Incoming packets from clients, in addition to the IP and TCP headers normally present, contain a GRE header.
The default
MSS value of 1460 doesn’t allow sufficient space for this additional GRE header, which causes IP packet fragmentation and packet reassembly failures.
The presence of the encapsulating GRE header (28 bytes) makes the total packet length equal to:
1460 bytes (payload: default MSS) + 20 bytes (TCP header) + 20 bytes (IP header) + 28 bytes (GRE header) = 1528 bytes
When the MTU is 1500, the now 1528-byte packets must be fragmented at the IP layer. The introduction of other fragmented packets has a direct effect on any upstream networking equipment, and the appliance itself.
Under light loads, fragmentation can be handled reasonably well. Under higher traffic loads, a linear increase in traffic volume leads to an exponential increase in reassembly failure rates. This increase often results in erratic and unpredictable results on the network.
