Three-Tiered Network Hierarchy Topology

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• hierarchical model breaks down a large and complex network design into smaller sections based on functions performed
  • each function can be assessed by network designers to identify the most efficient hardware and software to use to implement it
  • many corporate office networks follow Cisco’s design principles:
    • three-tiered hierarchy:
      • access
      • distribution
      • core

Access/Edge Layer

• the access/edge layer allows end user devices to connect to the network
• implemented for each site using structured cabling and wall ports for wired access and access points for wireless access
  • both are ultimately connected to workgroup switches
• switches deployed to serve the access layer are called LAN switches or data switches
  • end systems connect to switches in the access/edge layer in a star topology
  • there are no direct links between the access switches
• 1 Gbps access port and 10 Gbps uplinks ports are typical on a workgroup switch

Distribution/Aggregation Layer

• distribution/aggregation layer provides fault tolerant interconnections between different access blocks and either core or other distribution blocks
• each access switch has full or partial mesh links to each router or layer 3 switch in its distribution layer block
• distribution layer is often used to implement traffic policies
  • e.g., routing boundaries, filtering, or quality of service (QoS)
• a layer 3 switch is a capable of IP routing between virtual LAN (VLAN) subnets using hardware-optimized path selection and forwarding
  • used to implement the distribution/aggregation layer
  • have different capabilities to the layer 2 workgroup switches used in access tier
  • Aggregation switches have 10 Gbps interfaces and 40 Gbps (or 40/100 Gbps) uplink/backbone ports
  • works on the principle of “route once, switch many”
    • means that once a route is discovered,
      • it is cached with the destination MAC address
      • subsequent communications are switched without invoking the routing lookup
  • a router uses a generic processor and firmware to process incoming packets
  • but a layer 3 switch uses an application-specific integrated circuit (ASIC)
    • impacts the relative performance
    • much faster, but not always very flexible
  • cannot perform WAN routing
  • work with interior routing protocols only
  • often only supports Ethernet

Core Layer

The core layer provides a highly available network backbone.

• client and server computers should not be attached directly to the core
• purpose should be kept simple:
  • provide redundant traffic paths for data to continue to flow around the access and distribution layers of the network
• routers or layer 3 switches in the core layer establish a full mesh topology with switches in the distribution layer blocks

Collapsed Core

• Medium-sized networks might not need separate core and distribution layers
• in a two-tiered or collapsed core model, a monolithic core layer is implemented as a full mesh
  • impractical if there are a large number of core switches
    • design is less scalable