The MTU for the EoIP tunnel is 1500. Our working EoIP links are configured like this:
Radio on each end is assigned a static IP address, for example 10.1.1.1 at the AP and 10.1.1.2 at the station.
PPTP server is configured on AP end, and a user account(secret) is created that assigns an IP address to both the server and client end of the PPTP tunnel, for example, 10.2.1.1 on the server end of the PPTP link, and 10.2.1.2 on the PPTP client end of the link. The PPTP tunnel is encrypted. This has created a secure PPTP tunnel over the wireless link, with an MTU of 1460.
Of course, next I create a PPTP client, and have it dial the address of the AP's radio, which is 10.1.1.1 in this example. With the proper username and password in the client, the AP and station ends of the PPTP tunnel are assigned their 10.2.1.x addresses.
Next, I create an EoIP interface on the AP, and one on the client. The target address of each end of the EoIP tunnel is the IP address of the PPTP tunnel on the other end. In other words, the EoIP interface on the AP has a remote-address=10.2.1.2, and the EoIP interface on the client has a remote-address=10.2.1.1, creating an EoIP tunnel inside of the PPTP tunnel. The MTU of the EoIP tunnel is 1500.
From many readings of the MT documentation, I have gathered that the whole point of an EoIP tunnel is to allow unfragmented packets to travel over other links that may not be able to otherwise transfer unfragmented packets, and then to bond both ends into a "virtual" ethernet cable. Traffic immediately through an EoIP tunnel should not need to be specially handled, as the EoIP interface is supposed to fragment full ethernet packets to better fit through a "smaller" tunnel, and then reassemble the packets back to their original size transpartently on the other end. This was my understanding, anyhow, and in the above described example, it works perfectly.
The problems begin only when I try to do the same thing above using a PPPoE tunnel in place of the PPTP tunnel. The PPPoE tunnel that the EoIP tunnel is being created over has an MTU of 1488, but I have tried many different MTUs, and that should not matter anyway to an EoIP tunnel, as the EoIP tunnel's job is to deal with outside tunnels that are smaller than the packets that travel through the EoIP tunnel....
Isn't the purpose of the EoIP tunnel to allow full sized packets to pass, no matter what?
Edit: Also, both ends have an identical tunnel ID, and very different MAC addresses....