Asymmetric Link Quality from Border Router

[Alex-T-Stark]

I am facing a similar issue to the user in this discussion: #7670
Specifically, asymmetric links from the Border Router to its neighbors, where LQ In is nonzero (1-3), but LQ Out is zero.

Raspberry Pi BR:

• ot-br-posix 7975f40 5141b15
• OpenThread 1f2a315
• nRF52840 dongle RCP running nrf coprocessor
• OpenThread 1.3

nRF52840 based Zephyr Full Thread Devices:

• NCS 2.0.2
• (nrfconnect/sdk-openthread) aacd9eb
• OpenThread 1.3

The mesh is composed of 9 devices (1 border router & 8 Zephyr FTDs).
Each node utilizes mac filters to simulate a linear mesh:
BR <-> FTD1 <-> FTD2 <-> FTD3 <-> FTD4 <-> FTD5 <-> FTD6 <-> FTD7 <-> FTD8

BR output:

➜  ~ sudo ot-ctl rloc16
9400
Done
➜  ~ sudo ot-ctl router table
| ID | RLOC16 | Next Hop | Path Cost | LQ In | LQ Out | Age | Extended MAC     | Link |
+----+--------+----------+-----------+-------+--------+-----+------------------+------+
|  4 | 0x1000 |       63 |         0 |     0 |      0 |   1 | 0000000000000000 |    0 |
|  6 | 0x1800 |       63 |         0 |     0 |      0 |   1 | 0000000000000000 |    0 |
| 24 | 0x6000 |       63 |         0 |     0 |      0 |  16 | 0000000000000000 |    0 |
| 25 | 0x6400 |       63 |         0 |     0 |      0 |   1 | 0000000000000000 |    0 |
| 34 | 0x8800 |       63 |         0 |     3 |      0 |   3 | 5ab6a28ca6d93b27 |    1 |
| 36 | 0x9000 |       63 |         0 |     0 |      0 |  16 | 0000000000000000 |    0 |
| 37 | 0x9400 |       63 |         0 |     0 |      0 |   0 | 3eaaaacdb1995e7a |    0 |
| 44 | 0xb000 |       63 |         0 |     0 |      0 |   1 | 0000000000000000 |    0 |
| 49 | 0xc400 |       63 |         0 |     0 |      0 |   1 | 0000000000000000 |    0 |

Done
➜  ~ sudo ot-ctl neighbor table
| Role | RLOC16 | Age | Avg RSSI | Last RSSI |R|D|N| Extended MAC     |
+------+--------+-----+----------+-----------+-+-+-+------------------+
|   R  | 0x8800 |   2 |      -41 |       -42 |1|1|1| 5ab6a28ca6d93b27 |

Done
➜  ~ 

In FTD1 rloc 0x8800:

→ ~ ot rloc16
8800
Done
→ ~ ot router table
| ID | RLOC16 | Next Hop | Path Cost | LQ In | LQ Out | Age | Extended MAC     | Link |
+----+--------+----------+-----------+-------+--------+-----+------------------+------+
|  4 | 0x1000 |       24 |         6 |     0 |      0 | 180 | 0000000000000000 |    0 |
|  6 | 0x1800 |       24 |         5 |     0 |      0 | 180 | 0000000000000000 |    0 |
| 24 | 0x6000 |       63 |         0 |     3 |      3 |   0 | ee4d92f559e3ed3c |    1 |
| 25 | 0x6400 |       24 |         4 |     0 |      0 | 180 | 0000000000000000 |    0 |
| 34 | 0x8800 |       63 |         0 |     0 |      0 |   0 | 5ab6a28ca6d93b27 |    0 |
| 36 | 0x9000 |       24 |         1 |     0 |      0 | 180 | 0000000000000000 |    0 |
| 37 | 0x9400 |       24 |        56 |     3 |      0 |  11 | 3eaaaacdb1995e7a |    1 |
| 44 | 0xb000 |       24 |         2 |     0 |      0 | 180 | 0000000000000000 |    0 |
| 49 | 0xc400 |       24 |         3 |     0 |      0 | 180 | 0000000000000000 |    0 |

Done
→ ~ ot neighbor table
| Role | RLOC16 | Age | Avg RSSI | Last RSSI |R|D|N| Extended MAC     |
+------+--------+-----+----------+-----------+-+-+-+------------------+
|   R  | 0x6000 |   4 |      -32 |       -32 |1|1|1| ee4d92f559e3ed3c |
|   R  | 0x9400 |  18 |      -47 |       -47 |1|1|1| 3eaaaacdb1995e7a |

Done
→ ~ 

When FTD1 pings the mleid of FTD8, the message is properly routed through the mesh and back.
The BR however, cannot route outbound messages due to the LQ Out 0 to its neighbor, FTD1.
If the BR is reset via ot-ctl thread stop ot-ctl thread start, it sometimes establishes a symmetric quality 3-3 link with FTD1, and can briefly route messages through the mesh to FTD 8 and back - After a minute or two, the LQ Out degrades to 0, and outbound traffic is stopped.

Any further ideas on debugging this issue?