Network design

Topic 4LAN design

Agenda

• Modular design• Hierarchal model• Campus network design• Design considerations• Switch features

Problem of flat networks

• Large broadcast domains– Broadcast packets interrupt the CPU on each host• Routers, workstations and servers

• Large routing domains– CPU processing hit on routers which are required

to process updates and advertisements from many other routers

• Flat networks are adequate only for very small networks

Hierarchical design model

Each element has a specific set of functions and services and a specific role

Core layer• Performance

– High-speed backbone of the internetwork– Sufficient capacity– Low latency with a limited and consistent network diameter– No filtering and traffic inspection to slow down flows

• Interconnectivity– Connects campus networks to edge distribution (Internet services)– Highly available and highly reliable with redundant and hot

swappable components– Adapts to change quickly with fast converging protocols

• Scalable– Distribution layer switches (routers) can be added without

increasing the diameter– Collapsed core, where core and distribution functions combined

onto same device for smaller networks

Distribution layer

• Aggregates the data received from the access layer switches before it is transmitted to the core layer – Controls the flow of network traffic using policies

• Filtering (ACLs) to control traffic moving onto the backbone and between VLANs

– Defines the size of broadcast domains– Routes traffic between VLANs– Controls and optimises network traffic to the core

• Redistributes less optimal protocols to maintain optimal protocols across the core

• Uses summarised routes to core to simplify routing tables

• High availability and redundancy to ensure reliability– Two or more high performance switches, duplicate links and dual

power supplies

Access layer

• Switches and wireless access points to connect users

• VLAN access for applications with specific requirements such as voice

• Controls which devices are allowed to communicate on the network– Access layer should be controlled so that users can

not add a fourth layer by connecting extra switches, hubs, routers ‘adding a chain’…

– Switchport security– BPDUguard

Benefits of a hierarchical network

• Scalability• Availability and redundancy • Performance• Security• Manageability• Maintainability• Cost efficiency

Campus network design

Campus network design• Campus backbone

– high-performance, switched backbone that connects buildings and different parts of the campus

• Building distribution– Distribution layer switches which aggregates the switches in the wiring closets

of the building• Building access

– Workstations, IP phones and endpoints connected to access switches and wireless access point

• Server farm– Accessed over the core and provides internal server resources to users such as

application, file, print, e-mail, and Domain Name System (DNS) services. • Network management

– Access to management devices that support monitoring, logging, troubleshooting and security functions

• Edge distribution – at provides connectivity between the campus and the rest of the internetwork,

WAN services

Modular design

• Minimise costs by using only required features• Capacity planning – less bandwidth waste• Network management systems can be distributed• Simple and easy to understand• Testing simplified• Fault isolation is improved • Scalable and consistent• Facilitates change

Design guidelines

• Use hierarchical and modular models• Examine single points of failure and build in

redundancy• Characterise application and protocol traffic• Analyse bandwidth availability and determine

capacity required– Design the access layer first– Design the distribution layer next– Design the core layer

Redundancy• Duplicate network components to eliminate single points of failure

– Core and distribution router or switch, trunk links, power supplies– Redundant data centres!

• Expensive to deploy and maintain– Select a level of redundancy that matches the customer‘s requirements for

availability and affordability– Identify critical applications, systems, internetworking devices, and links– Analyse the customer's tolerance for risk and the consequences of not

implementing redundancy– Discuss with the customer the tradeoffs of redundancy versus low cost, and

simplicity versus complexity• Redundancy facilitates load balancing

– Requires routing protocol support • EIGRP and variance command for unequal load balancing• OSPF equal cost load balancing

Network diameter

• Network diameter is the number of devices that a packet has to cross before it reaches its destination. – The number of switches in the path between

endpoints• Keep the network diameter low to avoid high

impact from device latency – Device latency is the delay the packet incurs crossing

the switch. • STP/RSTP is optimised for a network diameter of

seven– Diameters greater that seven will produce errors

Designing links• Identify user communities

– Consider port density – number of switchports needed by the user community– Consider future growth– Consider the traffic flows generated from the network applications and the

locations of servers used

• Data Stores and Data Servers Analysis– Consider the location of data stores: such as servers, storage area networks

(SANs), network-attached storage (NAS)– Consider client-server traffic and use bandwidth aggregation and switch

forwarding rates to eliminate bottlenecks for traffic crossing many switches– Consider Server-server traffic and locate servers together to reduce high traffic

impact from rest of network

• Traffic flow analysis– Use traffic flow analysis to ensure capacity is sufficient

• Capacity on trunks • Internal forwarding rates on switches

Design capacity

• Bandwidth aggregation– Consider the bandwidth requirements of each

layer and aggregate links for more bandwidth– Link aggregation allows multiple switch port links

to be combined to achieve higher throughput between switches.

– EtherChannel, Cisco® proprietary link aggregation technology

Create topology diagrams

• A topology is a map of an internetwork– Indicates network segments, interconnection points

and user communities, servers and data stores– Indicates size and scope of networks– Types of internetworking devices• Access, distribution and core switches, APs, Routers

– Redundant paths and aggregated links

• Document network infrastructure in a topology diagram

Switch performance features• Port density

– Number of ports available on a single switch– Higher port densities use less space and power and require less uplink ports

and less port aggregation for uplinks• Forwarding rates

– How much data the switch can process per second – the processing ability– Wire speed is the data rate that each port on the switch is capable of attaining

Fa or Gig– Does the forwarding rate allow full wire speed across all ports? Is this

required? Yes for distribution switches, probably not for access layer switches• Link aggregation

– To reduce bottlenecks of traffic by allowing up to eight switch ports to be bound together for data communications

– 8 * 1Gb/s = 8 Gbps throughput on uplink– EtherChannel technology allows the grouping of switchports to create one

logical Ethernet link– Fault tolerance and high-speed links between switches, routers, and servers

Switch features• Switch form factors– Fixed configuration switches:

• Cannot be expanded with extra features such as ports– Modular configuration switches

• Buy a chassis of a particular size and modular line cards with the switchports – more flexible

– Stackable switches• StackWise® technology to connect up to nine switches using a

special backplane cable• Higher bandwidth throughput between the switches than

using line ports• Rack size

– thickness of the switch expressed in number of rack units– 1 rack unit (1U)

Other switch features

• Power over Ethernet– Switch delivers power using existing Ethernet cabling– Wireless Access Points and phones can be located

where cables are– Adds considerable cost to switch– Balance cost of switch to cost of power packs and

installing outlets• Multilayer switches• Perform layer 3 and 4 functions such as routing

and enforcing security policy with ACLs

Access layer switch features• Port security

– By number of hosts and by MAC address• VLANs

– For security domains and performance (voice VLAN)• Port speed and link aggregation

– Fast Ethernet or Gigabit Ethernet– Reserve extra switchports for aggregation, faster uplink connections

• Power over Ethernet (PoE)– Extra expensive – is it required for IP phones and Access Points?

• Internal forwarding rate– Does not need to be as high as combined switchport speed as end devices

unlikely to be fully used all the time• QoS support

– Classification of voice and video traffic in a converged network

Distribution layer switch features• Layer 3 support

– Inter-VLAN routing– Security policies – ACLS control where traffic can flow– QoS – classified traffic moves through priority queues more quickly

• Redundancy – Two power supplies which are hot swappable

• High forwarding rates– Layer 3 functions are processor intensive

• Link aggregation– Accept aggregated links from access layer switches– Ether-channel capabilities

• Performance– Move traffic to core with high speed aggregated links – Gig and 10 Gig

Core layer switch features• Very high forwarding rates

– Depends on number and location of devices and their traffic flows– Use traffic flow analysis to determine the rate required

• Link aggregation– to ensure adequate bandwidth support for aggregated 10 GbE

connections• Redundancy

– Fast convergence- the time to resume after hardware or link failure– Layer 3 protocols are faster to recover than layer 2 protocols – Hot-swappable hardware such as power supplies and fans to avoid

downtime• Quality of Service (QoS)

– Moving traffic through the network at the optimal rates for the type of traffic

Selecting switches• What are the business goals for performance level

or redundancy?– How many end devices?– How many access layer switches?– Estimate traffic that each end device generates– Select distribution switches able to process traffic

• Performance and forwarding rates , interfaces • What technology features are required?• Build in redundancy for distribution layer

– Select core switches able to process traffic crossing backbone• Performance and forwarding rates, interfaces• Build in redundancy for core layer

Agenda

• Modular design• Hierarchal model• Campus network design• Design considerations• Switch features