Cisco Chassis Architecture Overview with the 7600 Series

Introduction

Cisco has a huge range of products. It would seem that there is one Catalyst 7609 or 6506 device. But this is just a chassis that can be packed with completely different boards for different tasks.
And now the piece of iron is standing, working, but at some point suddenly something is amiss. And we don’t even know what can be replaced, what can be distorted. Or another task: the leadership sets the task of installing another card with additional 1GE ports, but right there you will break your head from the abundance of various boards and the spread in cost on them.
In this article, we will understand how a large whole device works, how its components interact, and most importantly, how to start searching for the component you want to expand the network core-level network device using the Cisco 7600 as an example.

The article is intended for a trained reader who presents how network devices of various levels work, superficially familiar with Cisco equipment.

Device Overview

The Cisco 7600 is the chassis family for networking. The device is modular, can be supplemented with various expansion cards. Boards from the Cisco 6500 platform are partially suitable. It is available in various modifications depending on the number of expansion slots: it happens 7603, 7604, 7606, 7609, 7613.

First, just look at how it works. All incomprehensible abbreviations will later be decrypted.

That is, in general, we have the following scheme: there is a Supervisor that performs all the intellectual activity, and there are other boards that connect to it. The connector is either a shared bus (Switching Bus) or a Switch Fabric. Here it must be said that there are 3 types of boards according to the type of interaction with supervisor:

• Classic Line Card - work only through the bus.
• Fabric-enbaled Line Card - can work both through the bus, and through the Switching Factory.
• Fabric-only Line Card - only work through Switch Fabric

Supervisor engine

The main board of the common chassis of the Cisco 7600 (and 6500) is the Supervisor Engine. You can say that this board contains all the brains of your router, without it nothing will work. Now two main types of these boards are used: Supervisor 720 and Supervisor 32. The first Cisco recommends using the network core, the second one on the edge nodes. In the future, we will consider the Supervisor 720.
The board itself is also modular. Here are its main subcomponents:

• MSFC (Multilayer Switch Feature Card) - is responsible for the main network protocols of the 2nd and 3rd level.
• PFC (Policy Feature Card) - works with the table of MAC addresses; determines at what level the packet needs to be processed - 2nd or 3rd, forwards packets to IP and MPLS; QoS and ACL are handled here.

Based on the models of these two subcards, almost the entire model range of supervisors of the 720 series is being built.

Also in the supervisor is integrated the so-called Switch Fabric, which is some connecting element with other boards. Unlike the common bus, this connection method is full-duplex, works on the principle of many-to-many. Just 720 in the name of the model and means the bandwidth of Switch Fabric - 720Gbps.

Switch fabric

A factory is a component that has interfaces with all the boards in the chassis. For compatibility, it can work in regular bus mode. This option is reserved for cards that do not support work through the factory, or for those who need a direct connection. We can say that the factory is some kind of switching table, only inside the chassis and for switching boards.
As we already said, the factory is integrated into Supervisor. Also, it can go and a separate fee. The factory itself is some interface for the exchange between the various Fabric-Enabled modules installed in the chassis. It is two-channel, works in full-duplex modes.
Switch Fabric can forward packets in different modes depending on what the expansion card supports: crossbar, dCEF (Cisco Express Forwarding, the same crossbar, only data is transmitted through the factory in a compact structure, in which it is convenient to view headers), bus (regular bus )

Here's a way to switch inside a Switch Fabric.

Additional expansion cards

The chassis can be equipped with additional boards that expand the capabilities to suit your needs. Like Supervisor, expansion cards may have sub-payments.
As it was written above, boards can exchange data with each other in crossbar, dCEF and bus modes. Examples of boards operating in Bus mode are FlexWan cards. These are cards with WAN interfaces (ISDN, E1 / T1, FastEthernet), as a rule, with not very high bandwidth. To be honest, I personally have never seen them in the chassis.
Modern cards that work with Switch Fabric in crossbar mode usually have a sub-slot with a CFC (Centralized Forwarding Card) data transfer module installed there. Cards operating in dCEF mode have a DFC module (Distributed Forwarding Card). The presence of subcards can again be shown by the teamshow module <1-6> . Just these boards are installed in case you need to expand the number of ports, go to other interfaces. Examples here are 48-port cards, motherboards with 10GE-ports.

Some useful ios commands

show fabric [additional] - view factory status, operation modes, errors.
show inventory [additional] - see installed components;
show modules [additional] - see boards installed in slots.

Materials used

A little about CEF technology
SuperVisor lineup
Features of the 65th series
About Switching Fabric
About CFC and DFC models