NC-SI Interface

The Mellanox Reference Platform design supports the Network Controller Sideband Interface (NC-SI) feature set. NC-SI is the DMTF-defined sideband protocol, and is fully documented in the DSP0222 – Network Controller Sideband Interface (NC-SI) Specification. The NC-SI is a board-level connection between the BlueField RBT port and the BMC MAC1 port. The NC-SI is based on RMII signaling, and runs at 100Mbps full duplex.

From a functional perspective, the NC-SI allows the BMC to communicate with the external network via the QSFP ports of the BlueField SoC. It is worth noting that while QSFP ports run at 100Gb/s, the NC-SI only runs at 100Mbps. From a packet format point of view, NC-SI traffic is simply Ethernet packets with one of two EtherTypes:

1. Control packets (EtherType=0x88F8) which contain information shared only between BMC and BlueField. This traffic does not get forwarded to the QSFP ports. One example of this traffic type is the “get link status” command sent by BMC to BlueField to query for port status.

2. Pass-through packets (all other EtherTypes) which contain information shared between the BMC and the external network. All pass-through packets contain L2 information pertaining to the BMC’s eth0 MAC address.

From a BMC point of view, the interface eth0 is the NC-SI. So, any information displayed by “ifconfig eth0” pertains to the NC-SI. The MAC address used for BMC’s eth0 is contained in the BMC Mezz EEPROM. The IP address used for BMC’s eth0 is configurable. The DHCP request and reply only resolve if the QSFP ports on the BlueField’s ConnectX®-5 side are up and active. If the QSFP ports are not up, then the BMC’s eth0 interface does not get an IP through DHCP.