8.2 Multi-Board Architectures

Multi-board architectures are needed since a single-processor/single-board system cannot scale to handle a large number of ports or increasing port speeds. While hardware acceleration could be used to address the port speed problem, there is a limit to the amount of board real estate available for high port densities.

Modularity is another reason for multi-board architectures. The customer needs to be able to add or remove ports from the system as his/her requirements change. In this scenario, an enterprise or service provider can purchase a low-end multi-board system with only a few I/O slots populated. Based on his need, the customer can add more cards to the system to expand the system’s capacity and capability without having to throw out the original investment. This is the “pay as you go” philosophy which many equipment vendors use. There are several variations of the multi-board architecture. This chapter focuses on the functional organization and interconnects using a Layer 3 switch/router, also termed the IP Switch (IPS) to illustrate the concepts.

8.2.1 Components of a Multi-Board System

The most common types of cards in a multi-board communications system are the control cards, switch fabric cards and transport cards (often called line cards). Control cards have the following functions:

  • Running control (e.g., routing protocols such as OSPF and BGP) and management protocols. The messages generated by these protocols are sent out on line card ports.

  • Building tables using information from control and management protocols for use by the line cards. The tables are provided to the line cards using card-to-card communication.

  • Providing an interface to management through tasks such SNMP, CLI, and TL1 Agent.

  • Performing system management functions such as shelf and chassis management and internal card-to-card communication monitoring. These are required to keep the system running.

  • Managing redundancy when switching to a new line card.

Line cards are responsible for:

  • Sending and receiving messages on the hardware ports.

  • Switching and forwarding traffic between line card ports using the tables built up by the control card.

  • Sending the received control and management messages to the control card.

  • Providing local control and management information to the control card, e.g., indication of a port failure.

  • Managing redundancy when switching to a new control card.

Most systems are designed to be modular by providing the media interface as a separate plugin module to the line card instead of integrating it on to the line card. This permits upgrade of the interfaces without having to upgrade the line card. For example, an 8-port Fast Ethernet module can be replaced by a single Gigabit Ethernet module without having to replace the line card (see Figure 8.4). This also implies that the module-to- line card interface is compatible with and able to handle the higher speed Gigabit Ethernet interface. For this chapter, the term line card indicates the line card plus its media module.

This functional separation of control card and line cards is applicable to several types of communications equipment, though the actual systems may use different terminology to describe them. Another way of looking at this is to visualize a split between the components of a single CPU system. The “closer to the wire” components are now housed in the line card, while the “closer to the application” components are housed on the control card.

The control card and line card correspond to the functional separation of control plane and data plane, as described in Chapter 3. This discussion assumes that part of management plane functionality is now included in the control card.

The third type of card is the switch fabric card. This is a card housing the functionality to switch traffic between different line cards. This card avoids the need for a “full- mesh” relationship between the line cards, in which a line card needs to be connected to every other line card. Instead, the line cards are now connected to the switch fabric. The switch fabric typically operates on fixed-size cells, so a line card will slice its packets into these fixed-size cells before sending them to the fabric.

There are two common architectures for multi-board IPS systems using the control card and the line cards:

  1. Single Control Card + Multiple Line Card Architecture

  2. Multiple Line Card Distributed Architecture

The following sections discuss these architectures in greater detail.



Designing Embedded Communications Software
Designing Embedded Communications Software
ISBN: 157820125X
EAN: 2147483647
Year: 2003
Pages: 126
Authors: T. Sridhar

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