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| 1.1 |  | The Hierarchical Network Design Model |
| 1.1.3 |  | Core layer example |

The core layer is the center of the network and designed to be fast and reliable. Access lists should be avoided in the core layer. Access lists add latency and end users should not have access directly to the core. In a hierarchical network, end user traffic should reach core routers only after those packets have passed through the distribution and access layers. Access lists may exist in those two lower layers.

Core routing is done without access lists, address translation, or other packet manipulation. Because of this, 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.

The 7000, 7200, and 7500 series routers feature the fastest switching modes available. These are the Cisco enterprise core routers. The 12000 series router is also a core router designed to meet the core routing needs of Internet Service Providers (ISPs). Unless the company is in the business of providing Internet access to other companies, it is unlikely a 12000 series router will be found in the telecommunications closet.

The Cisco 7000, 7200, and 7500 series routers are modular. This provides scalability since administrators can add interface modules when needed. The large chassis of this series can accommodate dozens of interfaces on multiple modules for virtually any media type. This makes these routers scalable and reliable core solutions.

Core routers achieve reliability through the use of redundant links, usually to all other core routers. When possible, these redundant links should be symmetrical having equal throughput, so that equal-cost load balancing may be used. Core routers need a relatively large number of interfaces to enable this configuration. Core routers achieve reliability through redundant power supplies. They usually feature two or more hot-swappable power supplies, which may be removed and replaced individually without shutting down the router.

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. This type of configuration is a full mesh. There are also two links between each router to provide redundancy. Core links should be the fastest and most reliable leased lines in the WAN:

- T1
- T3
- OC3
- Anything 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, interfaces can be added to the 7507 due to modularity.

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. Using ISDN BRIs as backup links can eliminate the capability of equal-cost load balancing.

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

| Web Links Internetworking Design Basics http://www.cisco.com/univercd/cc/td/doc/ cisintwk/idg4/nd2002.htm