Port Numbering

The following rules govern the number of (and the numbering of) ports that may be implemented on CAs, switches, and routers:

• CA:

  - As few as one or as many as 255 ports can be implemented on a CA.

  - They are numbered starting at one.

  - Each CA port must be assigned at least one 16-bit LID address by the SM at startup time. Each port must have a LID address that is unique within the subnet. The SM may optionally assign a range of LID addresses to a CA port. For more information, see “Why Assign a LID Range to a Port?” on page 137.

• Router:

  - A router may implement as few as two or as many as 255 ports. Although the specification doesn't say the lower limit is two ports, a router with one port would be a deadend. Consider that a router's job is to route packets received on a port attached to a link in one subnet through a port attached to a link within another subnet.

  - As with a CA, the ports are numbered starting at one.

  - Each router port must be assigned a 16-bit LID address by the SM at startup time. Each port must have a LID address that is unique within the subnet. The SM may optionally assign a range of LID addresses to a router port.

• Switch:

  - A switch may implement as few as three or as many as 255 ports. Although the specification doesn't say the lower limit is three ports, consider the following:

  - It is a requirement that all switches must implement port 0. This is the switch's management port through which managers (e.g., the SM) can access the switch's attributes. It's typically implemented as an internal port with no physical attachment to a link. The specification, however, doesn't rule out implementing a physical link as well.

  - A switch's job is to relay packets received from a port attached to one link in the subnet through a port attached to another link in the same subnet. This being the case, a minimalist switch design would have two ports (in addition to the management port, port 0).

  - As with a CA, the ports (other than port 0) are numbered starting at one.

  - The only switch port that is assigned a LID address is its management port, port 0.

  - Switch port 0 is only assigned a single LID address. It is never assigned a range of LID addresses.