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An NE40E with Port Rate Limit Enabled Discarded Large Packets Randomly Because of Inappropriate CBS Setting

Publication Date:  2013-10-08 Views:  107 Downloads:  0

Issue Description

An NE40E running VRP5.1-31002LSP08 was connected to a third-party access router through a 100 Mbit/s Ethernet port. CAR was set on the 100 Mbit/s Ethernet port. When users pinged the NE40E from its connected access router using large (10240-byte or longer) packets, the packets were randomly discarded. The NE40E port statistics showed that the inbound traffic is 2 Mbit/s while the outbound traffic is 4.5 Mbit/s. Users then pinged the access router from the NE40E using large packets and found that the packets were normally received.

Handling Process

Huawei performed ping tests on the NE40E and its connected access router using 56-byte packets. The packets were transmitted and received normally. The link was working normally.

Huawei then checked the CAR configuration for the NE40E port and obtained the following information.

traffic behavior AoZuWei-ratelimit                                              

 car cir 10000 cbs 10000 pbs 0 green pass yellow pass red discard 

After modifying the value of CBS to 15000, Huawei pinged the NE40E from the access router using 10240-byte packets. The packets were normally received and forward.

Root Cause

The set CBS value was inappropriate.

When CAR is used for traffic policing, CIR specifies the average traffic rate allowed and CBS specifies the maximum burst traffic size.

It is recommended that you set CBS to 1.5 to 2 times that of CIR. If you specify only CIR in configurations, then CBS takes the same value as CIR. In this case, CIR is set to 10000 and CBS is not specified but takes the same value as CIR. When the burst traffic is larger than 10000 bytes, they will be randomly discarded.


Set CBS to 15000.


Recommended CBS = (1.5 to 2) x CIR