Cisco chassis architecture overview using 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 stuffed with completely different boards for different tasks.
And here the piece of iron costs, works, but at some instant something wrong begins. And we do not even know what can be replaced, what to distort. Either another task: the management sets the task to install another card with additional 1GE ports, but then you will break your head from the abundance of various cards and the spread of value 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 need to expand the network device at the core of the network using the example of the Cisco 7600.

The article is designed for a prepared reader, representing how network devices at various levels, superficially familiar with Cisco equipment, work.


')

General overview of the device

The Cisco 7600 is a family of chassis for networking. The device is modular, can be supplemented with various expansion cards. Cards from the Cisco 6500 platform are partially suitable. Available in various versions depending on the number of expansion slots: sometimes 7603, 7604, 7606, 7609, 7613.

To get started, just take a look at how it works. All incomprehensible abbreviations will be deciphered later.

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

• Classic Line Card - work only through the bus.
• Fabric-enbaled Line Card - can work through a bus or through a Switching Factory.
• Fabric-only Line Card - only work via Switch Fabric

Supervisor engine

The main board of the Cisco 7600 common chassis (and even 6500) is the Supervisor Engine. You can say that all the brains of your router are collected in this board, nothing will work without it. Now there are two main types of these cards used: Supervisor 720 and Supervisor 32. First Cisco recommends using it in the network core, the second one on the border 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 basic network protocols of the 2nd and 3rd level.
• PFC (Policy Feature Card) - works with the table of MAC-addresses; determines at which level the packet is to be processed - 2nd or 3rd, forwards packets to IP and MPLS; QoS and ACL are processed here.

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

Also integrated into the supervisor is the so-called Switch Fabric, which is some connecting element with other cards. In contrast to the common bus, this connection method is full duplex, it works on the principle of many-to-many. Just 720 in the name of the model means Switch Bandwidth - 720Gbps.

Switch fabric

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

Here is the switching method inside the Switch Fabric.

Additional Expansion Cards

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

Some useful ios commands

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

Materials used

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