The answer is 1448. In a standard setup the maximum payload for a ICMP packet will be 1472 bytes (1500-20-8). 28 bytes for the IP (20) and ICMP (8) headers.
If you run GRE tunnels, you need to account for a 24 extra bytes overhead for Outer IP(20) and GRE(4).
1500-20-8-20-4=1448
about 1 year ago