Starting with ExtremeXOS 31.6, GRE tunnels support TCP Maximum Segment Size (MSS) adjustment. Turning on the TCP MSS adjustment on a GRE tunnel redirects TCP SYN and SYN ACK packets going into the GRE tunnel to the Control Protocol (CP) via the ACL implemented on fastpath. When passing the IP stack on CP, the netfilter rule is applied to those packets. If the TCP MSS value in the option is greater than the configured adjustment, the TCP MSS option value is rewritten to the configured one. However, if the option value is less than the configured value, there is no change. Doing this helps avoid end-to-end over GRE tunnel application failure.
When the TCP MSS adjustment is turned off, there is no interference on TCP SYN and SYN ACK packets going to the GRE tunnel since the ACL is removed. Those packets are fastpath forwarded.
You can turn on or off TCP MSS adjustment using the configure tunnel tunnel_name ip tcp adjust-mss [off | on tcp_mss_value] command for IPv4, or the configure tunnel tunnel_name ipv6 tcp adjust-mss [off | on tcp_mss_value] command for IPv6. The tcp_mss_value must be specified when turning the adjusment on.