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[EX/MX] "received pdu - length mismatch for lacp" LACP error received on interface

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Article ID: KB17674 KB Last Updated: 18 Nov 2020Version: 3.0
Summary:

The LACP error listed below was received on an interface that was connected to a server.

/kernel: %KERN-3: ge-0/0/1: received pdu - length mismatch for lacp : len 128, pdu 124

The purpose of this article is to provide more details about this message and its consequences.

 

Symptoms:

Errors were observed in the log indicating LACP pdu mismatch from the server connected to this interface.

/kernel: %KERN-3: ge-0/0/1: received pdu - length mismatch for lacp : len 128, pdu 124

 

Cause:

The reason for the error is non-compliance with the RFC for LACP specification as per 802.3ad-2000 by the peer devices.

 

Solution:

As per the RFC for LACP specifications, that is 802.3ad-2000, the maximum size of the allowed LACP frame is 128 bytes, including the FCS that is of additional 4 bytes. Hence, a PDU of size 124 is the maximum allowed.

Juniper strictly adheres to this RFC and hence will not accept any frames exceeding the mentioned length of 128 bytes. However, there are vendors who generate frames above the allowed limit, which leads to the errors in question when received on the interface of the Juniper device. In such situations, the error message is expected to indicate the receipt of such packets.

/kernel: %KERN-3: ge-0/0/1: received pdu - length mismatch for lacp : len 128, pdu 124

In such scenarios, it is expected that the Juniper device will not accept such frames and hence the LACP negotiation fails to establish and link aggregation is not successful. When working with such vendors where the possibility of oversized frames with LACP exists, it should be checked whether the vendors have any corrective upgrades, which bring the LACP implementation under the RFC. Another workaround would be to disable LACP on the aggregated links on both the sides.

The command to disable LACP configuration on the aggregated links is given here:

delete interfaces ae0 aggregated-ether-options lacp

A packet capture can be done and analyzed by using wireshark to identify if any such oversized frames are being received on the device that is causing the errors.

Note: The FCS is added by the underlying MAC layer and regular packet capture software remove the FCS and display only the LACP PDU.

 

Modification History:

2020-11-18: Added steps to resolve the issue in case the errors mentioned in the article are reported

 

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