Static Users under MA5200G cannot Send Mails with Attachments Because of MTU Val

Created: Apr 7, 2016 01:55:47Latest reply: Apr 7, 2016 04:05:52 837 1 0 0

Version: Independent of version.
Topology: users-L2-MA5200G-(POS)-S8505-Internet 
Symptom: Static users under MA5200G cannot send mails with attachments, but PPPOE users could. 

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Alan_brown     Created Apr 7, 2016 04:05:52 Helpful(0) Helpful(0)

1. From the symptoms of failure, PPPOE users could send mails normally, but static users could not; the difference between both users lies in the format and length of packet, so the problem may relate to fragments of packets.
2. According to checkup for topology, it is found that MA5200G connects to S8505 at the peer through POS port. If the packets are fragmented at POS, the problem alike may occur.
3. According to checkup for equipments at both sides, it is found that the MTU of POS port of S8505 at the peer is set to 1500, and that of MA5200G POS is set to 1492. The services are recovered by changing the MTU of POS port of MA5200G at the local to 1500.

For such failures that mails with attachments cannot be sent or large file cannot be loaded, they are often resulted from fragments of packets. The possible reasons include:
1. The network contains O/E converter, protocol translator, 2M transmission equipments, of which the early delivered do not support VLAN because packets of VLAN users are very large and they are discarded.
2. The network has a POS port, and its MTU of POS is set to 1492 in some equipments, and to 1500 in other equipments, so the difference in setting of both sides will cause the packets to be fragmented or discarded. The case is caused by this reason: the MTU of POS port of 8505 at the peer is set to 1500, and the MTU of MA5200G POS uses the default 1492; however, the users are static, so they use the default 1500 as MTU, and the big packets will be fragmented during sending out. For PPPOE users, the MTU after PPPOE negotiation is usually 1492, so they will not meet such a problem.

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