Content

| 1.1 |  | The Hierarchical Network Design Model |
| 1.1.4 |  | Distribution layer example |

The distribution layer enforces policies to limit traffic to and from the core. Distribution layer routers handle less traffic than core layer routers, therefore they need fewer interfaces and less switching speed. However, a fast core is useless if a slowdown of data transfer at the distribution layer prevents user traffic from accessing core links. For this reason, Cisco offers robust, powerful distribution routers, such as the 4000, 4500, and the 3600 series routers. These routers are modular, allowing interfaces to be added and removed depending on what is needed. However, the smaller chassis of these series are much more limiting than those of the 7000, 7200, and 7500 series.

Distribution layer routers bring policy to the network by using a combination of the following:

- Access lists
- Route summarization
- Distribution lists
- Route maps
- Other rules to define how a router should deal with traffic and routing updates

Many of these techniques are covered later in the course.

The figure shows two 3620 routers have been added at Core A, in the same wiring closet as the 7507. In this example, high-speed LAN links connect the distribution routers to the core router. Depending on the size of the network, these links may be part of the campus backbone and will most likely be fiber running 100 or 1000 Mbps. In this example, Dist-1A and Dist-2A are part of the Core A campus backbone. Dist-1A serves remote sites while Dist-2A, serves access routers at Site A. If Site A uses VLANs, Dist-2A may be responsible for routing between the VLANs.

Both Dist-1A and Dist-2A use access lists to prevent unwanted traffic from reaching the core. In addition, these routers summarize their routing tables in updates to Core A. This keeps the Core A routing table small and efficient.

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