A metropolitan cluster has member nodes in different parts of the city or in adjacent cities. The distance separating the nodes is limited by the network and data replication technology available. The distance between the two sites must be wide enough to reduce the likelihood of both sites getting destroyed by the same disaster.
Metropolitan clusters are useful to Internet service providers (ISPs) that have several sites along a fiber-optic network and to application service pro- viders (ASPs) that need to reach customers around the city. Like a campus cluster, a metropolitan cluster is also a single cluster, the main distinction being that the members are farther apart than in a campus cluster. Increasing the distance between cluster members causes problems. The latency caused by the distance interferes with the performance required for heartbeat links. Therefore, heartbeat links should be over dedicated and fast network connec- tions. DWDMs, repeaters, extended long wavelength GBICs, and cascaded SAN fabrics are used to access remote storage. You need dedicated dark fiber links between the two sites to be able to use the entire bandwidth for storage. The maximum distance allowed between DWDMs, FC switch ports, and repeaters depends on the vendor.
Several hindrances must be overcome when implementing a metropolitan cluster: