How does Identity Resolving Key work? Why does LESC only work when IRK is set to true(zephyr)?

[BenSiso]

We are using PC-BLE-DRIVER-JS for our central and zephyr in our peripheral  (833).
The central bonded with the peripheral only when the id (IRK) param in our central is set to true.
If IRK set to false the central will not bond with the peripheral.

Only on zephyr we needed to set it to true, on NRF52 we didnt need to set it.

How does IRK work? Why must it be set to enable bonding sec parameters?

code snap(from our central, on authenticate method):

:

    const secParamsCentral: SecurityParameters = {
      bond: true,
      mitm: true,
      lesc: true,
      keypress: false,
      // eslint-disable-next-line camelcase
      io_caps: this.adapter.driver.BLE_GAP_IO_CAPS_KEYBOARD_ONLY,
      oob: false,
      // eslint-disable-next-line camelcase
      min_key_size: 7,
      // eslint-disable-next-line camelcase
      max_key_size: 16,
      // eslint-disable-next-line camelcase
      kdist_own: {
        enc: true /** Long Term Key and Master Identification. */,
        id: true /** Identity Resolving Key and Identity Address Information. */,
        sign: false /** Connection Signature Resolving Key. */,
        link: false /** Derive the Link Key from the LTK. */,
      },
      // eslint-disable-next-line camelcase
      kdist_peer: {
        enc: true /** Long Term Key and Master Identification. */,
        id: true /** Identity Resolving Key and Identity Address Information. */,
        sign: false /** Connection Signature Resolving Key. */,
        link: false /** Derive the Link Key from the LTK. */,
      },
    };