Purpose of the SMI (QP0)

The SMI is a special UD QP. A separate SMI (QP0) is associated with each port on a CA or a router. On a switch, the SMI is only implemented on the switch management port (port 0).

The SMI was introduced in “The SM” on page 160. It only has one purpose: it is used by the SM and the SMAs within CAs, routers, and switches to send and receive SMPs. Refer to Figure 27-1 on page 763. “SMP MADs” on page 787 provides a detailed description of SMPs. SMPs are used in the following manner:

  1. The SM uses the SMI of a port on the device it is associated with to transmit a request SMP to a device's SMA instructing it to perform a specified action (referred to as a method) on a specified device attribute. As noted in the figure, the SM may either:

    - pass the SMP to the local device's SMA which, in turn, passes it to the port's SMI for transmission, or

    - pass the SMP directly to the port's SMI for transmission.

  2. The port's SMI SQ Logic transmits the SMP to the SMI associated with the destination CA. Upon packet receipt and validation, the port's SMI delivers the SMP to the SMA within the destination device.

  3. The SMA performs the specified method on the specified attribute.

  4. In most cases, as defined by the method/attribute combination, the SMA is required to return a response SMP back to the SM. To do so, the SMA reverses the addresses that were in the request packet's LRH:SLID and LRH:DLID fields and instructs the receiving port's SMI to send the response SMP back to the SM.

  5. Upon receipt of the response SMP, the SMI on the receiving port of the SM's associated device passes the response SMP back to the SM. As noted in the illustration, it may either pass it directly to the SM or may pass it to the device's SMA which, in turn, passes it to the SM.

Figure 27-1. Relationship of the SM, SMIs, and SMAs


In some circumstances, a device's SMA may send an unsolicited SubnTrap(Notice) SMP to the SM to notify it of an event that occurred within the device. Refer to “Traps” on page 790 for more information on Trap SMPs and also refer to “Switch Transmission of a Trap(Notice) SMP” on page 766 and “CA or Router Transmission of SubnTrap(Notice) SMP” on page 769.

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