Server--AR2831 (LNS) --S8512―8850 (LAC)--S8016---DSLAM---PC
The PC obtains the private IP address of the AR2831 through dialing up. The PC can ping through the IP address of the server on the private network that connects the AR2831. When the PC tries to download files from the server through FTP, the files in the server cannot display. It is displayed on the PC that it continues searching the server. The FTP connection then times out after a while.
To address the preceding causes, the following solutions are provided:
1. Log in to the intermediate devices between the PC and the server to check the ACL configurations. ACLs with ports 20 and 21 are not filtered. As a result, the problem is not caused by ACL configurations.
2. It is confirmed with engineers of the 8850 that the VPDN data is correctly configured.
3. When transmitting L2TP services, the AR2831 adds xx bytes into the packet header. It is possible that the 8850 discards the packets because the MTU becomes larger in the VT template. Change the MTU in the VT template to 1442. Then, the PC can download files from the server through FTP.
The PC can dial up successfully and obtain the private IP address of the AR2831. The PC can also ping through the IP address of the AR2831 server. This indicates that the L2TP is correctly configured on the AR2831. The causes of failing to download files from the server through FTP are as follows:
1. Ports 20 and 21 are adopted by the FTP to provide services. It is possible that port 21 is filtered by the intermediate device through ACL.
2. The VPDN data is wrongly configured on the 8850.
3. The MTU in the VT template on the AR2831 is incorrect.
During daily maintenance, if some webpages cannot be opened or download cannot be performed through FTP, it may matter the MTU on the interfaces.