Switching Basics and Intermediate Routing CCNA 3 Chapter 5

LAN Design Introduction LAN design has become more difficult Due to multiple media types and LANs Complexity has increased Three aspects of a network that need to be identified before designing a large LAN: An access layer that connects end users to a LAN A distribution layer that provides policy-based connectivity between end-user LANs A core layer that provides the fastest connection between distribution points

LAN Design LAN Design Goals Requirements of most networks designs: Functionality: the network must work as intended Scalability: the network must be expandable Adaptability: the network must be designed with a vision toward future technologies Manageability: the design must facilitate network monitoring and management to ensure stability

LAN Design LAN Design Considerations A virtual LAN (VLAN) is a group of devices on one or more LANs that communicate as if they were attached to the same wire To maximize bandwidth and performance, address these LAN design considerations: Function and placement of servers Collision domain issues Segmentation issues Broadcast domain issues

LAN Design LAN Design Considerations Servers are usually dedicated to one function such as email or file sharing Servers can be one of two types: Enterprise servers support all users on the network e-mail Domain Name System (DNS), the Internet-wide system of mapping names to IP addresses Workgroup servers support a specific set of users and offers services such as word processing and file sharing

LAN Design LAN Design Considerations Enterprise servers are usually placed in the main distribution facility (MDF) Traffic to enterprise servers should travel only to the MDF and not across other networks Workgroup servers should be placed in the intermediate distribution facilities (IDFs) closest to the users who access the applications on these servers Layer 2 switches in the MDF and IDF should have 1000Mbps (1Gbps) allocated bandwidth

LAN Design LAN Design Considerations Servers Are Typically Placed at a Point of Convergence in the Network, Such as Within an IDF or MDF

LAN Design LAN Design Considerations Ethernet nodes use carrier sense multiple access with collision detection (CSMA/CD) Each node must contend with all other nodes for access to the shared medium, or collision domain If two nodes transmit at the same time, a collision occurs The transmitted frames are destroyed and a jam signal is sent to all nodes on the segment Excessive collisions reduce bandwidth

LAN Design LAN Design Considerations Collisions Increase Multiplicatively with the Number of Hosts

LAN Design LAN Design Considerations Microsegmentation is when a single collision domain is split into smaller collision domains Reduces number of collisions on a LAN segment A broadcast occurs when the destination MAC address is FF:FF:FF:FF:FF:FF Single Broadcast Domain

LAN Design LAN Design Methodology LAN design should be done in a set of systematic steps: Step 1: Gather the requirements and expectations Users Corporate structure Skill level of people User attitudes towards computes and applications Documented policies of the organization Business information flow Data that is mission critical Protocols allowed on the network

LAN Design LAN Design Methodology Step 1: Gather the requirements and expectations (continued) Performance characteristics of current network Types of desktops supported Persons responsible for LAN addressing, naming, topology design, and configuration Current topology Human, hardware, and software resources How resources are linked and shared Financial resources of organization

LAN Design LAN Design Methodology Documentation of requirements allows for an informed estimate of costs and timelines for implementation Availability measures the usefulness of the network Factors affecting availability: Throughput Response time Access to resources Customers may have different definitions of availability As a network designer, goal is greatest availability at least cost

LAN Design LAN Design Methodology Step 2: Analyze the requirements of the network and its users Needs of users change Need for bandwidth increases Voice and video applications The network must reliably provide prompt and accurate information Information requirements of the users and organization must be met

LAN Design LAN Design Methodology Step 3: Decide on the overall LAN topology that will satisfy user requirements Star Extended star (most common) The Star Topology is a Special Case of the Extended Star Topology

LAN Design LAN Design Methodology Step 3: Decide on the overall LAN topology that will satisfy user requirements (continued) LAN topology design has three unique OSI model categories: Network layer (Layer 3) Data link layer (Layer 2) Physical layer (Layer 1) By looking at the OSI layer, the design engineer can properly incorporate products and technologies

LAN Design LAN Design Methodology Step 4: Document the physical and logical topology of the network Physical topology: the way the network components are connected Logical topology: the flow of data in the network, and the name and addressing schemes used

LAN Design LAN Design Methodology Important elements of LAN design documentation: OSI layer topology map LAN logical map LAN physical map Cut sheets, which show cable runs VLAN logical map Layer 3 logical map Address maps

LAN Design LAN Design Methodology VLAN Logical Design

LAN Design LAN Design Methodology IP Networks Are Displayed in a Layer 3 Logical Map

LAN Design LAN Design Methodology Address Maps Provide a Detailed View of IP Addresses for Key Devices and Interfaces

LAN Design Layer 1 Design One of the most important design considerations is the cables Most LAN cabling is based of FastEthernet or Gigabit Ethernet technology Both can utilize full duplex technology, giving concurrent, collision-free, two-way communication A logical bus topology that uses CSMA/CD can also be used with standard Ethernet

LAN Design Layer 1 Design Design issues at Layer 1: Type of cabling (copper or fiber optic) 100BASE-TX specifies Cat5e unshielded twisted-pair (UTP), limited to 100m per segment 100BAS-FX specifies multimode fiber with a length limit of 2 km TIA/EIA-568-A standard details layout and wiring connection schemes Media types: Cat5, Cat5e, Cat6 UTP and shielded twisted-pair (STP) that has shielding around wire pairs and another shield around all the wires in the cable, single-mode fiber, multi-mode fiber

LAN Design Layer 1 Design Design issues at Layer 1 (continued): Carefully evaluate strengths and weaknesses of topologies Layer 1 issues cause most network problems Use fiber-optic cable in the backbone and risers of a network Use Cat5e or Cat6 in horizontal runs Every device should be connected to a central location with a horizontal cabling run

LAN Design Layer 1 Design In a simple star topology with only one wiring closet, the MDF includes one or more horizontal cross-connect (HCC) patch panels HCC patch panels connect Layer 1 horizontal cabling with Layer 2 switch ports The uplink port on the LAN switch is connected to the Ethernet port on the Layer 3 router with a patch cable

LAN Design Layer 1 Design HCC Connects Layer 1 Cabling to Layer 2 Switch Ports

LAN Design Layer 1 Design When hosts in larger networks exceed the 100m distance limitation for Cat5e UTP, more than one wiring closet is required Multiple wiring closets means you have multiple catchment areas Secondary wiring closets are referred to as intermediate distribution facilities (IDFs)

LAN Design Layer 1 Design IDFs Connect via the MDF

LAN Design Layer 1 Design Vertical cabling is also called backbone cabling A vertical cross-connect interconnects IDFs to the central MDF Fiber-optic cable is normally used for the VCC because cable lengths are longer than the100m limit for Cat5e cable

LAN Design Layer 1 Design VCC Interconnects IDFs to the MDF

LAN Design Layer 1 Design The logical diagram is the basic road map of the LAN and includes these elements: Location and identification of MDF and IDF wiring closets Type and quantity of cables used to interconnect the IDFs with the MDF Number of spare cables that are available to increase bandwidth between wiring closets Detailed documentation of all cable runs, identification numbers, and port on which the run is terminated at the HCC or VCC Essential for troubleshooting network problems

LAN Design Layer 2 Design Purpose of Layer 2 devices is to switch frames based on destination MAC address Collisions and collision domain size negatively affect network performance Devices at Layer 2 (and Layer 3) determine the size of collision domains Microsegmentation reduces the size of collision domains and is implemented through the use of switches

LAN Design Layer 2 Design LAN Switches Provide Microsegmentation

LAN Design Layer 2 Design LAN switches allocate bandwidth on a per-port basis This supplies more bandwidth to vertical cabling, uplinks, and servers Referred to as asymmetric switching (provides switch connections between ports of unlike bandwidth) Symmetric switching provides switched connections between ports of similar bandwidth

LAN Design Layer 2 Design Desired capacity of vertical cable runs is greater than that of a horizontal cable run 100 Mbps is adequate on a horizontal drop Asymmetric LAN switches allow 100 Mbps and 1 Gbps on a single switch Next task in the design process is to determine number of 100 Mbps and 1 Gbps ports needed in the MDF and every IDF The number of hosts connected to a single port on a switch determines the size of the collision domain, affects bandwidth available to each host Collision domains can be eliminated by using one host per switch port

LAN Design Layer 3 Design A router is a Layer 3 device Creates unique LAN segments Allows communication between segments based on Layer 3 addresses, such as IP addresses Allows segmentation of LAN into unique physical and logical networks Allows for connectivity to WANs, such as the Internet Forwards data packets based on destination addresses Does not forward broadcasts Is the entry and exit point for a broadcast domain

LAN Design Layer 3 Design When to use a router: If the problem is a protocol issue instead of a contention issue If there are excessive broadcasts on the LAN If a higher level of security is needed However, Layer 3 switches can now perform many of these functions at nearly the same cost Expect Layer 3 switching to become pervasive in 10 years Layer 3 switches perform wire-speed routing, QoS, and security functions

LAN Design Layer 3 Design Physical Network Maps Ease Troubleshooting

LAN Design Layer 3 Design VLAN implementation combines Layer 2 switching and Layer 3 routing technologies Limits collision and broadcast domains Provides security with creation of VLAN groups that communicate only through a router Ports on a switch are assigned to different VLANs

LAN Design Layer 3 Design VLANs Are Essentially Switch Port Groupings

LAN Switches Introduction Early LAN switches did not support VLANs Second-generation switches supported VLANs, but relied on routers for inter-VLAN communication Third generation switches have the route processors built into the switches With the exception of access layer switches, switches are becoming almost indistinguishable from routers

LAN Switches Switched LANs and the Hierarchical Design Model Use of a hierarchical design model makes it more likely to meet the needs of a medium or large organization Layers of the hierarchical model: Access layer: gives users access to the network Distribution layer: provides policy-based connectivity Core layer: provides optimum transport between sites; often referred to as the backbone

LAN Switches Access Layer Overview Entry point to network for user workstations and servers Functions include MAC layer filtering and microsegmentation Layer 2 switches are used

LAN Switches Access Layer Overview The Access Layer is an Entry Point to the Network, Particularly for End Users

LAN Switches Access Layer Switches Access layer switches generally operate at Layer 2 of the OSI model Provide services such as VLAN membership Main purpose is to connect end users Should do this with low cost and high port density

LAN Switches Access Layer Switches Common legacy access layer switches used today Catalyst 1900 series Catalyst 2820 series Catalyst 2950 series (not a legacy switch) Catalyst 4000 series Catalyst 5000 series

LAN Switches Access Layer Switches Catalyst 1900 and 2820 switches were effective for small campus networks The 2950 series provides access for users and servers that require higher bandwidth Use FastEthernet and Gigabit Ethernet ports The 4000 and 5000 series include Gigabit Ethernet ports Effective in large campus networks

LAN Switches Access Layer Switches The Catalyst 4500 Series Is Often Used at the Access Layer in an Enterprise Network

LAN Switches Distribution Layer Overview The distribution layer is between the access and core layers Provides a boundary definition where packet manipulation can take place Segments layers into broadcast domains Can apply policies and access control lists to filter packets Prevents problems at the access layer from affecting the core layer Switches in this layer operate at Layer 2 and Layer 3

LAN Switches Distribution Layer Overview Distribution layer functions: Aggregation of the wiring closet connections Broadcast/multicast domain definition VLAN routing Any media transitions that need to occur Security

LAN Switches Distribution Layer Overview The Distribution Layer Is Typically Where Policy Is Incorporated

LAN Switches Distribution Layer Switches Distribution layer switches are the aggregation points for multiple access layer switches Must be able to accommodate the traffic from access layer devices Must have high performance Use Layer 2 and Layer 3 switching to delineate broadcast domains (multilayer switches that combine switching and routing functions) Combine VLAN traffic Focal points for decisions about traffic flow

LAN Switches Distribution Layer Switches Distribution layer often used legacy Cisco switches: Catalyst 2926G Catalyst 5000 series Catalyst 6000 series Current distribution layer switches: Catalyst 4500 Catalyst 4900 Catalyst 6500

LAN Switches Distribution Layer Switches The Catalyst 6513 Is About as Good as it Gets for a Distribution Layer Switch, Circa 2006

LAN Switches Core Layer Overview The core layer is a high-speed switching backbone If the core switch does not have a routing module, an external router is used for the Layer 3 function This layer should not perform packet manipulation, such as access list filtering Should have redundant paths Current trend is to use wire-speed Layer 3 switching at the core

LAN Switches Core Layer Overview Core Layer is Devoted to High-Speed Switching of Numerous Packets

LAN Switches Core Layer Switches Core Layer is the backbone of the campus switched network Switches in this layer can use various layer 2 technologies Ethernet ATM cell switching The core layer can be a routed (Layer 3) core Factors such as cost, need, and performance must be considered when choosing equipment

LAN Switches Core Layer Switches Core Layer legacy switches: Catalyst 8500 series IGX 8400 series Lightstream 1010 Current core layer switches: Catalyst 6500 series Cisco 12000 series routers (occasionally) Cisco CRS-1 (Carrier Routing System) devices

Summary LAN design typically focuses on: Functionality Scalability Manageability Adaptability Important elements of LAN design documentation: OSI topology map LAN logical map LAN physical map

Summary Important elements of LAN design documentation (continued): Cut sheets VLAN logical map Layer 3 logical map Address maps Layer 1 design issues include: Types of cables Overall structure of the cabling Media types such as Cat5, Cat5e, Cat6, single-mode fiber, and multi-mode fiber

Summary The logical diagram of a LAN includes MDF and IDF locations Type and quantity of cables used to connect IDFs to MDF Number of spare cables available to increase bandwidth between MDF and IDFs Layer 2 devices provide flow control, error detection and error correction, and reduce congestion in a network Microsegmentation of the network reduces the size of collision domains and reduces collisions

Summary Characteristics of routers: Layer 3 devices Create unique LAN segments Allow communication between segments based on Layer 3 addresses (IP, usually) Allow for segmentation of LAN into physical and logical networks Allow for connectivity to WANs such as the Internet

Summary VLAN implementation combines Layer 2 switching and Layer 3 routing technologies Limit collision domains and broadcast domains Originally used to create logical groupings according to function Now more often used to group IP subnets Routers or route processors enable communication between VLANs

Summary The hierarchical design model includes three layers: Access layer gives access to users in workgroups Distribution layer provides policy-based connectivity Core layer provides optimal transport between sites Access layer switches operate at Layer 2 of the OSI model Offer services such as VLAN membership Main purpose is to provide connectivity for end users Should have low cost and high port density

Summary Distribution layer switches delineate broadcast domains Combines VLAN traffic Focal point of decisions about traffic flow Operate at both Layer 2 and Layer 3 of the OSI model (called multilayer switches) Core layer switches provide a high-speed switching backbone Should not be used for packet manipulation (no ACLs) Should have redundant paths