Ring Topology

A ring topology can be fairly easily developed from a peer-to-peer network by adding one transmission facility and an extra port on two routers. This minor increment in cost provides route redundancy that can afford small networks the opportunity to implement dynamic routing protocols. Given that the cost of most transmission facilities is mileage sensitive, it would be wise to design the ring to minimize overall distances of those facilities. Figure 13-2 illustrates this WAN topology.

Figure 13-2: A ring-shaped WAN.

Figure 13-2: A ring-shaped WAN.

Usef Locality C

A ring-shaped WAN constructed with point-to-point transmission facilities can be used to interconnect a small number of sites and provide route redundancy at a potentially minimal incremental cost. The existence of redundant routes through the network means that the use of a dynamic routing protocol affords flexibility not available with static routing. Dynamic routing protocols can automatically detect and recover from adverse changes in the WAN's operating condition by routing around the impacted links.

Rings also have some basic limitations. First, depending on the geographic dispersion of the locations, adding an extra transmission facility to complete the ring may be cost prohibitive. In such cases, Frame Relay may be a viable alternative to dedicated leased lines, provided that its performance limitations are acceptable relative to the projected traffic loads.

A second limitation of rings is that they are not very scalable. Adding new locations to the WAN directly increases the number of hops required to access other locations in the ring. This additive process may also result in having to order new circuits. In Figure 13-2, for example, adding a new location (X) that is

Building Internetworks in geographic proximity to User Locations C and D would require terminating the circuit from location C to D. Two new circuits would have to be ordered to preserve the integrity of the ring: one running from C to X and the other from D to X.

The final limitation of a ring is its potential hop intensity. Each interior gateway router on a ring is only adjacent to two other interior gateway routers. The number of hops to any other location depends on the way that these locations were interconnected. From a routing perspective, this is not a good way to minimize the number of hops. It does offer route redundancy, but the hop count between any given source and destination address pair can vary widely.

The ring topology, given its limitations, is likely to be of value only in interconnecting very small numbers of locations. It is preferable to the peer-to-peer interconnection of locations only because of its capability to provide a redundant path to the locations within the ring.

0 0

Post a comment