Each layer of the hierarchical architecture contains special considerations. The following sections describe best practices for each of the three layers of the hierarchical architecture: access, distribution, and core.
When designing the building access layer, you must take into consideration the number of users or ports required to size up the LAN switch. Connectivity speed for each host should be considered. Hosts might be connected using various technologies such as Fast Ethernet, Gigabit Ethernet, or port channels. The planned VLANs enter into the design.
Performance in the access layer is also important. Redundancy and QoS features should be considered.
The following are recommended best practices for the building access layer:
■ Limit VLANs to a single closet when possible to provide the most deterministic and highly available topology.
■ Use RPVST+ if STP is required. It provides the best convergence.
■ Set VLAN Dynamic Trunking Protocol (DTP) to desirable/desirable with negotiation on.
■ Manually prune unused VLANs to avoid broadcast propagation.
■ Use VTP transparent mode, because there is little need for a common VLAN database in hierarchical networks.
■ Disable trunking on host ports, because it is not necessary. Doing so provides more security and speeds up PortFast.
■ Consider implementing routing in the access layer to provide fast convergence and Layer 3 load balancing.
■ Use the switchport host commands on server and end-user ports to enable PortFast and disable channeling on these ports.
As shown in Figure 3-6, 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 3-6 Distribution Layer
The distribution layer should not be limited on 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 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. Hot Standby Router Protocol (HSRP) 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 links between the distribution and core switches to allow for fast convergence and load balancing.
■ Build Layer 3 triangles, not squares as shown in Figure 3-7.
Figure 3-7 Layer 3 Triangles
■ Summarize routes from the distribution to the core of the network to reduce routing overhead.
Depending on the network's size, a core layer may or may not be needed. For larger networks, building distribution switches are aggregated to the core. This provides high-speed connectivity to the server farm/data center and to the Enterprise Edge (to the WAN and the Internet).
Figure 3-8 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.
The following are best practices for the campus core:
■ Reduce the switch peering by using redundant triangle connections between switches.
■ Use routing that provides a topology with no Layer 2 loops which are seen in Layer 2 links using spanning tree protocol.
■ Use Layer 3 switches on the core that provide intelligent services that Layer 2 switches do not support.
Figure 3-8 Core Switches
Server Farm/Data Center
Server Farm/Data Center
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