3.2.10. Pause Control Frame Filtering
The LL 40GbE IP core supports options to enable or disable the following features for incoming pause control frames. These options are available if you set the Flow control mode parameter to the value of Standard flow control or Priority-based flow control.
- Processing—You can enable or disable pause frame processing. If you disable pause frame processing, the IP core does not modify its behavior in response to incoming pause frames on the Ethernet link. You can enable or disable pause frame processing with the cfg_enable bit of the RX_PAUSE_ENABLE register. By default, RX pause frame processing is enabled.
Note: IP core variations that target an Arria 10 device do not process standard pause frames that are runts or have FCS errors, for any setting of the cfg_enable bit of the RX_PAUSE_ENABLE register.
- Filtering—If pause frame processing is enabled, the IP core automatically performs address filtering on incoming pause control frames before processing them. You set the matching address value in these registers:
- RX_PAUSE_DADDRL[31:0] at offset 0x707
- RX_PAUSE_DADDRH[15:0] at offset 0x708
- TX MAC filtering—For standard flow control only, Intel provides an additional level of filtering to enable or disable the TX MAC from responding to notification from the RX MAC that it received an incoming pause frame with an address match. Even if the RX MAC processes an incoming pause frame, you can separately set the TX MAC to ignore the RX MAC request to pause outgoing frames, by setting bit  of the TX_XOF_EN register to the value of 0. By default this register field has the value of 1.
- Pass-through—The LL 40GbE IP core can pass the matching pause packets through as normal traffic or drop these pause control frames in the RX direction. You can enable and disable pass-through with the cfg_fwd_ctrl bit of the RX_PAUSE_FWD register. By default, pass-through is disabled. All non-matching pause frames are passed through to the RX client interface irrespective of the cfg_fwd_ctrl setting.
The following rules define pause control frames filtering control:
- If you have disabled pause frame processing, by setting the cfg_enable bit of the RX_PAUSE_ENABLE register to the value of 0, the IP core drops packets that enter the RX MAC and match the destination address, length, and type of 0x8808 with an opcode of 0x1 (pause packets).
Note: IP core variations that target an Arria 10 device do not process standard Ethernet pause frames that are runts or have FCS errors.
- If you have enabled pause frame processing, and the destination address in the pause frame is a match, when the RX MAC receives a pause packet it passes a pause request to the TX MAC. The RX MAC only processes pause packets with a valid packet multicast address or a destination address matching the destination address specified in the RX_PAUSE_DADDR1 and RX_PAUSE_DADDR0 registers, or in the RX_PAFC_DADDRH and RX_PFC_DADDRL registers, as appropriate for the flow control mode . If you have turned on pause frame pass-through, the RX MAC also forwards the pause frame to the RX client interface. If you have not turned on pause frame pass-through, the RX MAC does not forward the matching pause frame to the RX client interface.
- In priority-based flow control, or in standard flow control if you have enabled TX MAC filtering, when the TX MAC receives a pause request from the RX MAC, it pauses transmission on the TX Ethernet link.
Pause packet pass-through does not affect the pause functionality in the TX or RX MAC. Pass-through applies equally to pause control frames that are runts or have FCS errors.
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