l2tp mtu(国外英文资料).docVIP

l2tp mtu(国外英文资料) Hello, yesterday met a user fault reporting, finished processing, to sum up, some doubts can not be confirmed, please help analysis. The user uses the VPDN for ADSL access, and the topology is described below: Central point SQL server - switch - HUAWEI 2631 router (LNS) - fiber - Metro network switches - Metro Network BAS (LAC) / HUAWEI 8850 point router, HUAWEI 2611-- users, PC The subscriber uses 2611 to dial PPPoE, and the address is converted after dialing. The PC equipped with SQL client software. After the branch point router dials, the user PC can access the central point of the SQL server, the Ping pass, but the SQL client cannot connect to the central point server. With Ping -l 1500 impassability, gradually reduce the message length of Ping, and finally confirm that the maximum can only be Ping 1434 length. Analysis of the reasons, because the user uses VPDN access center PPPoE, the user sends the message length in the LAN MTU for 1500 MSS was 1460, but due to the router of the PPPoE package, the user sends messages outside with PPPoE length 6byte+ppp length of 2byte, to the end user message transmission path maximum transmission unit reduces 8byte, BAS and terminal user center point LNS L2TP tunnel is established, BAS end user message PPPoE, and packaged into a L2tp message and then encapsulated into a layer of IP packet transmission to the user center router released after the L2TP package, turn into a normal Ethernet message sent to the SQL server. Look at the encapsulation of the ICMP packets of the Ping central server of the user The following intermediate L2TP package: IP 20byte+udp 8byte+L2TP 8byte+ppp 2 layer tunnel tunnel byte+ IP 20byte + ICMP 8byte =======66byte that is why the user can only Ping length below 1434, the client SQL client initiates a connection message length is greater than the data transfer can not be normal, but we know the mechanism of the adaptive network the transmission medium IP message transfer process,