Star Network Topology

A variant of the peer-to-peer topology is the star topology, so named for its shape. A star is constructed by homing all locations into a common location. One could argue that this, in essence, creates a two-tiered topology. The distinction between a star and two-tiered topology is that the center router in a star topology may also be used to interconnect the LANs installed at that location with each other as well as the WAN.

In a two-tiered topology, as discussed later in the chapter, the second tier router should be dedicated exclusively to interconnecting the transmission facilities of the other locations. More importantly, a two-tiered topology provides route redundancy by allowing multiple concentration points.

A star topology can be constructed using almost any dedicated transmission facility, including Frame Relay and point-to-point private lines. Figure 13-3 shows an example of a star-shaped WAN.

Figure 13-3: A star-shaped WAN.

A star topology WAN with point-to-point transmission facilities is much more scalable than a peer-to-peer or ring network. Adding locations to the star does not require the reengineering of existing transmission facilities. All that is required is to provide a new facility between the concentration router and the router at the new location.

The star topology rectifies the scalability problems of peer-to-peer networks by using a router to interconnect, or concentrate, all the other networked routers. This scalability is afforded at a modest increase in the number of routers, router ports, and transmission facilities compared to a comparably sized peer-to-peer topology. Star topologies may actually be developed with fewer facilities than ring topologies, as Figure 13-2 and Figure 13-3 demonstrate. The scalability of this topology would be limited by the number of ports that the router at the center of the star could support. Expansion beyond its capacity requires either a reengineering of the topology into a multitiered topology or the replacement of that router with a much larger unit.

Another benefit of a star topology is improved network performance. Overall network performance in a star topology is, in theory, always better than in either a ring or peer-to-peer network. This is because all network-connected devices are, at most, just three hops away from each other. These three hops are the router at the user's location, the concentrator router, and the router at the destination. This degree of consistency is unique to the star topology.

Note In very small wide-area networks, such as those with only two or three internetworked locations, you may be hard-pressed to perceive any difference between a star topology and a peer-to-peer topology. The benefits of a star topology become increasingly apparent as your network increases in size.

The two drawbacks to this approach are that it creates single points of failure and there is no route redundancy. The existence of a single point of failure means that all WAN communications can be disrupted if the concentrator router experiences a failure. The lack of route redundancy means that if the concentrator router fails, you are out of service until that failure is rectified. Dynamic routing protocols will not be able to calculate new paths through the network because there aren't any!

Of course, this limitation can be compensated for in a variety of ways. Ostensibly, this means a slightly more complex topology, such as the partial mesh, or even splitting the star into two smaller stars that are linked together. In the event of a failure, only half of the remote locations would be affected. Additionally, dial-on-demand technologies, such as ISDN and Switched 56, could be used to reestablish a limited amount of communications.

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