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Network Design and Planning

Hierarchical Network Design

Understanding traffic flow is an important step when designing a campus network, The network traffic then can be effectively moved and managed, and you can scale the campus network to support future needs. Ideally you network should be designed, so that the users resources are in the same building.

Traffic flows in a campus network can be classified as three types, based on where the network service or resource is located in relation to the end user. Table 12-2 lists these types, along with the extent of the campus network that is crossed going from any user to the service.

Table 12-2. Types of Network Services

Service Type Location of Service Extent of Traffic Flow
Local Same segment/VLAN as user Access layer only
Remote Different segment/VLAN as user Access to distribution layers
Enterprise Central to all campus users Access to distribution to core layers

Cisco have adopted a there layer hierarchical which makes the network easier to understand,troubleshoot and scale future changes, these are known as the building blocks

What are the building Blocks ?

– Access layer

– Distribution layer

– Core (backbone) layer

Access Layer

The access layer is were the end user connects to the network i.e. PC’s,Printers and IP Phones, the access access layer usually provide layer 2 VLAN’s between the users,sometimes called building access switches, should have the following capabilities:

  • QoS (marking, policing, etc.)
  • Scalable uplinks to higher layers
  • Security (802.1x, port security, DAI, etc.)
  • Multicast traffic management (IGMP Snooping)
  • Broadcast domain segmentation (VLANs)
  • Resiliency through multiple uplinks

Distribution Layer

The distribution-layer switches must be capable of processing the total volume of traffic from all the connected devices, the distribution layer usually is a Layer 3 boundary, where routing meets the VLANs of the access layer.

  • Multiple connections to upstream to Core and downstream to Access
  • Offers services such as
    – Gateway redundancy (HSRP/VRRP/GLBP)
    – Bandwidth aggregation (EtherChannel/802.3ad)
    – Load balancing
    – Topology summarization
  • High Layer 3 throughput for packet handling

Core Layer

A campus network’s core layer provides connectivity of all distribution-layer devices. The core, sometimes referred to as the backbone, must be capable of switching traffic as efficiently as possible. Core devices, sometimes called campus backbone switches, should have the following attributes:

  • Must be fast and reliable as all other blocks depend on it
  • Typically hardware accelerated Layer 3 Switches
  • Offers services such as
    – Wire speed forwarding
    – Fast convergence around a link or node failure
    – Efficient bandwidth utilization

Network Device Roles

To first understand how the different devices interact, we must understand what role different devices play in the network.

Hubs and Repeaters

  • Work at layer 1 of OSI mode
  • When a frame is received it is sent back out all ports– i.e. “multiport repeater”
  • Typically unintelligent and unmanaged
  • Does not inspect frame at all before forwarding
  • Accepts no user-defined configuration
  • Devices connected to a hub are in the same… Collision domain
    • i.e. Ethernet CSMA/CD Half-Duplex transmission Broadcast domain


Layer 2 Bridges & Switches

  • Work at layer 2 of OSI model can be managed or unmanaged
  • For Ethernet, “frames” are forwarded based on destination layer 2 MAC address
  •  “CAM” table used for decisions
  • Devices connected to a bridge/switch are… in the same broadcast domain but not in the same collision domain
  • Operates at Full-Duplex transmission

CAM Table Limitations

  • Switches use the MAC address (CAM) table to do destination based switching
  •  CAM table cannot be summarized like IP routing 50,000 hosts in the network, 50,000 MAC addresses per CAM per switch
  • When CAM is full, switch acts like a hub

Broadcast Domain Limitations

  • Devices in the same VLAN, or everyone in a flat network, are directly addressable via FFFF.FFFF.FFFF
  • Larger the broadcast domain, more likelihood of a “broadcast storm”
  • Limiting hosts per VLAN limits broadcast domain size
  • Usually one VLAN per /24 IP subnet is a good rule

Layer 3 Routers

  • Work at layer 3 of OSI model
  • “Packets” are forwarded based on destination layer 3 address
  • Rebuilds layer 2 frame header at every hop
  • All router links are in separate collision and broadcast domains
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One thought on “Network Design and Planning

  1. Pingback: How to configure a Catalyst 3750/3750-E/3750-X Series Switches Using LLDP (Link Layer Discovery Protocol) « Pete's Packet

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