The dual core is made up of redundant switches, and is bounded and isolated by Layer 3 devices. Routing protocols determine paths and maintain the operation of the core. As with any network, you must pay some attention to the overall design of the routers and routing protocols in the network. As routing protocols propagate updates throughout the network, network topologies might be undergoing change. The size of the network (the number of routers) then affects routing protocol performance, as updates are exchanged and network convergence takes place.
While the network shown in Figure 2-10 might look small with only two switch blocks of two Layer 3 switches (route processors within the distribution layer switches) each, large campus networks can have many switch blocks connected into the core block. Layer 2 devices are used in the core with usually only a single VLAN or subnet across the core. Therefore, all route processors connect into a single broadcast domain at the core. Each route processor must communicate with and keep information about each of its directly connected peers. Thus, most routing protocols have practical limits on the number of peer routers that can be connected.
Because two equal-cost paths from each distribution switch into the core, each router forms two peer relationships with every other router. Therefore, the actual maximum number of switch blocks that can be supported is half the number of distribution layer routers. For example, if routing protocols such as Open Shortest Path First (OSPF) and Enhanced Interior Gateway Routing Protocol (EIGRP) could support a maximum of 50 peers, only 25 switch blocks could be connected into the core. Also keep in mind that other types of campus modules (server blocks, WAN blocks, mainframe blocks, and so forth) connect into the core and create additional peer relationships.
One final core size consideration is related to the routing protocols and their support of equal-cost paths. In the case of dual core design, these paths must lead to isolated VLANs or subnets if a routing protocol supports two equal-cost paths. In other words, each path must be connected to a separate physical core switch. Two equal-cost paths are used in a dual core design with two Layer 2 switches. Likewise, a routing protocol that supports six equal-cost paths requires that the six distribution switch links be connected to exactly six Layer 2 devices in the core. Although this setup sounds complicated, it gives six times the redundancy and six times the available bandwidth into the core.
This leads to a final design point for the actual core switch—scale the core switch to match the incoming load. At a minimum, the core switch must be able to handle switching each of its incoming distribution links at 100% capacity.
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