Content

| 1.3 |  | Building-Block Approach |
| 1.3.3 |  | Building the core block |

A core is required when there are two or more switch blocks. The core block is responsible for transferring cross-campus traffic without any processor-intensive operations. All the traffic going to and from the switch blocks, server blocks, the Internet, and the wide-area network must pass through the core. Traffic going from one switch block to another also must travel through the core. It follows that the core handles much more traffic than any other block. Therefore, the core must be able to pass the traffic to and from the blocks as quickly as possible.

Different technologies such as frame, packet, or cell-based technologies can be used in the core, depending on specific needs of the organization. The examples and labs will be using an Ethernet core. Because the distribution switch provides Layer 3 functionality, individual subnets will connect all distribution and core devices.

The core can consist of one subnet. However, for resiliency and load balancing, at least two subnets are configured. Because VLANs terminate at the distribution device, core links are not trunk links and traffic is routed across the core. Therefore, core links do not carry multiple VLANs per link. One or more switches make up a core subnet. It is strongly recommended that a minimum of two devices be present in the core to provide redundancy. The core block can consist of high-speed Layer 2 devices or Layer 3 devices.

At a minimum, the media between the distribution switches and the core-layer switches should be capable of supporting the amount of load handled by the distribution switch. Also, at a minimum, the links between core switches in the same core subnet should be sufficient to switch the aggregate amount of traffic with respect to the input aggregation switch traffic. The design of the core should consider average link utilization, while still allowing for future traffic growth.

There are two basic core designs:

- Collapsed core
- Dual core

The collapsed core is characterized by a consolidation of the distribution- and core-layer functions into one device. A collapsed-core design is prevalent in small campus networks. Although the functions of each layer are contained in the same device, the functionality remains distinct. In a collapsed-core design, each access layer switch has a redundant link to the distribution-layer switch. Each access-layer switch may support more than one subnet. However, all subnets terminate on Layer 3 ports on the distribution switch.

Redundant uplinks provide Layer 2 resiliency between the access and distribution switches. Spanning tree blocks the redundant links to prevent loops. Redundancy is provided at Layer 3 by the dual distribution switches with Hot Standby Router Protocol (HSRP), providing transparent default gateway operations for IP. If the primary routing process fails, connectivity to the core is maintained.

A dual-core configuration is necessary when two or more switch blocks exist and redundant connections are required. Figure shows a dual-core configuration where the core contains only Layer 2 switches for the backbone.

A dual-core topology provides two equal-cost paths and twice the bandwidth. Each core switch carries a symmetrical number of subnets to the Layer 3 function of the distribution device. Each switch block is redundantly linked to both core switches, allowing for two distinct, equal path links. If one core device fails, convergence is not an issue because the routing tables in the distribution devices already have an established route to the remaining core device. The Layer 3 routing protocol provides the link determination across the core, while HSRP provides quick fail-over determination. Spanning tree is not needed in the core because there are no redundant links between the core switches.