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| 1.3 |  | Building-Block Approach |
| 1.3.4 |  | Layer 2 and 3 backbone scaling |

Switched Layer 2 Ethernet cores are very cost-effective, and they provide high-performance connectivity between switch blocks. The classic design model has several switch blocks, each supporting Layer 2 devices in a wiring closet that terminate into a Layer 3 device. A core composed of Layer 2 devices connects the Layer 3 devices.

The Spanning-Tree Protocol represents a practical limit to the scale of a Layer 2 switched backbone. As the number of core devices is increased, there is a corresponding increase in the number of links from the distribution switches to maintain redundancy. Because routing protocols dictate the number of equal-cost paths, the number of independent core switches is limited. Interconnecting the core switches creates bridging loops. Introducing the Spanning-Tree Protocol into the core compromises the high-performance connectivity between switch blocks. Ideally, Layer 2 switched backbones consist of two switches with no spanning tree loops in the topology.

The Layer 2 switched backbone provides redundancy without any spanning tree loops. Because the two core switches are not linked together, loops do not occur. Each distribution switch in every switch block has a separate path to each core switch. The dual connection between the core and distribution device provides each Layer 3 device with two equal-cost paths to every other router in the campus.

Most of the designs successfully follow the Layer 2/Layer 3/Layer 2 model. However, there are designs where the Layer 2/Layer 3/Layer 3 model is advantageous.

Implement a Layer 3 core, for the following reasons:

- Fast convergence
- Automatic load balancing
- Elimination of peering problems

Fast convergence
As the number of switch blocks and servers is increased each distribution-layer device must be connected to the core. Because there is a limit to the number of switch blocks in a dual Layer 2 core, increasing the number of connections means increasing the number of core devices. To maintain redundancy, the core devices must be connected. Bridging loops appear when interconnecting Layer 2 devices. To eliminate bridging loops in the core, the Spanning-Tree Protocol must be enabled. Unfortunately spanning tree has a convergence time of over 50 seconds. If the network core experiences a fault, Spanning-Tree Protocol convergence can disable the core for more than one minute.

If Layer 3 devices are implemented in the core, the Spanning-Tree Protocol is not necessary. In this design, routing protocols are used to maintain the network topology. Convergence for routing protocols takes from 5 to 10 seconds, depending on the routing protocol.

Automatic load balancing
Load balancing allows for a traffic-distribution pattern that best utilizes the multiple links that are providing redundancy. With multiple, interconnected Layer 2 devices in the core, the route for utilizing more than one path must be individually selected. The links must then be manually configured to support specific VLAN traffic. With Layer 3 devices in the core, routing protocols can be used to load balance over multiple equal-cost paths.

Elimination of peering problems
Another issue with the Layer 2 core in a very large network is that of router peering. Router peering mandates that the routing protocol running in a router maintains state and reachability information for neighboring routers. In this scenario, each distribution device becomes a peer with every other distribution device in the network. Scalability becomes an issue in the configuration because each distribution device has to maintain state for all other distribution devices.

By implementing Layer 3 devices in the core, a hierarchy is created, and the distribution device is not considered a peer with all other distribution devices. This type of core may appear in very large campus networks where the network supports more than 100 switch blocks.

Performance and cost issues
Implementing Layer 3 devices in the core is expensive. As stated earlier, the main purpose of the core is to move packets as quickly and efficiently as possible. Although Layer 3 devices can support switching of some protocols, both performance and equipment costs weigh in heavily as design considerations.