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An NE80E Fails to Load Balance Traffic, Causing the Packet Loss on a MAN.

Publication Date:  2013-01-07 Views:  18 Downloads:  0
Issue Description

Symptom 1: An SR pings the Internet but packet loss occurs randomly.

Symptom 2: When the SR uses different source addresses to ping the Internet, some packets are lost while some packets can be sent successfully.

Symptom 3: After the interface on which the ping operation is initiated and packet loss occurs is shut down and then re-opened, the number of lost packets drops a bit.

Handling Process

Based on symptom 1, network congestion may cause the packet loss.

Based on symptom 2, when the same router uses different source addresses to ping the Internet, the transmission device considers the ping packets carrying different source addresses to be of different flows. Most high-end routers link NE80Es perform per-session load balancing when forwarding traffic. This mode uses a hash algorithm that hashes the same flow to a specific link. In this case, the ping packets are hashed to different links. If a link is congested, the packets forwarded on this link are discarded. Therefore, symptom 2 occurs.

Based on symptom 3, since the interface is shut down and then re-opened, the routes converge and the SR recalculates the forwarding table and hashes the result. The newly hashed result may be different from the original result. When the ping packets are forwarded through a link that is selected based on the newly hashed result and the link is not congested, the packet loss is removed.

In view of the preceding analysis, some links on the MAN are congested.

Root Cause

The on-site Huawei technical support personnel check the traffic on all backbone links on the MAN and find that there are three POS links between the NE80E and the P320. The downstream traffic from the MAN is heavy, and there is a defect in load balancing on the NE80E VRP5.10 12002L. When the routes destined for the same IP address share one next hop, traffic cannot be evenly distributed across the three POS links. As a result, one of the links is congested.

Solution

Evenly distribute the downstream traffic to the three POS links using static routes. The link congestion is removed and the packet loss is resolved.

END