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FAQ-Web Pages Cannot Be Displayed Completely Owing to the MTUs

Publication Date:  2012-07-27 Views:  42 Downloads:  0
Issue Description
In an office, an NE40E V300R002C06B285 serves as a PE, 7750 devices of company A serve as Ps, and the NE40E are connected to different upper-level 7750 devices through six links to implement interconnection over a VPLS network in a city When a branch accesses the servers at the headquarters, some login dialog boxes cannot be displayed. Such a case occurs after addresses are reallocated by the company. 
 
Alarm Information
 Null 
Handling Process
1. View the MTU on each interface of the NE40E. They are all 1500 bytes by default.
2. Six uplinks are connected from the NE40E to different 7750 devices of company A. The MTU on some interfaces interconnected to the NE40E is the default value 9212 bytes and the MTU on the other interfaces is 1514 bytes.
3. Change the MTU from 1514 bytes to 1522 bytes. The problem is solved. 

 
Root Cause
 It is doubted that some Web pages cannot be displayed owing to the MTU. 
Suggestions
 1. The MTU defined by company A is different from what is defined by Huawei. The MTU defined by company A refers to the entire packet length, and is 14 bytes greater than the default MTU (1500 bytes) defined by Huawei. The default MTU defined by company A is 9212 bytes. If there are multiple uplinks on the NE40E, packets with different destination addresses take different paths after being hashed. Supposing that the MTU on a link that packets take is 1514, the size is greater than 1514 after a 4-byte label is added to 1514-byte packets and the 7750 device discards these packets. Therefore, some Web pages cannot be displayed after the customer reallocates addresses.
2. If the length of packets on the interface is greater than the MTU, the 7750 device of company A can receive them, but the 7750 device neither fragments them nor forwards them. Like the popular processing mechanism in the industry, the NE40E does not fragment these packets but can directly forward them.
3. Since two MPLS labels (eight bytes in total) need to be added to the Ethernet packet header for the VPLS service, the interface MTU of the SR12 needs to be changed to 1500 + 14 + 8= 1522 if the NE40E uses the default MTU 1500. 

 

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