There are two ways you can change the default LCP MRU value.
You can override the MRU value by configuring the testvar lanMtu to a different value other than 'default'. This changes both the maximum size packet that will be used on the LAN as well as the LCP MRU.
testvar lanMtu 1440
In this example, the MRU used by PPP LCP will be 1440. CDRouter will not sent packets on the LAN greater than 1440 without IPv4 fragmentation.
The second option for changing the LCP MRU is to explicity set its value using the testvar pppForceLcpMRU. If this testvar is set, the PPP LCP MRU with be set based on the this value regardless of the lanMtu setting. This can be a work around for routers that will not negotiate a LCP MRU less than 1500 bytes.
testvar pppForceLcpMRU 1500
NOTE: The pppForceLcpMRU testvar requires CDRouter version 1.7.2 or greater.