Enterasys Networks X-Pedition XSR CLI - Manual (Page 241)

Other IP Commands
XSR CLI Reference Guide 5-165
ip tcp adjust-mss
This command sets the Maximum Segment Size (MSS) for TCP SYN (synchronize) packets. When
the XSR terminates PPPoE traffic, a PC connected to the FastEthernet interface may have problems
accessing Web sites if the PCʹs Maximum Transmission Unit (MTU) setting is too high. The MTU
contains maximum segment size (MSS) values for TCP packets transmitted by the PC.
Some Web sites do not perform Path MTU discovery correctly. To address this issue, the XSR
automatically sets the TCP MSS to 1452 when using PPPoE ports. This forces both TCP peers to
send 1492 byte packets so Path MTU discovery never has to deal with PPPoEʹs 1492‐byte MTU.
This is a sub‐command of Interface mode and is configured with the following commands:
• interface fast/gigaethernetx.x
• ip address negotiated
• encapsulation ppp/mux pppoe
• ip mtu 1492
• ip tcp adjust-mtu 1400
Setting the MSS will cause all TCP SYN packets with the MSS option being modified if the option
value exceeds the configured MSS.
Syntax
ip tcp adjust-mss mss
Mode
PPPoE Interface configuration: XSR(config-if)#
Default
1452 bytes
Example
The following example configures a PPPoE client with an MSS of 1452 bytes on F1.1:
XSR(config-if<F1.1>)#ip address 192.168.100.1.255.255.255.0
XSR(config-if<F1.1>)#ip tcp adjust-mss 1452
XSR(config-if<F1.1>)#no ip address
XSR(config)#interface dialer 1
XSR(config-if<D1>)#ip address negotiated
XSR(config-if<D1>)#ip mtu 1492
XSR(config-if<D1>)#ip nat outside
XSR(config-if<D1>)#encapsulation ppp
XSR(config-if<D1>)#dialer pool 1
XSR(config-if<D1>)#dialer-group 1
XSR(config-if<D1>)#ppp authentication pap
XSR(config-if<D1>)#ppp pap sent-username frizz password 7 141B1309000528
XSR(config)#ip nat inside source list 101 dialer 1 overload
XSR(config)#ip route 0.0.0.0.0.0.0.0 Dialer1
XSR(config)#access-list 111 permit ip 192.168.100.0.0.0.0.255 any
mss Range of MSS: 512 to 1452.