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Designing a Network Topology

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Introduction• During the topology design phase

• you identify networks and interconnection points• the size and scope of networks, • And the types of internetworking devices

that will be required, but not the actual devices.• This chapter provides tips for both campus and enterprise WAN

network design and focuses on hierarchical network design, which is a technique for designing scalable campus and WAN networks using a layered, modular model.

Designing a Network Topology• Network design is an art, not a science• There are no rules• There are no precisely correct formulas• It always depends on requirements /needs

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Hierarchical Network Design• Design network in discrete layers.• Each layer focused on specific

functions-which allow the organization to choose the right systems and features for the layer.

• Use high speed WAN routers to carry traffic across the enterprise backbone.

• Middle-speed routers can connect buildings at each campus and switches connects user devices and servers within building.

Enterprise WANBackbone

Campus A Campus B

Campus C

Building C-1 Building C-2

Campus C Backbone

Core Layer

Distribution

LayerAccess Layer

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Hierarchical Design Model• A core layer of high-end routers and switches that are optimized for

availability and speed• A distribution layer of routers and switches that implement policies

and segment traffic• An access layer that connects users via hubs, switches, and other

devices

Hierarchical Network Levels Access

The access layer is where workstations connect to hubs/switches The access layer can include routers, switches, bridges, shared-media hubs , wireless

access point. For internetworks that include small branch offices, the access layer can provide access

into the corporate VLANs may be used to create separate broadcast domains at this level With a layered design, a failure in an access layer device will only affect those devices

directly attached to it In multistory building for example, each floor would be isolated this way

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Hierarchical Network Levels

• Distribution• Is the separation point between the access and core layers of

the network.• Has many roles –

• control access to resource for security reason• control network traffic that traverses the core for performance reason.

• This layer defines broadcast domain.• Allow the core layer to connect sites that run different protocols.• To improve routing protocol performance, this layer can

summarize routes from the access layer.

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Hierarchical Network Levels• Core

• The high-speed backbone of the internetwork.• It is critical for connectivity- need to design with redundancy.• When configure routers in this layer- should use routing features that

optimize packet throughput.• Should avoid packet filters that will slow down the packet.

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Why Use a Hierarchical Model?• Reduces workload on network devices

• Avoids devices having to communicate with too many other devices • The workload required of the CPUs on the devices can be burdensome.• Example in large flat (switched) network , broadcast packets are giving trouble- broadcast packet interrupts the CPU on each device

within the broadcast domain and demands processing time on every device

• Design a modular topology that limits the number of communicating routers-reduce router advertisements.

• Minimize costs- purchase the appropriate devices for each layer of the hierarchy • Enable accurate capacity planning within each layer of the hierarchy-reducing wasted bandwidth.• Modularity enables user to design network with simple and easy to understand .

• Simplicity – minimize the need for extensive training for network operation.

• Facilitates changes- as elements in a network require change, the cost pf upgrading the network may just be based on one small subset out of the overall network.

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Flat VS Hierarchical

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Flat Network• With a flat network design, there is no hierarchy.• In a flat network all connecting devices are on the same level

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Flat Network design • Flat is adequate for a very small network• No hierarchy• Each interconnecting device has essentially the same job• The network is not divided into layers or modules]• Easy to design and implement, maintain as long as it is small.• Not suit for big network since lack of hierarchy make troubleshooting

difficult- need to inspect entire network

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Flat Versus Hierarchy

Flat Loop Topology

Headquarters in Medford

Grants Pass Branch Office

Ashland Branch Office

Klamath Falls Branch Office

Headquarters in Medford

Ashland Branch Office

Klamath Falls Branch Office

Grants Pass Branch Office

White City Branch Office

Hierarchical Redundant Topology

Flat WAN topologies

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• WAN for small company can have few sites connected in a loop.• Each site has a WAN router that connects to 2 other adjacent sites via point-to-point

links.• When more than 1 link fails, some sites will be isolated.

LAN Flat topology

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The typical architecture for a small LAN is workstations, printers, and servers attached to one or more hubs or to a small switch in a flat topology.

Flat LAN• The workstations, printers, and servers here use a MAC process, such as Ethernet's

carrier sense multiple access collision detect (CSMA/CD), controlling access to the shared bandwidth.

• These devices are all part of the same bandwidth and broadcast domain and have the capability to impact the throughput of other devices and cause delay in traffic delivery.

• For networks with high bandwidth requirements- numerous users and/or traffic-intensive applications, network designers recommend attaching the workstations, printers, and servers to switches rather than hubs.

• hubs work at the physical layer (Layer 1) and switches work at the data link layer (Layer 2), the network is segmented into multiple smaller collision domains.

• This means that a small number of devices compete for bandwidth at any one time, rather than a "free-for-all" in which everyone competes for the bandwidth.

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Mesh VS Hierarchical

Mesh topologies• Network designer recommend mesh topology to meet availability

requirements. • 2 types of mesh topologies ;

• Full mesh• Every router /switch is connected to every other router/switch• Provide complete redundancy, good performance

• Partial mesh• Has fewer connection

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Mesh topology• Disadvantages;

• Expensive to deploy and maintain • Hard to optimize, troubleshoot and upgrade• Have scalability limits for groups of routers that broadcast routing updates or

service advertisement.• As the number of router CPU adjacencies increases, the amount of bandwidth and CPU

resources devoted to processing updates increases.

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Mesh Designs

Partial-Mesh Topology

Full-Mesh Topology

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A Partial-Mesh Hierarchical Design

Headquarters (Core Layer)

Branch Offices (Access Layer)

Regional Offices

(Distribution Layer)

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Problem with mesh topology • Expensive to deploy and maintain• Hard to optimize, troubleshoot and upgrade if not design with simple

hierarchical model.• Have scalability limits for groups of routers that broadcast routing

updates. • Rule of thumb- ND should keep broadcast traffic at less than 20% of the traffic

on each link.• This rule limits the number of adjacent routers that can exchange routing

tables and routing updates.

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Mesh ..continue

Guidelines for Hierarchical Network Design

• First – Control the diameter of a hierarchical enterprise network topology.

• In most cases, 3 major layers are sufficient. • Core, Distribution, Access• Why need to control the diameter?

• It will provides low and predictable latency. • Help predicting the routing paths, traffic flows and

capacity requirements.• It also make troubleshooting and network

documentation easier.

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Continue..• Second- strict control of the network topology at the access layer

should be maintained. • The access layer is the most susceptible to violations-since user have the

tendency to add network to the internetwork inappropriately. • Exp: a network admin at a branch office might connect the branch network to

another branch –adding the fourth layer-adding a chain-common mistake.

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Chain and Backdoor at the Access layer

Continue..• Third- avoid backdoors – is a connection between devices in the same

layer.• A backdoor can be an extra router, bridge, or switch added to connect two

networks.• It is should be avoided since they can cause unexpected routing and switching

problems and make network documentation and troubleshooting more difficult.

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Continue..• Finally- design the access layer first, followed by distribution layer

then the core layer.• Why access layer?

• Can accurately perform capacity planning for the distribution and core layers.• Can also recognize the optimization techniques you will need for the

distribution and core layers.

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Redundant Network Design Topologies • Meet network availability requirement• How ?

• Duplicating the network elements

• The goal for redundancy – to eliminate any single point of failure on the network by duplicate any required component whose failure could disable critical applications.

• Components: core router , switch , a link between two switches , a power supply etc….

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Where to implement RNDT• Inside campus networks

• Meet availability goals for users accessing local services

• Between layers of the hierarchical model • On the edge of the enterprise network to ensure high availability for

Internet, extranet and VPN access.

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Drawback of redundancy • Add complexity

• To the network topology • To network addressing • To routing

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What inside Redundant network • Need to have backup path for packet to travel when primary link is

down.• Backup path have duplicates devices and link as in primary path.• 2 aspects of the backup path that we need to look into:

• How much capacity the backup path supports• How quickly the network will begin to use the backup path

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Redundant network – load sharing• Second goal for redundant network is to improve performance by

supporting load sharing across parallel links. • Load sharing/load balancing – allows two or more paths to share the

traffic load.

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References• http://www.edrawsoft.com/Network-Architecture.php• http://www.erg.abdn.ac.uk/~gorry/eg3561/intro-pages/protocols.html• http://study-ccna.com/collision-broadcast-domain• http://www.google.com/images