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Fragments and CRC Errors Increased on a 10M Electrical Port on NE40E After a Service Cutover

Publication Date:  2013-09-25 Views:  15 Downloads:  0
Issue Description
Some of Vendor C's devices were replaced by Huawei NE40Es at site C. Huawei and vendor C's equipment were interconnected using 10M electrical modules. After a service cutover, fragments, undersized packets, and CRC errors gradually increased on the NE40E.
Handling Process

Errored packets were generally caused by poor link quality. In this case, the network cables and modules were reused from vendor C's equipment, which had proved reliable. The links also worked properly.
Huawei suspected that port working modes were inconsistent at both ends. The configurations of the peer NE were unknown, so Huawei completed the following steps to infer the working mode of the peer port.
1. Checked the working mode of the port on NE40E.
The port on the NE40E was configured to work in 10M full-duplex mode. The ports at both ends were up, indicating that port rates were normal.
2. Suspected that the ports at both ends worked in different duplex modes.
The collision detection mechanism is effective for the half-duplex mode, whereas the full-duplex mode does not have such a mechanism. If a port that works in half-duplex mode and is sending packets detects packet sending by the peer end, the port stops sending packets. As a result, the peer end receives incomplete packets.
3. Set the port on the NE40E to work in auto-negotiation mode.
The port on the NE40E was up and worked in 1G full-duplex mode. This negotiation result indicated that the peer port supported the 1 Gbit/s rate and was enabled with auto-negotiation.
4. Confirmed that the peer port worked in half-duplex mode when the problem occurred.
According to the electrical port auto-negotiation mechanism in the IEEE 802.3 protocol, a negotiation result is based on the port with lower capability.
When the port on the NE40E was set to work in full-duplex mode and the peer port was set to work in auto-negotiation mode, as was the case in this problem scenario, the duplex mode negotiation result for the peer port was half-duplex.

The following explains why no packet errors occurred on the peer port:
Vendor C's equipment: Either the rate or the duplex mode of a port can be configured with a fixed value or configured to work in auto-negotiation mode.
Huawei's NE40E: Both the rate and duplex mode of a port are configured with fixed values or configured to work in auto-negotiation mode.
Root Cause
The ports at both ends worked in different modes.
Solution
a. Set the port on the NE40E to work in auto-negotiation mode, so both ports work in 1G full-duplex mode.
b. Set the port on the NE40E to work in 10M half-duplex mode, so both ports work in 10M half-duplex mode. (Note: Some equipment may not support the half-duplex mode.)
Suggestions
Before a service cutover in such a scenario, obtain port configurations at both ends.

END