Packet Routing During Discovery

Problem

Before all ports have been assigned LID addresses and switch Forwarding Tables have been initialized by the Master SM, the SM must use a different method to direct a packet to its destination. It uses a directed-route SMP. There are two basic scenarios:

Four Possible Scenarios

There are four possible scenarios:

Solution: Directed-Route SMP

Refer to Table 31-2 on page 877. Upon receipt of an SMP with the DLID field set to FFFFh (the Permissive LID, or PLID), the CA or router port at the other end of the link delivers the packet to its SMI for processing. If the device at the other end of the link is a switch, the PLID address causes the switch port to internally deliver the SMP to the SMI of port 0 for processing. In addition to the PLID in its DLID field, the directed-route SMP originated by the SM contains the following:

• The SLID field is set to the LID of the port that originates the packet (or the PLID if a LID address has not yet been assigned to it).

• The DLID field is set to one of the following:

  - LID address of the switch management port on the switch at the end of the initial part of the path to the destination port. The Forwarding Tables in one or more switches in the initial part of the path have already been set up by the Master SM. Those switches can therefore use their Forwarding Tables to get the packet to the switch management port at the end of the initial part of the path to the destination port.

  - If the switch Forwarding Tables and the management port's LID in the switches on the first part of the path have not yet been set up, the packet's DLID is set to the PLID address.

• The DrSLID (directed-route SLID) field is set to the address of the port that the SM uses to transmit the SMP request packet:

  - If the SM resides within or behind a CA, DrSLID is set to either the LID address of the port or the PLID address (if the port has not yet been assigned a LID address).

  - If the SM resides within a router, DrSLID is set to either the LID address of the router port or the PLID address (if the port has not yet been assigned a LID address).

  - If the SM resides within a switch, DrSLID is set to either the LID address of the switch's management port or the PLID address (if the port has not yet been assigned a LID address).

• The DrDLID field is set to the address of the ultimate destination port. This is one of the following:

  - The LID address of the ultimate destination port (if it has already been assigned a LID address).

  - The PLID address (if the ultimate destination port has not yet been assigned a LID address).

• InitialPathArray. Some of the switches along the path do not have Forwarding Tables and therefore will not know through which exit port the packet should be forwarded. This array substitutes for the missing Forwarding Tables, providing the exit port number to be used on each unconfigured switch along the path between the source and destination ports.

• ReturnPathArray. During the packet's forward progress towards the destination port, each unconfigured switch along the way records which of its ports the packet was received on. When the destination port transmits its response packet back to the source port, the unconfigured switches use the contents of this array as a trail of bread crumbs to determine the exit port the packet must pass through on each switch during its return journey.

• Direction bit (D bit).

  - 0 = forward movement of the request packet toward the responder. Tells each of the unconfigured switches to use the next entry in the Initial Path Array to determine which exit port to forward the packet through.

  - 1 = reverse movement of the resulting response packet toward the requester. Tells each of the unconfigured switches to use the next entry in the Return Path Array to determine which exit port to forward the packet through.

• Hop Count. Indicates the number of links the packet must traverse to get across the unconfigured portion of the path.

• Hop Pointer. Initialized to 0 and used to select which element of the Initial Path Array will provide the next switch exit port number on the outward-bound journey, and which element of the Return Path Array will provide the next switch exit port number on the response packet's homeward-bound journey back to the SM's port.