Chapter 1: Configuration Configuring IP and Ethernet interfacesPage 1-62Use User Defined MTU allows the operator to specify an MTU value to useto override any MTU that may be determined in the LCP phase of PPPoEsession setup. If this is selected, the user is able to enter an MTU value up to1492. However, if the MTU determined in LCP negotiations is less than thisuser-specified value, the SM will use the smaller value as its MTU for thePPPoE link.Timer Type Keep Alive is the default timer type. This timer will enable a keepalive thatwill check the status of the link periodically. The user can set a keepaliveperiod. If no data is seen from the PPPoE server for that period, the link istaken down and a reconnection attempt is started. For marginal links, thekeep alive timer can be useful so that the session will stay alive over periodicdropouts. The keepalive timer must be set such that the session can outlastany session drop. Some PPPoE servers will have a session check timer oftheir own so that the timeouts of the server and the SM are in sync, toensure one side does not drop the session prematurely.Idle Timeout enables an idle timer that checks the usage of the link from thecustomer side. If there is no data seen from the customer for the idle timeoutperiod, the PPPoE session is dropped. Once data starts flowing from thecustomer again, the session is started up again. This timer is useful for userswho may not be using the connection frequently. If the session is idle forlong periods of time, this timer will allow the resources used by the sessionto be returned to the server. Once the connection is used again by thecustomer, the link is reestablished automatically.Timer Period The length in seconds of the PPPoE keepalive timer.TCP MSS Clamping If this is enabled, then the SM will alter TCP SYN and SYN-ACK packets bychanging the Maximum Segment Size to be compatible with the currentMTU of the PPPoE link. This way, the user does not have to worry about MTUon the client side for TCP packets. The MSS is set to the current MTU – 40(20 bytes for IP headers and 20 bytes for TCP headers). This will cause theapplication on the client side to not send any TCP packets larger than theMTU. If the network is exhibiting large packet loss, try enabling this option.This may not be an option on the PPPoE server itself. The SM will NOTreassemble IP fragments, so if the MTUs are incorrect on the end stations,then MSS clamping will solve the problem for TCP connections.
