LID Address Space

The LID assigned to a port is a 16-bit value assigned by the SM. This means there are 64K LIDs available for assignment to the ports that populate a specific subnet. The 64K overall LID address range is subdivided as follows:

• LID address 0000h is reserved and must never be used.

• LID addresses 0001h–BFFFh (a 48K minus 1 address range) are set aside for assignment as unicast addresses. A packet with a unicast DLID is always delivered to the single target port that is assigned that address. It is never delivered to more than one port. When a switch receives a packet wherein the DLID field contains a unicast LID address, it consults its unicast Forwarding Table and forwards the packet through the port indicated by the matching entry. For more detail on unicast packet forwarding, refer to “Switch Unicast Packet Forwarding” on page 669.

• LID addresses C000h–FFFEh (a 16K minus 1 address range) are set aside for assignment as multicast addresses. Usage of a multicast DLID permits a packet to be broadcast to multiple destinations. When a switch receives a packet wherein the DLID field contains a multicast LID address, it consults its Multicast Forwarding Table and forwards the packet through all of the ports indicated by the selected table entry. A detailed description of multicast operations can be found in “Switch Multicast Packet Forwarding” on page 675 and “Multicasting” on page 563.

• LID address FFFFh is referred to as the Permissive LID (PLID) and has a special use during the discovery process. A detailed description of PLID usage can be found in the chapter entitled “Discovery” on page 871.