Cisco WS-SUP720= User Guide - Page 3

Chapter 4 Configuring a Supervisor Engine 720 Configuring and Monitoring the Switch Fabric Functionality Understanding How the Switch Fabric Functionality Works These sections describe how the switch fabric functionality works: • Switch Fabric Functionality Overview, page 4-3 • Forwarding Decisions for Layer 3-Switched Traffic, page 4-3 • Switching Modes, page 4-3 Switch Fabric Functionality Overview The switch fabric functionality is built into the Supervisor Engine 720 and creates a dedicated connection between fabric-enabled modules and provides uninterrupted transmission of frames between these modules. In addition to the direct connection between fabric-enabled modules provided by the switch fabric funtionality, fabric-enabled modules also have a direct connection to the 32-Gbps forwarding bus. Forwarding Decisions for Layer 3-Switched Traffic Either a PFC3 or a Distributed Feature Card 3 (DFC3) makes the forwarding decision for Layer 3-switched traffic as follows: • A PFC3 makes all forwarding decisions for each packet that enters the switch through a module without a DFC3. • A DFC3 makes all forwarding decisions for each packet that enters the switch on a DFC3-enabled module in these situations: - If the egress port is on the same module as the ingress port, the DFC3 forwards the packet locally (the packet never leaves the module). - If the egress port is on a different fabric-enabled module, the DFC3 sends the packet to the egress module, which sends it out the egress port. - If the egress port is on a different nonfabric-enabled module, the DFC3 sends the packet to the Supervisor Engine 720. The Supervisor Engine 720 fabric interface transfers the packet to the 32-Gbps switching bus where it is received by the egress module and is sent out the egress port. Switching Modes With a Supervisor Engine 720, traffic is forwarded to and from modules in one of the following modes: • Compact mode-The switch uses this mode for all traffic when only fabric-enabled modules are installed. In this mode, a compact version of the DBus header is forwarded over the switch fabric channel, which provides the best possible performance. • Truncated mode-The switch uses this mode for traffic between fabric-enabled modules when there are both fabric-enabled and nonfabric-enabled modules installed. In this mode, the switch sends a truncated version of the traffic (the first 64 bytes of the frame) over the switch fabric channel. • Bus mode (also called flow-through mode)-The switch uses this mode for traffic between nonfabric-enabled modules and for traffic between a nonfabric-enabled module and a fabric-enabled module. In this mode, all traffic passes between the local bus and the supervisor engine bus. Table 4-1 shows the switching modes used with fabric-enabled and nonfabric-enabled modules installed. OL-3999-08 Catalyst 6500 Series Switch Cisco IOS Software Configuration Guide, Release 12.2SXF 4-3