The default Transmission Control Protocol (TCP) maximum segment size (MSS) setting for a Cisco router that is originating data destined for a remote IP network is 536 bytes. The MSS is specified in the TCP SYN packet during the TCP handshake. MSS values can be used to restrict outgoing TCP segments to a segment size that is small enough to avoid fragmenting the IP datagram, thereby avoiding the performance problems that can occur as a result of IP fragmentation.

The smallest maximum transmission unit (MTU) that can be used on an IPv4 network is 576 bytes. The 536byte default MSS value is therefore derived by subtracting the 20byte TCP header and the 20byte IP header from that MTU value.

It is important to note that some firewall rules are capable of stripping TCP options from a segment. If a firewall is configured to strip TCP options from a segment, the MSS value that is applied to a TCP segment by the router will not be used. If you have NetFlow enabled, you can issue the show ip cache flow command to view statistics that include IP packet size distribution.

The default TCP MSS setting for a Cisco router that is sending data destined for a local LAN is 1460 bytes. The typical default MSS value for PCs communicating on a LAN is 1500 bytes.

The lowest value you can use to enable an MSS for TCP connections that originate from a router is 68 bytes. To configure an MSS value for TCP segments that originate from a router, you should issue the ip tcp mss mssvalue command in global configuration mode.

Router(config)#ip tcp mss ? <68-10000> MSS

By issuing the ip tcp adjust-mss mssvalue command (where mssvalue is a value in the range from 500 through 1460) in interface configuration mode, you can configure an MSS for TCP segments that do not originate from the router but that are being forwarded by the router.

The highest value you can use to enable an MSS for TCP connections that originate from a router is 10000 bytes. To configure a 10000byte MSS value, you should issue the ip tcp mss 10000 command in global configuration mode. However, you cannot configure a 10000byte MSS for TCP segments that are simply being forwarded on a router interface, because the maximum MSS you can configure for TCP segments that are being forwarded on a router interface is 1460.

Router(config-if)#ip tcp adjust-mss ? <500-1460> Maximum segment size in bytes