CSS

What Is CSS?

A Cluster Switch System (CSS) is a popular horizontal virtualization technology that consolidates two switches with clustering capabilities into a cohesive logical switch. CSS offers numerous benefits, including simplified configuration and management, increased bandwidth, and the implementation of inter-chassis link redundancy.

The Significance of CSS

To ensure high reliability, traditional campus networks rely on device and link redundancy. However, this approach introduces challenges such as low link efficiency and high network maintenance costs. CSS technology addresses these issues by virtualizing two physical switches into a single logical switch, simplifying network deployment and reducing maintenance expenses. CSS offers the following key advantages:

1. Simplified Configuration and Management: With CSS, the two devices are combined into a logical switch, eliminating the need for complex protocols like MSTP and VRRP. This simplifies network configuration, and administrators can manage and configure all member switches within the CSS from a single access point.

2. Bandwidth Expansion and Inter-Chassis Link Redundancy: CSS allows for the addition of new switches to the network, creating a CSS with the existing switches. Multiple physical links on member switches can be aggregated to increase uplink bandwidth. Inter-device link aggregation provides backup and redundancy, enhancing network reliability.

3. Long-Distance Clustering: CSS enables the formation of a CSS between switches located far apart. This allows for simplified network structures and reduced device management and maintenance costs. By connecting aggregation switches in different buildings, the CSS provides improved reliability with dual links to the external network.

Two Stages of Development of CSS

The development of CSS technology has progressed through two stages:

1. Traditional CSS: This stage involves two CSS connection modes: clustering utilizing CSS cards on Main Processing Units (MPUs) and clustering using service ports.

2. Cluster Switch System Generation 2 (CSS2): CSS2 establishes a CSS by connecting CSS cards on Switch Fabric Units (SFUs). It introduces several advancements compared to traditional CSS, including support for 1+N backup of MPUs, in addition to the functionalities provided by traditional CSS.

Basic Concepts of CSS

1. Master Switch and Standby Switch:

• Master Switch: The master switch assumes the responsibility of managing the entire CSS and there is only one master switch in a CSS.

• Standby Switch: The standby switch serves as a backup to the master switch. In the event of a master switch failure, the standby switch takes over all services and assumes the role of the master. A CSS has only one standby switch.

2. CSS Link: A CSS link facilitates data communication between the master and standby switches in a CSS. It can be a single link or a bundle of multiple links.

3. CSS ID: A CSS ID is assigned to each CSS member switch and must be unique within the CSS. The CSS ID defaults to 1 for a switch, but if you are setting up a CSS between two switches, one switch's CSS ID needs to be manually set to 2 to avoid conflicts.

4. CSS Priority: CSS priority determines the role election of a CSS member. A higher priority value increases the chances of a member being elected as the CSS master. The default CSS priority for a switch is 1.

Master Switch Election

The election of the master switch follows these rules:

1. The switch that starts first and enters the single-chassis CSS running state becomes the master switch.

2. If two switches start simultaneously within a 20-second time difference, the switch with a higher CSS priority becomes the master switch.

3. If two switches start simultaneously and have the same CSS priority, the switch with a smaller MAC address becomes the master switch.

4. If two switches start simultaneously, have the same CSS priority, and the same MAC address, the switch with a smaller CSS ID becomes the master switch.

Synchronization of System Software Version and Configuration File

CSS technology provides automatic synchronization for system software versions. Switches can set up a CSS as long as their system software versions are compatible.

If the standby switch has a different system software version from the master switch, it downloads the system software from the master switch, restarts with the new version, and rejoins the CSS.

Configuration file synchronization is a crucial aspect of CSS. It ensures that all member switches function as a unified entity:

1. Upon startup, the standby switch synchronizes its configuration file with the master switch.

2. During CSS operation, user configurations performed on the master switch are recorded in its configuration file and synchronized to the standby switch.

The configuration file synchronization mechanism ensures that all member switches have identical configuration files. In the event of a CSS split scenario, configurations on the master switch are preserved, resulting in two separate single-chassis CSSs with the same configurations. If a switch restarts, any unsaved configurations on that switch are lost.

When a switch enters the CSS state, it automatically creates a backup of its original configuration file by appending the extension ".bak" to the filename. This allows the switch to restore previous configurations if the CSS function is disabled. For example, if the original configuration file is named "switch.cfg," the backup configuration file will be named "switch.cfg.bak."

To restore the original configurations of a switch after disabling the CSS function, one must remove the ".bak" extension from the backup configuration file, specify the configuration file without the extension for the next startup, and then restart the switch.