Details:

This issue impacts the following Module:

[J7VCL] 5-port CPSW at 2.5G on ports 2-4

The issue with ALE is due to CPSW frequency and IET operation with short express traffic and pre-empted packets that get pre-empted between 60-69 bytes on non-10G capable ports.

If an IET pre-emptible packet get interrupted at 60-69 bytes, the lookup will occur when the next chunk arrives. The CPSW only gives the ALE 64 bytes from the pre-emptible MAC.

As a result, a short express traffic lookup will start at the end of a 64 byte express traffic, but when the pre-empted queue continues, the pre-empted traffic will complete the 64 bytes and attempt a lookup for the pre-empt packet. But this lookup is less that 64 clocks from the express lookup start, so the express lookup will be aborted(express traffic dropped) and start the new lookup for the pre-empted traffic.

Rules to induce the issue:

1. You are in IET (Interspersed Express Traffic) mode on ports not capable of 5/10G operation
2. Remote express packets can be preempt packets as low as 60 bytes
3. Pre-empt packet traffic that is 128 bytes or more.
4. Express traffic that interrupts the pre-empt traffic between 60-69 bytes.
5. A short express traffic immediately followed by the continuation of the pre-empt traffic.
   1. Gap between express frame and pre-empt frame be its minimum.
6. The CPSW frequency is at its lowest capability for the speeds required.

Workaround(s):

During IET negotiation, tell the remote to fragment at 128 bytes.