The Transmission Control Protocol (TCP) Maximum Segment Size (MSS) Adjustment feature enables the configuration of the maximum segment size for the transient packets that traverse a router, specifically TCP segments in the SYN bit set, when PPP over Ethernet (PPPoE) is used in the network. PPPoE truncates the Ethernet maximum transmission unit (MTU) 1492, and if the effective MTU on the hosts is not changed, the router in between the host and the server can terminate the TCP sessions.
The ip tcp adjust-mss command specifies the MSS value on the intermediate router of the SYN packets in order to avoid truncation.
The MSS is not supported on the Catalyst 3750 (Cat3750) series switches in order to adjust the MSS value of the TCP SYN packets.
The error counters that increase on the Cat3750 series switch, such as oversize or giants, confirm that the problem is with the MTU. The only solution is to set the MTU correctly all along the path.
Refer to these documents for more information: