What is default value for HDLC MTU?

 If fragmentation is not supported, the two Frame Relay endpoints MUST support an MTU size of at least1526 which is based on adding the PPP Max-Rceive-Unit size with the PPP header size with the Max L2TP

 Header Size with the Frame Relay header size (PPP header size is the protocol field size plus HDLC framing bytes, which is required by L2TP). 

To avoid packet discards on the Frame Relay interface, the RECOMMENDED default Frame Relay MTU is1564 based on a PPP default MRU of1500.  The means to ensure these MTU settings are left to   implementation.

1564 Bytes