The core block

The core block
Typically, you would have only one subnet configured on the core network. However, for
redundancy and load balancing, you could have two or more subnets configured.
Switches can trunk on a certain port or ports. This means that a port on a switch can be a
member of more than one VLAN at the same time. However, the distribution layer will handle
the routing and trunking for VLANs, and the core is only a pass-through after the routing has
been performed. Because of this, core links do not carry multiple subnets per link; the distribution
layer does.
A Cisco 6500 or 8500 switch is recommended at the core, and even though only one of those
switches might be sufficient to handle the traffic, Cisco recommends two switches for redundancy
and load balancing. You could consider a 4000 or 3550 Catalyst switch if you don’t need
the power of the 6500 or the 8500.
Collapsed Core
A collapsed core is defined as one switch performing both core and distribution layer functions;
however, the functions of the core and distribution layer are still distinct. The collapsed core is
typically found in a small network.
Redundant links between the distribution layer and the access layer switches, and between
each access layer switch, can support more than one VLAN. The distribution layer routing is the
termination for all ports. Figure 12.9 shows a collapsed core network design.