1.1 The Hierarchical Network Design Model
1.1.3 Core layer example
As the center of the network, the core layer is designed to be fast and reliable. Access lists are avoided in the core because they add latency, or delay. Moreover, end users should not access the core directly. Consider an apple; you can not get to the seeds in an apple's core without going through the skin first. In a hierarchical network, end users' traffic should reach core routers only after those packets have passed through the distribution and access layers, where access lists may be applied.

Because core routing is done without access lists, address translation, or other packet manipulation, it may seem as though the least powerful routers would work well for so simple a task. However, the opposite is true. The most powerful Cisco routers serve the core because they have the fastest switching technologies and the largest capacity for physical interfaces.

Marketed by Cisco as enterprise core routers, the 7000, 7200, and 7500 series routers feature the fastest switching modes available. The 12000 series router is also a core router, but it is designed to meet the core routing needs of Internet service providers (ISPs). Unless your company is in the business of providing Internet access to other companies, you are not likely to see a 12000 series router in your telecommunications closet.

Unlike some routers, such as the Cisco 2500 series, the 7000, 7200, and 7500 series routers are modular, so interface modules can be added as needed. The large chassis of this series can accommodate dozens of interfaces on multiple modules for virtually any media type, which makes these routers scalable, reliable core solutions.

One way that core routers achieve reliability is through using redundant links, usually to all other core routers. When possible, these redundant links should be symmetrical (i.e., they should have equal throughput) so that equal-cost load balancing can be used. That is why core routers need a relatively large number of interfaces. Another way that core routers achieve reliability is through redundant power supplies. Core routers usually feature two or more "hot-swappable" power supplies, which may be removed and replaced individually without bringing down the router.

The figure presents a simple core topology using 7507 routers at three key sites in an enterprise. Each Cisco 7507 is directly connected to every other router by two links, which makes this configuration a full mesh. Core links should be the fastest, most reliable, and most expensive leased lines in the WAN: T1, T3, OC3, or better. If redundant T1s are used for this WAN core, each router needs four serial interfaces for two point-to-point connections to each site. Ultimately, the design requires even more than this because other routers at the distribution layer will also need to connect to the core routers. Fortunately, you can easily add interfaces to the 7507s because they are modular.

You can see that with the high-end routers and WAN links involved, the core can become a huge expense, even in a simple example such as this. Some designers will choose not to use symmetrical links in the core to reduce cost. In place of redundant lines, packet-switched and dial-on-demand technologies, such as Frame Relay and ISDN, may be used as backup links. The trade-off for saving money by using such technologies is performance. For instance, if you use ISDN BRIs as backup links, you lose the capability to do equal-cost load balancing.

The core of a network does not have to exist in the WAN. In some cases, a LAN backbone may also be considered to belong to the core layer. Campus networks, or large networks that span an office complex or adjacent buildings, might have a LAN-based core. In this case, switched Fast Ethernet and Gigabit Ethernet are the most common core technologies, and they are usually run over fiber. Enterprises switches, such as the Catalyst 4000, 5000, and 6000 series, shoulder the load in LAN cores because they switch frames at Layer 2 much faster than routers can switch packets at Layer 3. In fact, as modular devices, these switches can be equipped with route switch modules (RSMs), adding Layer 3 routing functionality to the switch chassis.