Hierarchical Design

Objectives

• Describe the benefits of using a hierarchical design model

• Identify the three layers that make up the hierarchical model

• Describe the functions typically performed at each layer

BackboneCampus

BackboneBuilding

Local WorkgroupsRemote Workgroups

Distribution

Access

Core

Three-Layer Model Components

Site A

Site B

Site C

Core-Layer Functions

• Optimized transport between remote sites

– Redundant paths

– Load sharing

– Rapid convergence

– Efficient use of bandwidth

BackboneCampus

BackboneBuilding

Distribution-Layer Functions

• Policy-based connectivity

– Control access to services

– Define path metrics

– Control network advertisements

BackboneBuilding

Access-Layer Functions

• Connect workgroups to backbones– Provide logical segmentation– Group users with common interests – Isolate broadcast traffic from the workgroup– Distribute services between multiple CPUs

Hierarchical model• divide nodes into levels; intra-level nodes talk to each other

mostly; level functional independent

• inter-level communications is summarised

• adv. : better control, scalability & modularity

• control - grouping ease network management

• scalability - traffic contained within region

• modularity -same cost/method for adding node

• adding new nodes won’t disturb other levels ; network can grow smoothly & uniformly

Hierarchical model (Cont.)

• disadv. : address administration overhead

• recommended for enterprise networks

• many hierarchical models exist

• Cisco network model

core level :high speed transport for internetwork

distribution level :implement management policy

access level :connectivity to desktop PCs

Benefits of a Hierarchical Design Model

• Scalability

• Ease of implementation

• Ease of troubleshooting

• Predictability

• Protocol Support

• Manageability

Flat model

• every node is equal and talks to each other

• advantage is plug-and-play for small network

• disadvantage is lack of scalability due to excessive communications and CPU overhead

• may require a major upgrade at certain point if network continues to grow

• generally not recommended as an enterprise network architecture

Site A

Site B

Site C

Core WAN

One-Layer Design-Distributed

CentralSite

RemoteSite

Core WAN

One-Layer Design-Hub and Spoke

Site ACampusBackbone

Building A2Building A1

SiteB Site C Core WAN

Two-Layer Design

Data Center

RemoteSite 1

Redundant Two-Layer Hierarchy

• Use for cost-effective LAN redundancy

Guidelines for effective deployment

• Do not mesh the network within a layer

• Do not place end stations on backbones

• Keep 80% of the traffic local to the workgroup

• Use network features at the appropriate layer

Logical topology

• logical topology may differ from physical

• choices depend on network protocols used :

flat (L2 protocol)

hierarchical (L3 protocol)

• flat logical structure :

based on low cost switches; bridges are out

plug-and-play; no address administration

not scaleable; need to contain broadcast

Logical structure (Cont.)

• hierarchical logical structure :

based on expensive routers or L3 switches

need address administration

scaleable; better control and manageability

• several logical structures can co-exist in a multiple protocol network