there is a need for some geographical concentration, a two-level design is appropriate. This produces a “star of stars” pattern. Again, the pattern chosen based on leased line technology will be considerably different from the pattern based on Frame Relay technology. When planning simpler networks, the three-layer model should still be considered as it may provide for better network scalability. The hub at the center of a two-layer model is also a core, but with no other core routers connected to it. Likewise, in a single-layer solution the area hub serves as the regional hub and the core hub. This allows easy and rapid future growth as the basic design can be replicated to add new service areas.
Content 2.3 WAN Design 2.3.6 Other WAN design considerations Many enterprise WANs will have connections to the Internet. This poses security problems but also provides an alternative for inter-branch traffic.Part of the traffic that must be considered during design is going to or coming from the Internet. Since the Internet probably exists everywhere that the enterprise has LANs, there are two principal ways that this traffic can be carried. Each LAN can have a connection to its local ISP, or there can be a single connection from one of the core routers to an ISP. The advantage of the first method is that traffic is carried on the Internet rather than on the enterprise network, possibly leading to smaller WAN links. The disadvantage of permitting multiple links, is that the whole enterprise WAN is open to Internet-based attacks. It is also difficult to monitor and secure the many connection points. A single connection point is more easily monitored and secured, even though the enterprise WAN will be carrying some traffic that would otherwise have been carried on the Internet.If each LAN in the enterprise has a separate Internet connection, a further possibility is opened for the enterprise WAN. Where traffic volumes are relatively small, the Internet can be used as the enterprise WAN with all inter-branch traffic traversing the Internet. Securing the various LANs will be an issue, but the saving in WAN connections may pay for the security.Servers should be placed closest to the locations that will access them most often. Replication of servers, with arrangement for off-peak inter-server updates, will reduce the required link capacity. Location of Internet-accessible services will depend on the nature of the service, anticipated traffic, and security issues. This is a specialized design topic beyond the scope of this curriculum.
Content Summary An understanding of the following key points should have been achieved: