route-map is using.
May be someone have suggestions...
And remember - UDP!