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An MA5200G and a Non-Huawei Router Cannot Ping Each Other by Sending Large ICMP Packets due to MTU Restriction

Publication Date:  2013-01-07 Views:  58 Downloads:  0
Issue Description

Networking: MA5200G---(WDM device)---1XXXX (a non-Huawei device)

Version: MA5200G V3.1 2332

Problem description: When the MA5200G pings the IP address of the interface on the 1XXXX by sending ICMP packets larger than 1472 bytes, the ping operation fails. When the 1XXXX pings the IP address of the interface on the MA5200G by sending packets larger than 1500 bytes, the ping operation fails.

Handling Process

Check the MTU value of each interface on the MA5200G and the 1XXXX and find that both the MTU values are 1548.

When the MA5200G pings the IP address of the interface on the 1XXXX by sending ICMP packets of 1473 bytes, the ping operation fails. This is because the IP packet length = 1473 bytes (the length of a packet at the application layer) + 8 (the length of the ICMP packet header) + 20 bytes (the length of the IP packet header) = 1501. The IP packets smaller than 1548 bytes are not fragmented.

The length of the data frame = 1473 bytes (the length of a packet at the application layer) + 8 bytes (the length of the ICMP packet header) + 20 bytes (the length of the IP packet header) + 18 bytes (the length of the frame header and trailer) = 1519 bytes. The transmitting device (the WDM device) does not support 1519-byte frames. The data frame is thus discarded by the WDM device. As a result, the ping operation fails.

When the MA5200G pings the IP address of the interface on the 1XXXX by sending ICMP packets of 1472 bytes, the ping operation succeeds. This is because the length of the data frame = 1472 bytes (the length of a packet at the application layer) + 8 bytes (the length of the ICMP packet header) + 20 bytes (the length of the IP packet header) + 18 bytes (the length of the frame header and trailer) = 1518 bytes. The transmitting device (the WDM device) supports the maximum of 1518-byte frames.

When the 1XXXX pings the IP address of the interface on the MA5200G by sending packets of 1501 bytes, the ping operation fails. This is because the packets sent in the ping operation are considered as IP packets by default. Additionally, the length of the frame header and trailer at the data link layer is 1519 bytes. Therefore, the packets are discarded on the transmitting device, causing the ping operation to fail. By default, packets used in the ping operation are packets at the application layer on the MA5200G. The packet is thus added with headers at the transport layer and the network layer.

Root Cause

The ping operation fails due to the MTU restriction.

Solution

Change the MTU value of the interface through which the MA5200G and the 1XXXX are connected to 1500 bytes. In this way, the MA5200G can ping the 1XXXX by sending packets larger than 1472 bytes and the 1XXXX can ping the MA5200G by sending packets larger than 1500.

At the network layer, IP packets longer than 1500 bytes are fragmented before the 18-byte frame header and trailer are encapsulated into the frame. The frame is 1518 bytes, which equals the length of the longest frame that the transmission device supports. The ping operation thus succeeds.

Suggestions
When the MA5200G pings the 1XXXX, the specified length is the length of the data field in an ICMP packet. When the 1XXXX pings the MA5200G, the specified length is the length of the IP packet.

END