Low price per port and high port density can govern switch choice for wiring closet environments, but high-performance wire-rate multilayer switching drives the Campus Core design. Using Campus Core switches reduces the number of connections between the Building Distribution layer switches and simplifies the integration of the Server Farm module and Enterprise Edge modules. Campus Core switches are primarily focused on wire-speed forwarding on all interfaces and are differentiated by the level of performance achieved per port rather than by high port densities.

As a recommended practice, deploy a dedicated Campus Core layer to connect three or more buildings in the Enterprise Campus, or four or more pairs of Building Distribution switches in a very large campus. Campus Core switches are typically multilayer switches.

Issues to consider in a Campus Core layer design include the following:

■ The performance needed in the Campus Core network.

■ The number of high-capacity ports for Building Distribution layer aggregation and connection to the Server Farm module or Enterprise Edge modules.

■ High availability and redundancy requirements. To provide adequate redundancy, at least two separate switches (ideally located in different buildings) should be deployed.

Another Campus Core consideration is Enterprise Edge and WAN connectivity. For many organizations, the Campus Core provides Enterprise Edge and WAN connectivity through Edge Distribution switches connected to the core. However, for large enterprises with a data center, the Enterprise Edge and WAN connectivity are aggregated at the data center module.

Typically, the Campus Core switches should deliver high-performance, multilayer switching solutions for the Enterprise Campus and should address requirements for the following:

■ Gigabit density

■ Data and voice integration

■ LAN, WAN, and metropolitan area network (MAN) convergence

■ Scalability

■ High availability

■ Intelligent multilayer switching in the Campus Core, and to the Building Distribution and Server Farm environments.

Large Campus Design

For a large campus, the most flexible and scalable Campus Core layer consists of dual multilayer switches, as illustrated in Figure

Multilayer-switched Campus Core layers have several best-practice features:

■ Reduced multilayer switch peering (routing adjacencies): Each multilayer Building Distribution switch connects to only two multilayer Campus Core switches, using a redundant triangle configuration. This implementation simplifies any-to-any connectivity between Building Distribution and Campus Core switches and is scalable to an arbitrarily large size. It also supports redundancy and load sharing.

■ Topology with no spanning-tree loops: No STP activity exists in the Campus Core or on the Building Distribution links to the Campus Core layer, because all the links are Layer 3 (routed) links. Arbitrary topologies are supported by the routing protocol used in the Campus Core layer. Because the core is routed, it also provides multicast and broadcast control.

■ Improved network infrastructure services support: Multilayer Campus Core switches provide better support for intelligent network services than data link layer core switches could support.

Small and Medium Campus Design Options

A small campus (or large branch) network might have fewer than 200 end devices, and the network servers and workstations might be connected to the same wiring closet. Because switches in a small campus network design may not require high-end switching performance or much scaling capability, in many cases, the Campus Core and Building Distribution layers can be combined into a single layer, as illustrated on the left of Figure below. This design can scale to only a few Building Access layer switches. A low-end multilayer switch provides routing services closer to the end user when multiple VLANs exist. For a very small office, one low-end multilayer switch may support the LAN access requirements for the entire office.

For a medium-sized campus with 200 to 1000 end devices, the network infrastructure typically consists of Building Access layer switches with uplinks to Building Distribution/Campus Core multilayer switches that can support the performance requirements of a medium-sized campus network. If redundancy is required, redundant multilayer switches connect to the Building Access switches, providing full link redundancy, as illustrated on the right of Figure as mentioned above.