The architecture of earliest routers was based on that of a computer as shown in Fig 2. It has a shared central bus, central CPU, memory and the Line cards for input and output ports. Line cards provide MAC-layer functionality and connect to the external links. Each incoming packet is transferred to the CPU across the shared bus. Forwarding decision is made there and the packet then traverses the shared bus again to the output port.
To remove the first bottleneck, some router vendors introduced parallelism by having multiple CPUs and each CPU now handles a portion of the incoming traffic. But still each packet has to traverse shared bus twice. Very soon, the design of router architecture advanced one step further as shown in Fig 3. Now a route cache and processing power is provided at each interface and forwarding decisions are made locally and each packet now has to traverse the shared bus only once from input port to the output port.
Even though CPU performance improved with time, it could not keep pace with the increase in line capacity of the physical links and it is not possible to make forwarding decisions for the millions of packets per second coming on each input link. Therefore special purpose ASICs(Application Specific Integrated Circuits) are now placed on each interface which outperform a CPU in making forwarding decisions, managing queues and arbitration access to the bus.
But use of shared bus still allowed only one packet at a time to move from input port to output port. Finally, this last architectural bottleneck was eliminated by replacing shared bus by a crossbar switch. Multiple line cards can communicate simultaneously with each other now. Fig 4. shows the router architecture with switched backplane.
Multi-service support: Most of the network backbones support both ATM and IP traffic and will continue to do so as both technologies have their advantages. Therefore routers must support ATM cells, IP frames and other network traffic types in their native modes, delivering full efficiency of the corresponding network type.
Guarantee short deterministic delay: Real time voice and video traffic require short and predictable delay through the system. Unpredictable delay results in a discontinuity which is not acceptable for these applications.
Quality of Service: Routers must be able to support service level agreements, guaranteed line-rate and differential quality of service to different applications, or flows. This quality of service support must be configurable.
Multicast Traffic: Internet traffic is changing from predominantly point-to-point to multicast and therefore routers must support large number of multicast transmissions simultaneously.
High Availability: High speed routers located in the backbones handle huge amounts of data and can not be turned down for upgrades etc. Therefore features such as hot-swappable software tasks- allowing in-service software upgrades are required.