Network Layer Overview

The Network Layer logic is implemented in routers and CAs (not in switches) and, in a router, is responsible for routing packets (both unicast and multicast) between IBA subnets. Some characteristics of the Network Layer:

• The specification does not define multi-protocol routing (i.e., routing IBA packets over non-IBA fabric types), nor does it specify how raw packets are routed between IBA subnets.

• SMPs do not transit routers (because subnet management occurs at the subnet level).

• The packet's ICRC (Invariant CRC) field covers the packet fields that are not altered as the packet is routed through the fabric.

• The packet's VCRC (Variant CRC) field covers all the fields of a packet, including those that may be changed by switches or routers.

• SL mapping is used to achieve the desired QoS within the subnet, while the packet's TClass field is used to achieve the desired QoS when crossing routers from one subnet to another.

• As the packet transits a subnet, the SLID/DLID fields remain unchanged.

• As the packet transits a router from one subnet to another, the router alters the contents of some fields. For example:

  - The LRH:SLID is changed to the LID of the router port through which the packet will be output.

  - The DLID is changed to the LID of either:

  - the next router ingress port along the path to the destination,

  - or, if this is final leg of the journey, to the LID of the destination CA port.

  - Based on the packet's GRH:TClass, the router chooses the SL that will be used when the packet is injected into the next subnet.

• Support for multi-pathing. Multiple GUIDs may be assigned to a port (the 64-bit GUID forms the lower half of a 128-bit GID address assigned to a port). The Routing Table within a router may be set up to provide different paths to a CA port based on which GID is used in the packet's GRH:DGID address.

• Multicast support. When a router's Network Layer receives a packet, it examines the packet's GRH:DGID address to determine if it's a multicast address. If it is, the Network Layer determines through which of its ports the packet should be forwarded.

• For a description of the actions performed by a CA's Network Layer, refer to “Network Layer” on page 107.