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LAN Design Goals. Frequent goals of network design: Functionality - the network must work Scalability - the network must be able to grow Adaptability - network must be able to adapt to new technologies Manageability - network must facilitate network monitoring and management.
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LAN Design Goals • Frequent goals of network design: • Functionality - the network must work • Scalability - the network must be able to grow • Adaptability - network must be able to adapt to new technologies • Manageability - network must facilitate network monitoring and management
Components of LAN Design • Critical factors when designing a LAN • Function and placement of servers • Contention • Segmentation • Bandwidth vs. broadcast domains
Function and Placement of Servers • Servers categorised as Enterprise or Workgroup • Enterprise servers • Support all users on a network (e.g. email, DNS) • Should be placed at the MDF • Workgroup servers • Support specific group of employees • Should be placed at IDF nearest workgroup
IDF LAN design considerations To maximize available LAN bandwidth and performance: • The function and placement of servers • Collision detection issues • Segmentation issues • Broadcast domain issues MDF: Main Distribution Facility IDF: Intermediate Distribution Facility
Cabrillo College – MDF/IDF Map MDF IDF MDF: Main Distribution Facility IDF: Intermediate Distribution Facility
LAN design considerations Server Placement • Servers can be categorized into two distinct classes: • Enterprise servers (located in a Data Centre) • Workgroup servers (Located in specific departments within the intranet) • An enterprise server supports all the users on the network by offering services, such as e-mail or Domain Name System (DNS) that everyone in an organization would need because it is a centralized function. • A workgroup server supports a specific set of users, offering services such as word processing and file sharing. Other examples might include applications that are specific to a group of users.
LAN design considerations Server Placement • Enterprise servers should be placed in the main distribution facility (MDF). • Traffic to the enterprise servers travels only to the MDF and is not transmitted across other networks. • Ideally, workgroup servers should be placed in the intermediate distribution facilities (IDFs) closest to the users accessing the applications on these servers. • By placing workgroup servers close to the users, traffic only has to travel the network infrastructure to an IDF, and does not affect other users on that intranet network segment. • Layer 2 LAN switches located in the MDF and IDFs should have at least 100 Mbps or more allocated to these servers.
Contention on an Ethernet Network • Contention refers to excessive collisions on Ethernet, caused by too many devices, each with a great demand, on a single network segment • Collisions are overhead on Ethernet – more collisions means less data gets through • Contention solved by segmentation
A broadcast domain is a logical division of a computer network, in which all nodes can reach each other by broadcastat the data link layer A broadcast domain can be within the same LAN segment or it can be bridged to other LAN segments. • In terms of current popular technologies: Any computer connected to the same Ethernet repeater or switch is a member of the same broadcast domain. • Further, any computer connected to the same set of inter-connected switches/repeaters is a member of the same broadcast domain. • Routers and other higher-layer devices form boundaries between broadcast domains.
A Bandwidth or collision domain is a section of a network where data packets can collide with one another when being sent on a shared medium or through repeaters, in particular, when using early versions of Ethernet. • A network collision occurs when more than one device attempts to send a packet on a network segment at the same time. • Collisions are resolved using carrier sense multiple access with collision detection in which the competing packets are discarded and re-sent one at a time. This becomes a source of inefficiency in the network
Network Design Methodology • A systematic, step-by-step approach: • Gathering the users’ requirements and expectations • Analysing requirements • Designing the Layer 1, 2 and 3 structure • Documenting the logical and physical network implementation
Layer 1 Design • Issues: • Type of cabling to be used • Layout of cable • Distance limitations • Use fibre-optic for backbones / vertical runs, UTP for horizontal runs • Most problems caused by Layer 1 issues • Cable installation must meet standards
Layer 2 Design • Common Layer 2 Devices:
Layer 2 design • Collisions and collision domain size are two factors that negatively affect the performance of a network. • Micro-segmentation of the network reduces the size of collision domains and reduces collisions. • Micro-segmentation is implemented through the use of bridges and switches. • The goal is to boost performance for a workgroup or a backbone. • Switches can be used with hubs to provide the appropriate level of performance for different users and servers.
Layer 3 design • Routers can be used to create unique LAN segments and also allow for connectivity to wide-area networks (WANs), such as the Internet. • Layer 3 routing determines traffic flow between unique physical network segments based on Layer 3 addressing. • Routers provide scalability because they serve as firewalls for broadcasts. • They can also provide scalability by dividing networks into subnetworks, or subnets, based on Layer 3 addresses. • VLAN implementation combines Layer 2 switching and Layer 3 routing technologies to limit both collision domains and broadcast domains. • VLANs can also be used to provide security by creating the VLAN groups according to function and by using routers to communicate between VLANs.
Switched LANs, access layer overview The hierarchical design model includes the following three layers: • The access layer provides users in workgroups access to the network. • The distribution layer provides policy-based connectivity. • The core layer provides optimal transport between sites. • The core layer is often referred to as the backbone.
Access layer switches • Access layer switches operate at Layer 2 of the OSI model and provide services such as VLAN membership. • The main purpose of an access layer switch is to allow and connect end-users into the network. • An access layer switch should provide this functionality with low cost and high port density. • Catalyst 1900 series • Catalyst 2820 series • Catalyst 2950 series • Catalyst 4000 series • Catalyst 5000 series
Distribution Layer • The purpose of this layer is to provide a boundary definition in which packet manipulation can take place. • Networks are segmented into broadcast domains by this layer. • Policies can be applied and access control lists can filter packets. • The distribution layer also prevents problems from affecting the core layer. • Switches in this layer operate at Layer 2 and Layer 3. • The distribution layer includes several functions such as the following: • Aggregation of the wiring closet connections • Broadcast/multicast domain definition • Virtual LAN (VLAN) routing • Any media transitions that need to occur • Security
Distribution layer switches 6500 2926G • Distribution layer switches are the aggregation points for multiple access layer switches. • The switch must be able to accommodate the total amount of traffic from the access layer devices. • The distribution layer combines VLAN traffic and is a focal point for policy decisions about traffic flow. • For these reasons distribution layer switches operate at both Layer 2 and Layer 3. • The following Cisco switches are suitable for the distribution layer: • Catalyst 2926G • Catalyst 5000 family • Catalyst 6000 family
Core Layer • The core layer is a high-speed switching backbone. • If they do not have an associated router module, an external router is used for the Layer 3 function. • This layer of the network design should not perform any packet manipulation. • Packet manipulation, such as access list filtering, would slow down the switching of packets. • Providing a core infrastructure with redundant alternate paths gives stability to the network in the event of a single device failure.
Core Layer Switches Lightstream 1010 8540 • In a network design, the core layer can be a routed, or Layer 3, core. • Core layer switches are designed to provide efficient Layer 3 functionality when needed. • Factors such as need, cost, and performance should be considered before a choice is made. • The following Cisco switches are suitable for the core layer: • Catalyst 6500 series • Catalyst 8500 series • IGX 8400 series • Lightstream 1010
VLANs • A VLAN is a logical grouping of devices or users • Devices or users can be grouped by function, department, or application, regardless of their physical segment location • VLAN configuration is done at the switch and/or router via software • VLANs are not standardized and require the use of proprietary software from the switch vendor
VLANs vs. Typical LANs • A typical LAN is configured according to the physical infrastructure it is connecting • Users are grouped based on their location in relation to the hub they are plugged in to and how the cable is run to the wiring closet • The router interconnecting each shared hub typically provides segmentation and can act as a broadcast firewall • The segments created by switches do not split broadcast domains
