1. Topology: it can be referred to the attachment. NE40-8 acts as PE and S8505 only serves as L2 transparent transmission.
2. S8505 attached VPN users can access most of websites, but they cannot open www.sina.com.cn, www.wowchina.com. The downloading speed is slow. One day they could open www.163.com,www.online.sh.cn, but the next day they cannot open www.163.com,www.online.sh.cn.
1. get packet at user access point: when PC accesses www.163.com and www.sina.com.cn, the result of MSS negotiation is 1460. When visiting www.163.com, the maximum of returned IP packet length total length=1492 bytes, while www.sina.com.cn returns tcp previous segment lost.
2. PC cannot ping -l 1472 -f www.sina.com.cn, but it can ping -l 1468 -f www.sina.com.cn.
3. Adjust mpls mtu of GE link between NE80E and GSR as 1530 and it is still problematic.
4. Change MTU as 1492 through the change of registration table and visit www.sina.com.cn again. It can open the webpage and visit www.wowchina.com.
5. The link between S8505 and the client connects to the client through O/E converter and NE80E/NE40-8/S8505 attaches VIP. MTU cannot be changed. The client decides to come the equipment room to handle the problem.
6. Attach PC under S8505 and simulate the user (MTU is adjusted as 1500). As a result it cannot visit www.sina.com.cn,www.wowchina.com and www.163.com,www.online.sh.cn.
7. Adjust MTU of GE link between NE80E and GSR as 1536 and increase MPLS MTU; it is still problematic. MTU is changed as 1600 and MPLS MTU is increased, it is problematic.
8. Adjust PE as NE80E and it is not problematic. Set one test user at the node.
9. The difference between PE as NE40-8 and PE as NE80E: GSR returns the packet to NE80E that is more than 4 bytes. NE80E, NE40-8 and S8505 are in the same equipment room while GSR is in other area, GE link between GSR and NE80E is problematic.
10. During the test, enable mirroring on NE40 and get packet between NE80E and NE40; after www.sina.com.cn returns continuation information of http, PC returns tcp dup ACK information.
11. Enable mirroring on NE80E and get packet that GSR returns to NE80E; but NE80E configures with rainer and it does not support mirroring.
12. Check optical module of NE80E and it is 10km. From the distance, there is transmission between NE80E and GSR. Contact the engineer and confirm such phenomenon. The transmission device is from Huawei and then the transmission engineer confirms MTU. MTU of GE is 1522 by default (transmission MTU is the maximum frame length that the link layer permits, not IP MTU of TCP/IP).
13. Ask client to increase MTU and change it as 9600. The problem is solved.
1. Common IPV4 user can access network normally. MPLS VPN users cannot access network normally. It is possible that Path MTU is too small.