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The Web Server Can Be Pinged by the NE80E VPLS but No Web Page Can Be Displayed

Publication Date:  2012-07-27 Views:  62 Downloads:  0
Issue Description
The following is the network diagram:
NE80E1-G3/0/0-----T320-------T320----NE80E2----server
|
pc
Martini VPLS is run between NE80Es and the devices of Manufacturer J. The server can be pinged from the PC, but the web pages provided by the server cannot be opened. 
Alarm Information
 Null 
Handling Process
1. Perform local switching on the PE connected to the Web server. The web pages provided by the Web server can be opened on the attached PC. Therefore, the problem about the Web server is ruled out. Since the VPLS service is normal, the problem about configurations is also ruled out. No web page can be opened because large-size packets cannot be directly transmitted owing to the MTU supported by the devices of manufacturer J and they are fragmented before being transmitted to the Web server.
2. Change the MTU on the devices of manufacturer J. The MTU on the NE80E was then 1500 while the MTU on the devices of manufacturer J was 1514. After the MTU on the NE80E is changed to 1514, the problem still exists and the OSPF state between the NE80E and the device of manufacturer J is changed to exchange. Later it was learned that the MTU 1514 on the devices of manufacturer J referred to the layer 2 MTU. The customer took 1514 for the layer 3 MTU. Then, the customer changed the MTU on NE80E to 1514 for the purpose of consistent MTUs on the two sides. Thus, the OSPF state between NE80E and the devices of manufacturer J was changed to exchange. Let the MTU on NE80E remain 1514 and change the MTU on manufacturer J’s devices to 1528. The OSPF state becomes normal. 1500-byte packets, however, cannot be pinged during a VPLS test. Observe the MTU on the interface of the devices of manufacturer J and find protocl mpls MTU=1502. Theoretically, MPLS packets are encapsulated into VPLS packets after a label is appended to MPLS packets. The MPLS layer MTU must be greater than or equal to 1504 so that 1500-byte MPLS packets can be transparently transmitted. There is only a difference of two bytes between 1502 and 1504.Change the layer 2 MTU on the devices of manufacturer J to 1552. After that, large packets can pass and Web pages can be opened. 
Root Cause
 The possible causes are as follows:
1. The P and PE are configured incorrectly.
2. The Web server is faulty.
3. The link MTU is set incorrectly. 
 
Suggestions
 1. The RFC requires that the MTU in DD packets needs to be negotiated. If the MTU in DD packets is less than the MTU on the local interface, the neighbors are disconnected. If ospf mtu-enable is not configured on the interface, the negotiated MTU is ignored.
2. The negotiation principle for the MTU in DD packets varies with manufacturers.
3. The MTU definition also varies with manufacturers. Before connecting a Huawei device to a third-party’s device, make sure that the MTU on the third-party’s device refers to the layer 3 MTU or layer 2 MTU. 

END