Fault Symptom
When the Huawei device networks with devices from other vendors deploy Layer 3 MPLS IPv6 VPN service by using the Ethernet interface, it is found that the packet larger than 1492 bytes cannot be transmitted between private network users. Users cannot access certain websites or download files through FTP.
Run the ping command, and find that the ping fails when the payload of the specified ICMP is larger than 1464 bytes.
Fault Analysis
1. The default MTU of an Ethernet interface is 1500 bytes. When forwarding data, MPLS IPv6 VPN inserts a 4-byte or 8-byte MPLS packet header between the IP header and the Layer 2 frame header. That is, a 4-byte label is added during the forwarding between the penultimate hop and the tail-end hop; a 8-byte label is added in data forwarding between other P devices.
2. The link layer does not know the MPLS processing. By default, the link layer still receives data packets with the maximum size of 1500 bytes. Then, packets of 1492 to 1500 bytes is too long after the MPLS packet header is added to the packets. Consequently, the link layer cannot receive them, and data forwarding is adversely affected.
Procedure
1. Adjust the MTU value of the physical interfaces on other vendors' devices. The MTU value should be at least 1508 bytes.
2. By default, an Ethernet interface on the Huawei device can send and receive large frames. No adjustment is required on the Huawei device.
Summary
When large packets cannot be received, check whether the MTU of the inbound interface is too small.
