Jumbo Frames
BCM5718 Programmer’s Guide
Broadcom
®
January 29, 2016 • 5718-PG108-R
Page 116
Jumbo Frames
The BCM5718 family supports jumbo Ethernet frames in both the receive (RX) and transmit (TX) paths and in
conjunction with LSO. The jumbo frame architecture and the software interface is nearly identical to that of the
legacy NetXtreme family controllers which also supported jumbo frames. The related jumbo architectural
changes for the BCM5718 family are described in this section.
Following are the feature highlights.
The maximum jumbo frame length supported by the BCM5718 family is 9622B, which may be broken down as
follows:
• 9600B MTU payload
• 14B Ethernet Header
• 4B Ethernet FCS
• Optional 4B of VLAN Tag
• 9622B maximum size limit applies equally to both TX and RX paths
Transmit Side:
• CRC checksum offload of jumbo frames is permitted.
• TCP and IP checksum offload of jumbo frames is permitted.
• TCP segmentation offload (TSO), a.k.a. large segment offload (LSO), of jumbo frames is permitted.
• Jumbo frames are to be constructed out of standard send buffers.
• There is a single send buffer ring which jumbo and standard frames share. The driver may inter-mix jumbo
and standard frames in the send ring without restriction.
• The TX MBUF on-chip memory has be upsized to 22K bytes.
• The behavior of TX jumbo frame processing remains identical to that of standard TX frame processing:
– A TX frame is first completely DMAed into the TX MBUF memory and only then will it be transmitted
onto the wire.
– The TX EMAC treats jumbo frames exactly the same as it treats a standard frame, except for being
cognizant of the length.
– Full-Duplex and Half-Duplex behavior remains the same.
– Host coalescing timing remains identical to that of standard frames.
– A new jumbo frame flag is introduced in the send buffer descriptor (SBD). The driver must set this flag
bit to indicate a jumbo frame (i.e., frame length > 1514 bytes without CRC and VLAN tag fields)
– In Multiple Send Queue mode, all 16 send queues are permitted to post jumbo frames. Thus there are
16 send ring control blocks (RCBs) available in this mode.
Receive Side:
• An additional BD producer ring (the jumbo producer ring) has been introduced.
• The receive side retrieves buffers only from the jumbo producer ring in turn to post a received jumbo frame.
Similarly, the receive side only retrieves buffers from the standard producer ring in turn to post standard
sized frames.