1. Topology A: PC-S8016-NE80-M40-GSR, enable MPLS.
Topology B: PC-S8016-NE80-M40-transmission--transmission-GSR, enable MPLS.
2. PC can access www.huawei.com but cannot open support.huawei.com、www.sohu.com、www.online.sh.cn. These sites can ping through small packets.
1. At first users provide topology A and consider interconnection of NE80 and M40 MPLS.
2. Engineers can access network, but client engineers, GSR engineers and M40 engineers cannot. Disable MPLS LDP of NE80 and M40, all users can access network. Ping external network and packets more than 1468 bytes cannot be pinged.
3. MTU of MPLS is problematic. Change physical MTU of GE port at M40 as1600. MTU of MPLS is1522 and MTU of inet keeps 1500. It is still problematic.
4. Adjust physical MTU of GE port at M40 as1522, mtu of inet 1500 and MTU of MPLS as1508. MPLS MTU of GE port at downstream of GSR is set as 1508, MTU as 1500 and physical port MTU as 1522. Because Ethernet frame of M40 does not compute CRC with 4 bytes. It is still problematic.
5. Enlarge the link of GSR and M40, physical MTU, MPLS MTU of M40. It is still problematic.
6. Mirror at NE80 and capture dual-packet of GE port at NE80.
PC has TCP syn packet and the website responds tcp ack packet. PC sends http get packetbut does not receive http continuation packet of the website. My computer is normal and DF field of returned packet is not changed. If the packet is larger than 1468 bytes, there is transmitted echo request packet and no echo reply packet.
7. The device at upstream is problematic. Change MTU of the link of GSR and M40 as1550. GSR and IS-IS neighborhood of M40 cannot tbe built. Change MTU of the port at downstream of GSR as 1500 and IS-IS neighborhood is built at once. When IS-IS neighborhood cannot be built, return route will go to another GSR and will not choose interconnected link of GSR and M40, all users can access network.
9. The topology is actually topology B. Between then there is transmission of Z client and MTU of GE port is 1500.
10. Capture packets of engineers and client engineers and it is found that SYN of engineers carries MSS=1260, ACK of website carries 1460 and www.huawei.com carries 536. SYN of client engineers carries MSS=1460 but ACK of website carries vacant value. Considering MSS=MTU-20 (TCP header)-20 (IP header) and transmission MTU=1500. Packet is discarded at transmission, use the following ways:
1) Change MTU of GSR and M40 as 1496 or 1492.
2) GSR naked fiber and M40 interconnect and not transmit.
11. Use the second ways and all users can access network normally.
12. MTU of transmission device of Z company is 1500 and cannot be changed.
1. When clients accesses network, it is asymmetric traffic and packet. Traffic at upstream is little and packet is small (only several hundred bytes).
2. When PC and network establish FTP connection, SYN packet carries MSS parameter to negotiate the maximum packet length, MSS=MTU-20 (TCP header)-20 (IP header) and gets the smaller one.）If the server agrees MSS parameter, ACK packet does not carry MSS parameter or carry its own MSS, client makes the choice.
3. Once two ports negotiate MSS value, it will choose this one as the maximum packet to send and avoid fragmentation.
4. In topology B, enable MPLS. Link layer GE payload supports 1504 bytes at least. If the server returns packet with 1460 bytes, it sends Ethernet interface as 1504. At the moment transmission MTU is 1500. Transmission does not recognize MPLS protocol or support fragmentation. The packets will be discarded.
5. Engineers can access network because negotiated MSS is not larger than1260 bytes.
6. All clients can access www.huawei.com because negotiated MSS is not larger than 536 bytes of www.huawei.com.
It is suggested to upgrade transmission device and support the maximum MTU. In order to make MPLS VPN load other service, MTU of Ethernet should not less than 1508 bytes.