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Certain Webpages Cannot Be Opened After GRE Tunnel Is Enabled on the NE40

Publication Date:  2012-07-27 Views:  54 Downloads:  0
Issue Description
Version: Version 5.30 (NE40&80 V300R002C01B599)
Networking: PC-------NE20----Network------NE40----------Web Server
Symptom of the fault: The user cannot open the webpages of the web server though even large packets can be successfully pinged. 
 
Alarm Information
Null
Handling Process
1. When the GRE tunnel is not enabled between the NE20 and the NE40, webpages can be normally opened.
This indicates that the failure to open webpages is related to GRE.
2. Reset the GRE tunnel at both ends. Change the next hop of the network segment of the PC on the NE40 to a physical port so that the PC does not transmit packets through the tunnel.
In this case, webpages can be normally opened. This indicates that the fault is probably related to the tunnel processing mechanism of the NE40.
3. Change the next hop of the route on the NE20 to a physical port. After the change, the NE40 transmits packets downstream still through the tunnel.
In this case, webpages still cannot be opened.
4. Through the preceding analysis, it is deduced that the fault is related with the tunnel processing mechanism of the NE40.
5. Capture packets for analysis. The result shows that the size of a TCP packet is 1460 bytes. Plus the 20-byte TCP header and the 20-byte IP header, the size of a TCP packet reaches 1500. Packets sent from the NE40 are subject to GRE encapsulation. Therefore, the size of a packet reaches 1532.
6. Change the MTU value to 1532 and enable the GRE tunnel at both ends.
After the preceding operations, webpages can be normally opened. 
 
Root Cause
After the GRE tunnel is enabled on the NE40, the size of packets exceeds the MTU value, 1500, because GRE encapsulates a 20-byte header to each packet. 
Suggestions
GRE features unidirectional tunnels. During troubleshooting, you need to analyze both incoming and outgoing packets. Because GRE encapsulation increases the length of a packet, you also need to consider the MTU value. Through analysis of the packets captured, you can learn the size of packets and find out whether outgoing packets are transmitted normally. 

END