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Hierarchical models enable you to design internetworks that use specialization of function combined with a hierarchical organization. Such a design simplifies the tasks required to build a network that meets current requirements and can grow to meet future requirements. Hierarchical models use layers to simplify the tasks for internetworking. Each layer can focus on specific functions, allowing you to choose the right systems and features for each layer. Hierarchical models apply to both LAN and WAN design.
Benefits of the Hierarchical Model
- Cost savings
- Ease of understanding
- Modular network growth
- Improved fault isolation
After adopting hierarchical design models, many organizations report cost savings because they are no longer trying to do everything in one routing or switching platform. The model's modular nature enables appropriate use of bandwidth within each layer of the hierarchy, reducing the provisioning of bandwidth in advance of actual need.
Keeping each design element simple and functionally focused facilitates ease of understanding, which helps control training and staff costs. You can distribute network monitoring and management reporting systems to the different layers of modular network architectures, which also helps control management costs.
Hierarchical design facilitates changes. In a network design, modularity lets you create design elements that you can replicate as the network grows. As each element in the network design requires change, the cost and complexity of making the upgrade are contained to a small subset of the overall network. In large, flat network architectures, changes tend to impact a large number of systems. Limited mesh topologies within a layer or component, such as the campus core or backbone connecting central sites, retain value even in the hierarchical design models.
Structuring the network into small, easy-to-understand elements improves fault isolation. Network managers can easily understand the transition points in the network, which helps identify failure points.
Today's fast-converging protocols were designed for hierarchical topologies. To control the impact of routing-protocol processing and bandwidth consumption, you must use modular hierarchical topologies with protocols designed with these controls in mind, such as Open Shortest Path First (OSPF).
Hierarchical network design facilitates route summarization. EIGRP and all other routing protocols benefit greatly from route summarization. Route summarization reduces routing-protocol overhead on links in the network and reduces routing-protocol processing within the routers.
Hierarchical Network Design
As shown in Figure 2-1, a traditional hierarchical LAN design has three layers:
- The distribution layer provides policy-based connectivity.
- The access layer provides workgroup and user access to the network.
Figure 2-1. Hierarchical Network Design Has Three Layers: Core, Distribution, and Access
Each layer provides necessary functionality to the enterprise campus network. You do not need to implement the layers as distinct physical entities. You can implement each layer in one or more devices or as cooperating interface components sharing a common chassis. Smaller networks can "collapse" multiple layers to a single device with only an implied hierarchy. Maintaining an explicit awareness of hierarchy is useful as the network grows.
Core Layer
The core layer is the network's high-speed switching backbone that is crucial to corporate communications. The core layer should have the following characteristics:
- Fast transport
- High reliability
- Redundancy
- Fault tolerance
- Limited and consistent diameter
- Quality of service (QoS)
When a network uses routers, the number of router hops from edge to edge is called the diameter. As noted, it is considered good practice to design for a consistent diameter within a hierarchical network. The trip from any end station to another end station across the backbone should have the same number of hops. The distance from any end station to a server on the backbone should also be consistent.
Limiting the internetwork's diameter provides predictable performance and ease of troubleshooting. You can add distribution layer routers and client LANs to the hierarchical model without increasing the core layer's diameter. Use of a block implementation isolates existing end stations from most effects of network growth.
Distribution Layer
The network's distribution layer is the isolation point between the network's access and core layers. The distribution layer can have many roles, including implementing the following functions:
- QoS
- Security filtering
- Address or area aggregation or summarization
- Departmental or workgroup access
- Broadcast or multicast domain definition
- Routing between virtual LANs (VLAN)
- Media translations (for example, between Ethernet and Token Ring)
- Redistribution between routing domains (for example, between two different routing protocols)
- Demarcation between static and dynamic routing protocols
- Filtering by source or destination address
- Filtering on input or output ports
- Hiding internal network numbers by route filtering
- Static routing
The distribution layer provides aggregation of routes providing route summarization to the core. In the campus LANs, the distribution layer provides routing between VLANs that also apply security and QoS policies.
Access Layer
The access layer provides user access to local segments on the network. The access layer is characterized by switched and shared-bandwidth LAN segments in a campus environment. Microsegmentation using LAN switches provides high bandwidth to workgroups by reducing collision domains on Ethernet segments. Some functions of the access layer include the following:
- High availability
- Port security
- Broadcast suppression
- QoS
- Rate limiting
- Virtual access control lists (VACL)
- Spanning tree
- Trust classification
- Auxiliary VLANs
You implement high-availability models at the access layer. The later section "Network Availability" covers availability models. The LAN switch in the access layer can control access to the port and limit the rate at which traffic is sent to and from the port. You can implement access by identifying the MAC address using ARP, trusting the host, and using access lists.
For small office/home office (SOHO) environments, the entire hierarchy collapses to interfaces on a single device. Remote access to the central corporate network is through traditional WAN technologies such as ISDN, Frame Relay, and leased lines. You can implement features such as dial-on-demand routing (DDR) and static routing to control costs. Remote access can include virtual private network (VPN) technology.
Hierarchical Model Examples
You can implement the hierarchical model by using either routers or switches. Figure 2-2 is an example of a switched hierarchical design in the enterprise campus. In this design, the core provides high-speed transport between the distribution layers. The building-distribution layer provides redundancy and allows policies to be applied to the building-access layer. Layer 3 links between the core and distribution switches are recommended to allow the routing protocol to take care of load balancing and fast route redundancy in the event of a link failure. The server-distribution layer provides redundancy and allows access to the servers to be filtered. For example, Cisco Unified CallManager servers are placed in the server farm, and the server distribution is used to control access to the IP Telephony servers.
Figure 2-2. Switched Hierarchical Design
Figure 2-3 shows examples of a routed hierarchical design. In this design, the enterprise network connects to the WAN core. WAN distribution routers provide site redundancy to the remote sites. The selected routing protocol (EIGRP or OSPF) provides Layer 3 load balancing from the remote sites to the core.
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