Assignment of Port's Subnet ID and GUID(s)

Introduction

Refer to Figure 8-3 on page 147. The upper 64 bits of a port's assigned GID is its assigned Subnet ID, while the lower 64 bits is the port's assigned GUID. The next two sections describe how these two values are assigned to the port.

Port's Subnet ID Assignment

General

During configuration, the SM assigns the same Subnet ID (sometimes referred to as the GID Prefix or Subnet Prefix) to every port within the subnet that it manages. It is stored in each port's PortInfo.GidPrefix attribute element.

Default Subnet ID

The IBA specification doesn't say what default value the PortInfo.GidPrefix attribute contains immediately after power-up prior to Subnet ID assignment by the SM. It's the author's opinion that it must contain what is referred to as the default GID prefix, consisting of:

• 1111 1110 10b (FE8h) in its upper 10 bits,

• followed by 54 bits of zero.

As previously described in “Link-Local Unicast Address: Packet Cannot Leave the Subnet” on page 149, any GID with its Subnet ID set to the default GID prefix is referred to as a link-local address. Routers must not pass a packet containing a link-local DGID outside of the local subnet.

Port's GUID(s) Assignment

Introduction

Refer to Figure 8-3 on page 147. As previously mentioned, the lower 64 bits of a port's assigned GID consists of its GUID. Every port on a CA or router, and switch port 0 (its management port) has at least one GUID assigned at design-time by the CA, switch, or router manufacturer. The SM may optionally assign additional GUIDs to a port.

Port's Default GUID Is Hardwired

Although a port is assigned addresses by the SM after power-up, there is one identifier that persistently (i.e., it will survive a power cycle) identifies each of the following:

• Each device.

• Each port on a CA or router.

• Port 0 (the management port) on a switch.

This identifier is referred to as the EUI-64. Refer to Figure 8-8 on this page. A portion of the Extended Unique Identifier (EUI)-64 is assigned by a registration authority operating under the auspices of the IEEE organization. The EUI-64 consists of:

• 24-bit company ID value assigned by IEEE. Bit 6, the Universal/Local scope bit, is always set to one (Universal scope ID, not assigned to anything else in the universe) in the value assigned by the IEEE.

• 40-bit extension ID assigned by the company that “owns” the assigned company ID. The interpretation of the company-assigned extension is outside the scope of the specification. As an example, it may represent the device ID and manufacturer-assigned serial number.

Where Are Port's GUIDs Stored?

The port's GUID(s) is stored in its PortInfo.GUIDInfo attribute element. This attribute element is actually a table containing a minimum of one GUID (the permanent one assigned to the port by the device manufacturer). This hardwired, read-only GUID is stored in entry 0 of the table. The size of the table is indicated by the PortInfo.GUIDCap attribute element. As described in the next section, the SM may assign additional GUIDs to a port.

Multiple GUID Assignment Permits Router Multipathing

Description

Assigning additional GUIDs to a port permits the port to be addressed (in the GRH:DGID field) using a DGID with its GUID portion (i.e., the lower 64 bits) set to any of the port's assigned GUIDs. The Routing Tables in routers can then be set up to route the packet via a different path (i.e., multipathing) depending on which of the port's assigned GUIDs is used as the lower 64 bits of the packet's DGID address.

As mentioned above in “Where Are Port's GUIDs Stored?” on this page, each CA and router port implements the PortInfo.GUIDInfo attribute element. This is a table with n entries, where n is defined by the read-only value in the PortInfo.GUIDCap attribute element. Entry 0 contains the port's hardwired, read-only EUI-64. Assuming that the table has more than just one entry, the SM may optionally write additional GUID addresses into the additional table entries.

Additional Port GUIDs Must Flip Universal/Local Bit

Any additional GUIDs assigned by the SM must have the Universal/Local bit (see Figure 8-8 on page 156) cleared to zero to indicate that it's a locally scoped GUID.

Additional Port GUIDs Are Not Persistent

It should be noted that SM-assigned GUIDs are not persistent (i.e., they will not survive a power cycle).