There is a separate Layer 2 access level when the devices at the Layer 3 distribution layer are capable of Layer 2 switching. The access layer reduces the complexity of the network block by separating the functions of the devices and provides scalability when more ports are required as the network grows. To ensure network modularity, the routers serving this cluster should be dedicated to the cluster and sized to the task. Simplification argues for the reduction of subnets and routed interfaces in the cluster since the service is common. If multiple server clusters are implemented across the network, using a single subnet within the cluster simplifies the configuration of the entire network. The addition of static subnets in the direction of the cluster increases the configuration complexity with little benefit in terms of availability unless the subnets terminate on different routers, which in turn implies separate modular clusters. A separate management subnet is created but is unrelated to the service address configuration. Separate subnets simplify diagnostic activities, but this benefit is achievable with address partitioning within the subnet. Port densities for smaller full featured routers can be inadequate to scale to the connectivity requirements of even this small cluster when the extra ports for management, troubleshooting, and testing are considered.
An alternative design uses the smaller high density integrated switching and routing platforms that are becoming popular as routing functions have moved into commodity Application-Specific Integrated Circuits (ASIC).
When selecting this type of configuration, bandwidth and inter-switch traffic capacity must be considered. In a load balanced configuration under fault conditions, approximately half the call load can travel on the inter-switch link. The inter-switch link must be redundant to prevent a single failure from causing a bifurcated network, and if a Link Aggregation Group (LAG) is used to eliminate potential spanning tree loops, the individual link bandwidth must still be capable of supporting the required traffic.
