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| 1.3 |  | Building-Block Approach |
| 1.3.1 |  | The switch block |

Network building blocks, as shown in Figure , can be any one of the following fundamental campus elements or contributing variables.

Campus elements:

- Switch block
- Core block

Contributing variables:

- Server block
- WAN block
- Mainframe block
- Internet connectivity

The switch block contains a set of logically grouped switches and associated network devices. This set provides a balance of Layer 2 and Layer 3 services. Although the current generation of LAN switches is replacing shared-media concentrators, LAN switches are not replacements for Layer 3 devices. Therefore, the switch block consists of both switch and router functionality.

Layer 2 switches in the wiring closets connect users to the network at the access layer and provide dedicated bandwidth to each port. The access-layer devices merge into one or more distribution-layer devices. The distribution device provides Layer 2 connectivity between access switches and acts as a central connection point for all the switches in the wiring closets. The distribution layer also provides Layer 3 functionality, which supports routing and networking services. The distribution layer shields the switch block against failures in other parts of the network as shown in Figure .

The distribution device can be one of the following:

- A switch and external router combination
- A multilayer switch

If the switch block experiences a broadcast storm, the router prevents the storm from propagating into the core and across the rest of the network. Each block is protected from the other blocks when failures occur. However, the switch block, in which the broadcast storm occurs, still experiences network problems until the device generating the broadcasts is found and removed from the network.

A switch may support one or more subnets. Remember that a subnet must reside within one broadcast domain. This means that all stations residing in or ports configured on the same VLAN are assigned network addresses within the same subnet.

The broadcast-isolation feature of VLANs is the characteristic that allows VLANs to be identified with subnets. For example, the IP ARP propagates only within the VLAN of the originating request. All subnets terminate on Layer 3 devices, such as a router or a route switch module (RSM). To connect to devices in other VLANs, the frame must traverse a Layer 3 device. Using this model, VLANs should not extend beyond the distribution switch.

Access devices have redundant connections, or uplinks, to the distribution switch to maintain resiliency. The Spanning-Tree Protocol allows these redundant links to exist while preventing undesirable loops in the switch block. The Spanning-Tree Protocol terminates at the boundary of the switch block.