Cisco Router Hardware
The Cisco router product line has three flavors. Cisco routers are available as modular, fixed or combination configurations. Along with full router configuration Cisco offers router platforms on personal computer (PC) card format. Additionally, Cisco combines routers and small hubs into one device suitable for small office installations. Key to a successful implementation of Cisco routers in a networking environment is proper placement and configuration of the router. Each Cisco router offering is suited for a specific function. These functions are depicted in Figure 2.1 as core, distribution and access. These functional characteristics make up Cisco’s router internetwork architecture.
- Cisco Router Network Architecture Early on in the development of internetworks, an architecture emerged. This architecture for deploying routers was documented into an architecture which Cisco employs and preaches to its customer base. The architecture relies on the ability of the processor in the router and its need for processing routes, filters and physical connections. The architecture places the larger Cisco 7x00 series and 12000 series routers at the center or core of the network. The 4x00 series routers are at the net layer of the network architecture called the distribution layer. Finally, the 25xx, 100x, 7x0 and 200 series routers constitute the access layer of the architecture. While these assignments to the three different layers of the architecture make sense it does not mean that 7x00 series routers can not be used as a distribution or access router. Likewise, in some cases the 4500 and 4700 series router platforms may be used as a core or access router. However, the smaller fixed and combination routers are most suited for the access layer and will not perform the physical or logical requirements of the core or distribution routers.
The outer layer of the architecture is the access layer. It is at this layer that end users gain access to the network resources connected by the routers. A typical example for using access routers is in large buildings or campuses. As depicted in Figure 2.4, access routers connect workgroups and/or floor segments within a building to the distribution router. Access routers also provide remote dial-up connectivity for temporary connections.
- Core The routers that comprise the core layer of the architecture are often referred to as the backbone routers. These routers connect to other core routers providing multiple paths over the backbone between destinations. These routers carry the bulk of WAN traffic between the distribution routers. Core routers are usually configured with several high speed interfaces as shown in Figure 2.2. However, the introduction of ATM and interface cards providing up to OC-12 speeds (622Mbps), core routers may only require two physical interfaces. However, as the section on ATM configuration will reveal, multiple subinterfaces are allowed on each physical interface. The need for the core router to manage many high speed interfaces is still a requirement even with only two physical ATM interfaces.
The use of Packet over SONET is another alternative to proving a high-spped core using Cisco routers. In large WANs and MANs it is common to have the backbone built on SONET rings with OC-3, OC-12 and OC-48 connections. Packet over SONET allows for the transmission of IP direct over the SONET network without the use of ATM. This provides a great incentive to corporations that have yet to embrace ATM but have a need for high speed and bandwidth over their backbone. Using Packet over SONET as the backbone transport requires an investment in only routers versus ATM which requires investments in routers and switches.
- Distribution The distribution router functions as the main conduit for a location back to the core. As an example, in Figure 2.3, the distribution router acts as a core router for a campus environment but as a distribution router for a building. Or the distribution router may act solely as a distribution router for a region or campus managing only the transmission of data between the core and the access layers.