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XSR CLI Reference Guide 5-165
ip tcp adjust-mss
ThiscommandsetstheMaximumSegmentSize(MSS)forTCPSYN(synchronize)packets.When
theXSRterminatesPPPoEtraffic,aPCconnectedtotheFastEthernetinterfacemayhaveproblems
accessingWebsitesifthePCʹsMaximumTransmissionUnit(MTU)settingistoohigh.TheMTU
containsmaximumsegmentsize(MSS)
valuesforTCPpacketstransmittedbythePC.
SomeWebsitesdonotperformPathMTUdiscoverycorrectly.Toaddressthisissue,theXSR
automaticallysetstheTCPMSSto1452whenusingPPPoEports.ThisforcesbothTCPpeersto
send1492bytepacketssoPathMTUdiscoveryneverhas
todealwithPPPoEʹs1492‐byteMTU.
Thisisasub‐commandofInterfa cemodeandisconfiguredwiththefollowingcommands:
• interface fast/gigaethernetx.x
• ip address negotiated
• encapsulation ppp/mux pppoe
• ip mtu 1492
• ip tcp adjust-mtu 1400
SettingtheMSSwillcauseallTCPSYNpacketswiththeMSSoptionbeingmodifiediftheoption
valueexceedstheconfiguredMSS.
Syntax
ip tcp adjust-mss mss
Mode
PPPoEInterfaceconfiguration:XSR(config-if)#
Default
1452bytes
Example
ThefollowingexampleconfiguresaPPPoEclientwithanMSSof1452bytesonF1.1:
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
RangeofMSS:512to1452.