Have 3 devices:
WAN PPP interface with static external IP 188.8.131.52 & real MTU/MRU 1500
WAN PPPoE interface with static external IP 184.108.40.206 & real MTU/MRU 1492
WAN ethernet interface with static external IP 220.127.116.11 & real MTU 1500
All devices with ROS 6.33.5
From DEVICE 1 established IPIP tunnels to DEVICE 2 & to DEVICE 3
Tunnels parameters are absolutely identical for all devices (except for the IP address & MTU).
While examined the best possible MTU for all tunnels set it all to 1500 and started Ping with DF flag from DEVICE_1 to tunnel IP address of DEVICE_2.
Found that, contrary to the logic, tunnel from D_1 to D_2 passes packets up to 1500 bytes without fargmentation, but tunnel from D_1 to D_3 passes packets up to 1480 bytes, which in general is normal. But I am confused by the behavior of the tunnel from D_1 to D_2. I thought that this tunnel dynamically fragmenting packets at the time of encapsulation to be assembled on the end. Set tunnel MTU to 1600 only confirmed my assumption (and even delighted). But tunnel between D_1 and D_3 still passed whole packets maximum of 1480 bytes
After reading a lot of documentation I thought possible provider of D_3 just removes the DF flag from incapsulated packets (it happens), but assumption has not been confirmed because packets more than 1600 bytes are discarded anyway.
Could not find anything about automatic fragmentation at the documentation about IPIP tunnels.
It is also not clear what is the reason that this mechanism (if any) does not work for the tunnel between D_1 and D_3.
How did it actually?
Trying to examine the document but think not quickly able to understand it, and not the fact that it will help: http://www.cisco.com/c/en/us/support/do ... pfrag.html
Next, I try to put all the settings that can be involved.