Stacking is a method of joining multiple physical access switches into a single logical switch. Switches are interconnected by stackwise interconnect cables, and a master switch is selected. The switch stack is managed as a single object and uses a single IP management address and a single configuration file. This reduces management overhead. Furthermore, the switch stack can create an EtherChannel connection, and uplinks can form MECs with an upstream VSS distribution pair.
Distribution Layer Best Practices
As shown in Figure 7-4, the distribution layer aggregates all closet switches and connects to the core layer. Design considerations for the distribution layer include providing wire-speed performance on all ports, link redundancy, and infrastructure services.
Figure 7-4 Distribution Layer
The distribution layer should not be limited in terms of performance. Links to the core must be able to support the bandwidth used by the aggregate access layer switches. Redundant links from the access switches to the distribution layer and from the distribution layer to the core layer allow for high availability in the event of a link failure. Infrastructure services include quality-of-service (QoS) configuration, security, and policy enforcement. Access lists are configured in the distribution layer.
The following are recommended best practices at the distribution layer:
- Use first-hop redundancy protocols (FHRPs). Hot Standby Router Protocol (HSRP), Virtual Router Redundancy Protocol (VRRP), or Gateway Load Balancing Protocol (GLBP) should be used if you implement Layer 2 links between the Layer 2 access switches and the distribution layer.
- Use Layer 3 routing protocols between the distribution and core switches to allow for fast convergence and load balancing.
- Only peer on links that you intend to use as transit.
- Build Layer 3 triangles, not squares, as shown in Figure 7-5.
Figure 7-5 Layer 3 Triangles
- Use the distribution switches to connect Layer 2 VLANs that span multiple access layer switches.
- Summarize routes from the distribution to the core of the network to reduce routing overhead.
- Use Virtual Switching System (VSS) to eliminate the use of Spanning Tree Protocol and the need for an FHRP.
Core Layer Best Practices
Depending on the network’s size, a core layer might or might not be needed. For larger networks, building distribution switches are aggregated to the core. This is called a collapsed core. This core layer provides high-speed connectivity to the server farm or data center and to the enterprise edge (to the WAN and the Internet).
Figure 7-6 shows the criticality of the core switches. The core must provide high-speed switching with redundant paths for high availability to all the distribution points. The core must support gigabit speeds and data and voice integration.
Figure 7-6 Core Switches
The following are best practices for the campus core:
- Reduce switch peering by using redundant triangle connections between switches.
- Use routing that provides a loop-free topology.
- Use Layer 3 switches on the core that provide intelligent services that Layer 2 switches do not support.
- Use two equal-cost paths to every destination network.