ly a 16-port switch that connects each of the fabric-enabled modules via the fabric bus. Because the SFM is a switch unto itself, modules commu- nicate concurrently, much the same way multiple computers can communicate on a Chapter 17: Cisco 6500 Multilayer Switches switched Ethernet network. By contrast, the 32 Gbps bus operated in such a way that all modules received all packets, regardless of the destination module (similar to the way computers communicate on an Ethernet network connected with a hub). Because it controlled the crossbar fabric bus, the SFM could only reside in certain slots. One of the major downsides to this design was that a highly redundant installa- tion required two slots for the supervisors and an additional two slots for the SFMs. In a nine-slot chassis, this left only five slots for line cards or service modules. The Supervisor-720 solved the slot-density problem by incorporating the SFM into the supervisor module. Now, a highly resilient installation requires only two slots for supervisors. However, note that because the Supervisor-720 includes the SFM's func- tionality, it must reside in the SFM's slots. For example, on a redundant 6509, the Supervisor-2 modules reside in slots 1 and 2, while the SFMs reside in slots 5 and 6. Supervisor-720 modules must reside in slots 5 and 6, which frees up slots 1 and 2 for line cards or service modules. Buses The 6500 series switch backplane is composed of the following four buses: D bus The data bus, or D bus, is used by the EARL chipset to transfer data between modules. The speed of the D bus is 32 Gbps. The D bus is shared much like a traditional Ethernet network, in that all modules receive all frames that are placed on the bus. When frames need to be forwarded from a port on one mod- ule to a port on another module, assuming the crossbar fabric bus is not in use or available to the modules, they will traverse this bus. R bus The results bus, or R bus, is used to handle communication between the mod- ules and the switching logic on the supervi