Switch/Router Architectures

6Architectures with Bus-Based Switch Fabrics: Case Study—Fore Systems Powerhub Multilayer Switches

6.1 Introduction

This chapter describes the architectures of the Fore Systems PowerHub multilayer switches [FORESYSWP96]. The PowerHub multilayer switches (PowerHub 7000 and 6000) perform both Layer 2 and Layer 3 forwarding and support a wide range of physical media types for network connectivity. The PowerHub supports Layer 3-based VLANs, IP routing (RIP and OSPF), and IP multicast among other features.

The PowerHub employs a software-based packet processing engine (with multiple RISC processors) for both route processing and packet forwarding. The main argument for this software-based solution at the time the architecture was proposed is that it is very flexible to allow routing and management protocols, packet forwarding features to be added or updated, and software bugs to be fixed with simple software fixes, upgrades, or download.

It was viewed at that time that network devices (switches, routers, switch/routers, etc.) that use ASICs to process and forward packets are fixed-function devices that do not have the kind of flexibility required when the system requires enhancements or modifications. As an example, when the designer requires the switch to be enhanced with IEEE 802.1Q VLAN tagging for Ethernet frames, most ASIC-based switches will require parts of the switch to be replaced with new ASICs incorporating this new tagging feature. However, the PowerHub, being software-based, ...