The SSN1PQ1 board is installed in slot 1 of the OptiX OSN 3500, and the N2PQ1 boards are installed in slots 2, 3, 4, and 5. The N1PQ1 board in slot 1 is the protection board.
When you enable TPS protection, the SSN2PQ1 boards are not available. This is because these boards are not listed in the Available Working Board option.
If the N2PQ1 and N1PQ1 are involved in TPS protection, the protection board must be the N2PQ1. In this case, you can configure the TPS protection group by changing the N1PQ1 in slot 1 to an N2PQ1 in slot 2, 3, 4, or 5.
When TPS protection is enabled, the N2PQ1 can protect the N1PQ1, but the N1PQ1 cannot protect the N2PQ1. This is because the N2PQ1 supports the E13 function, that is, conversion between E1 and E3, in addition to the N1PQ1 function. Therefore, you cannot use the N1PQ1 to protect the N2PQ1.