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After E1 Interfaces Are Bundled on an NE20, MCUs Report Alarms Out of Order

Publication Date:  2019-04-24 Views:  107 Downloads:  0

Issue Description

Local MCU---100M---NE20---2 x E1 interface bundling----long-haul link transmission-----remote MCU

Alarm Information

1. No alarm is generated on the NE20.

2. No alarm is generated on the local MCU.

3. The remote MCU reports alarms out of order. Specifically, mosaic occurs on the screen, or the sound is discontinuous.

Handling Process

1. Verify the fault symptom.

(1) Configure an MP group on the 2 x E1 interface bundling on the NE20. The remote MCU reports alarms out of order. Specifically, mosaic occurs on the screen, or the sound is discontinuous.

(2) Ping the link from the local MCU to the NE20. No packets are lost during the test on 1500-byte packets.

(3) Ping the MP-Group link on the NE20. No packets are lost during the test on 1500-byte packets, and the delay is normal.


2. Remove 2 x E1 interface bundling and test single E1 links.

(1) Ping the first E1 link on the NE20. No packets are lost during the test on 1500-byte packets, and the delay is normal.

(2) Ping the second E1 link on the NE20. No packets are lost during the test on 1500-byte packets, and the delay of the second E1 link is much larger than that of the first E1 link.

(3) Use the first E1 link on the NE20 to test the MCU. The alarm on the remote MCU is cleared. No mosaic occurs on the screen. Due to insufficient bandwidth of the single link, frames are lost occasionally.

(4) Use the second E1 link on the NE20 to test the MCU. The alarm on the remote MCU is cleared. No mosaic occurs on the screen. Due to insufficient bandwidth of the single link, frames are lost occasionally.


3. Reproduce the fault after E1 bundling.

4. Analyze the causes of the alarm.

(1) Communicate with the MCU. It is found that the alarm is generated because the MCU check on the received packets shows that the sequence of packet sending is inconsistent with that of packet receiving. If too many out-of-order packets exist, mosaic may occur on the screen and frames may be lost.

(2) Check the NE20 product document. It is found that the MP-Group interface is deployed in load balancing mode. The sequence of packet sending and that of packet receiving are inconsistent due to factors such as the link quality.

Root Cause

1. When the MP-Group interface is bound to multiple links, the physical features and packet fragment sizes may differ, which causes inconsistency between the sequence of packet sending and that of packet receiving.

2. The out-of-order IP packets are automatically reassembled based on the fragment offset field of the IP header. However, if there are strict requirements for packet sequence and the out-of-order packets are not effectively reassembled, the MP packet disorder may lead to severe network performance deterioration and even cause the network to be unavailable.

Solution

1. Perform the following operations to enable packet order-preserving on the MP-Group interface:

(1) Enter the MP-Group interface view.

(2) Run the ppp mp reorder command to enable packet order-preserving function on the MP-Group interface.

(3) Restart the restart command to restart the MP-Group interface or run the shutdown command and then the undo shutdown command on the MP-Group interface.

2. Check whether the tested service is normal and the fault is rectified.

Suggestions

1. If the MP-Group interface is bound to multiple links, link delay may differ due to different link quality. This causes inconsistency between the sequence of packet sending and that of packet receiving.

2. The out-of-order IP packets are automatically reassembled based on the fragment offset field of the IP header. However, if there are strict requirements for packet sequence and the out-of-order packets are not effectively reassembled, the MP packet disorder may lead to severe network performance deterioration and even cause the network to be unavailable.

The inconsistency between the sequence of packet sending and that of packet receiving on the remote MCU causes alarm disorder.

END