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The Catalyst 6500 is a modular chassis switch manufactured by Cisco Systems, capable of delivering speeds of up to 400 million packets per second [1].

A 6500 comprises a chassis, power supplies, one or two supervisors, line cards and service modules. A chassis can have 3, 4, 6, 9 or 13 slots each with the option of one or two modular power supplies. The supervisor engine provides centralised forwarding information and processing, the line cards provide port connectivity and service modules allow for devices such as firewalls to be integrated within the switch.

Contents 1 Supervisor 2 Operating System 2.1 CatOS 2.2 Native IOS 2.3 Modular IOS 3 Methods of Operation 3.1 Classic Bus 3.2 cef256 3.3 dcef256 3.4 cef720 3.5 dcef720

The 6500 Supervisor is comprised of a Multilayer Switch Feature Card (MSFC) and a Policy Feature Card (PFC). The MSFC runs all software processes, such as routing protocols. The PFC makes forwarding decisions in hardware.

The supervisor also includes bootflash for the Cisco IOS software, a connection to the switching fabric and classic bus.

The 6500 currently supports three operating systems. CatOS, Native IOS and Modular IOS.

CatOS is supported for layer 2 (switching) operations only. To be able to perform routing functions (e.g. Layer 3) operations, the switch must be run in hybrid mode. In this case, CatOS runs on the Switch Processor (SP) portion of the MSFC and IOS runs on the Route Processor (RP). To make configuration changes, the user must then manually switch between the two environments.

While CatOS does have some functionality missing [2], it's generally considered obsolete compared to running a switch in Native Mode.

Cisco IOS can be run on both the SP and RP. In this instance, the user is unaware of where a command is being executed on the switch, even though technically two IOS images are loaded -- one on each processor. This mode is the default shipping mode for Cisco products and enjoys support of all new features and line cards.

Modular IOS is a version of Cisco IOS that employs a modern UNIX-based kernel to overcome some of the limitations of IOS [3]. Additional to this is the ability to perform patching of processes without rebooting the device and in service upgrades.

The 6500 has five major modes of operation. Classic, cef256, dcef256, cef720 and dcef720.

The 6500 classic architecture provides 32 Gbps centralised forwarding performance [4]. The design is such that an incoming packet is first queued on the line card and then placed on to the global data bus (dBus) and is copied to all other line cards, including the supervisor. The supervisor then looks up the correct egress port, access lists, policing and any relevant rewrite information on the PFC. This is placed on the result bus (rBus) and sent to all line cards. Those line cards for whom the data is not required terminate processing. The others continue forwarding and apply relevant egress queuing.

The speed of the classic bus is 16gb full duplex (hence the 32gb) and is the only supported way of connecting a Supervisor 32 engine to a 6500.

This method of forwarding was first introduced with the Supervisor 2 engine. When used in combination with a switch fabric module, each line card has an 8gb connections to the switch fabric and additionally a connection to the classic bus. In this mode, assuming all line cards have a switch fabric connection, an ingress packet is queued as before and its headers are sent along the dBus to the supervisor. They are looked up in the PFC (including ACLs etc) and then the result is placed on the rBus. The initial egress line card takes this information and forwards the data to the correct line card along the switch fabric. The main advantage here is that there is a dedicated 8gbps connection between the line cards. The receiving line card queues the egress packet before sending it from the desired port.

The '256' is derived from a chassis using 2x8gb ports on 8 slots of a 6509 chassis. 16 * 8 = 128 * 2 = 256. The number is doubled to the switch fabric being 'full duplex'.

dcef256 uses distributed forwarding. These line cards have 2x8gb connections to the switch fabric and no classic bus connection.

Unlike the previous examples, the line cards holds a full copy of the supervisors routing tables locally, as well as its own L2 adjacency table (i.e. MAC addresses). This eliminates the need for any connection to the classic bus or requirement to use the shared resource of the supervisor. In this instance, an ingress packet is queued, but its destination looked up locally. The packet is then sent across the switch fabric, queued in the egress line card before being sent.

This mode of operation acts identically to cef256, except now there are 2x20gb connections to the switch fabric and there is no need for a switch fabric module (this is now integrated in to the supervisor). This was first introduced in to the Supervisor Engine 720.

The '720' is derived from a chassis using 2x40gb ports on 9 slots of a 6509 chassis. 40 * 9 = 360 * 2 = 720. The number is doubled to the switch fabric being 'full duplex'. The reason we use 9 slots for the calculation instead of 8 for the cef256 is that we no longer need to waste a slot with the switch fabric module.

This mode of operation acts identically to dcef256, except now there is 2x20gb connections to the switch fabric.