LAN Design and the Hierarchical Network Model
CCNA focuses on networks for SMBs (small, medium businesses).
A hierarchical design model is recommended.
Easier to manage and expand.
Problems are solved more quickly.
Hierarchical design divides the network into 3 layers.
Core, (CL).
Distribrution, (DL).
Access, (AL).
Each layer provides specific functions.
This "modularity" facilitates scalability and performance.
Access Layer, (AL): lowest
Interfaces with the end device (user).
Includes routers, switches, bridges, hubs and wireless APs.
Provides a means of connecting devices to the network and controlling which one communicate on the network.
Distribution Layer, (DL):
Aggregates data received from the AL before transmitting to the CL for routing.
Controls traffic flow using policies and delineates broadcast domains with VLANs defined at the AL.
VLANs allow traffic segmentation (separate subnetworks).
DL switches are typically high-performance devices that have high availability and redundancy to ensure reliability.
Core Layer, (CL):
The high-speed backbone or the internetwork.
Critical for interconnectivity between distribution layer devices ? needs to be highly available and redundant.
Often connects to Internet resources.
Aggregates tfc fm all devices, so it must be capable of forwarding large amounts of data quickly.
Note: smaller networks often combine the distribution and core layers.
Three Logical Laye3rs are separated into a well-defined hierarchy.
It is much harder to see these layers physically.
Benefits of a Hierarchical Network:
Scalability:
Hierarchical networks scale very well.
The modularity allows you to replicate design elements.
Expansion is easy to plan and implement.
Redundancy:
As a network grows, availability becomes more important.
Availability increases dramatically with hierarchical networks.
E.G. AL switches connect to 2 DL switches. If one DL switch fails, the AL switch can switch to the other one.
Redundancy is limited is at the access layer. Typically, end devices do not connect to multiple switches.
Performance:
Properly designed networks can achieve near wire speed btwn all devices.
Security:
AL switches can be configured to provide control over which devices are allowed to connect to the network.
More advanced security policies available at the DL.
Some AL switches support L3 functionality, but it is usually the job of the DL switches, because they can process it much more efficiently.
Manageability:
Changes can be repeated across all devices in a layer because they presumably perform the same functions.
Deployment of new switches is simplified because configs can be copied with few modifications.
Consistency within each layer simplifies troubleshooting.
Maintainability:
Because of their modularity and scalability, hierarchical networks are easy to maintain.
This also means these networks are less expensive.
With other designs, manageability becomes increasingly complicated as the network grows.
Principles of Hierarchical Network Design:
Hierarchical design is no guarantee of good design.
Simple guidelines help differentiate btwn well-designed and poorly designed hierarchical networks.
Network Diameter:
Usually the first thing to consider.
The # of devices a packet crosses to reach its destination.
Small diameter ensures low and predictable latency.
Bandwidth Aggregation, (adding together):
Combines links btwn switches to achieve up throughput.
Cisco has a proprietary link aggregation technology called EtherChannel.
Aggregated links are indicated by multiple dotted lines with an oval or a single, dotted line with an oval.
Can be used at every layer (less common @ AL).
Redundancy:
Redundancy can be provided in a number of ways.
E.G. 2x connections btwn devices, or 2x devices.
Redundant links can be expensive.
Designing redundancy starts at the AL. You ensure that you accommodate all network devices ? 3 of AL switches.
This helps determine 3 of DL switches ? CL switches.
What is a Converged network?
SMBs are increasingly running voice, video and data.
Convergence is the process of combining these.
Until recently this was limited to large enterprises.
Legacy (older) equipment hinders convergence.
Because analog phones have not yet been replaced, you will also see legacy PBX telephone and IP-based systems.
Convergence is now easier and less expensive.
With a convergence there is just one network to manage.
This costs less to implement and manage.
IT cabling requirements are simplified.
Convergence also creates new opportunities.
You can tie voice and video directly into an employee's PC.
No need for an expensive phone or video equipment.
Softphones (Cisco IP |Communicator) offer a lot of flexibility.
With software businesses can quickly convert to converged networks with little capital expense.
With cheap webcams videoconferencing can be added.
Separate Voice, Video and Data Networks:
Voice networks contain isolated phone lines running to a PBX (Private BDXT Exchange) switch located in a Telco wiring closet PSTN (Public Switch Telephone Network).
Telco closet often separate fm the data and video closets.
New phone ? a new line to the PBX.
Using a properly designed hierarchical network voice lines can be added with little or no impact.
Now that networks can accommodate the BW it makes sense to converge.
Considerations for Hierarchical Network Switches:
Tfc Flow Analysis:
The process of measuring BW usage and analyzing it for performance tuning, planning, and HW improvement.
To select the appropriate gear in a hierarchical network, you need to spec out tfc flows, users and servers.
Networks must be designed with an eye on growth.
Done using tfc flow analysis software.
Should consider port densities and forwarding rates to ensure adequate growth capability.
Analysis Tools:
Many tfc flow analysis tools are available.
E.G. Solarwinds Orion 8.1 NetFlow Analysis.
User Communities Analysis:
Identifies user grpings and their impact on net performance.
Affects port density and tfc flow, which influences the selection of network switches.
Typically users are grped according to job function.
E.G. HR one floor and Finance on another.
Each dept. has different users and needs, and requires access to different resources through the network.
Choose switches that have enough ports to meet the dept needs and pwrful enough to accommodate tfc.
Good network design also factors in the growth.
Investigate the tfc generated by end-user applications.
Some user communities generate a lot, some do not.
The location of the user communities influences where data stores and server farms are located.
By locating users close to their servers, you can reduce network diameter, reducing the impact on other users.
However, usage is not always bound by department or physical location.
Data Stores and Data Servers Analysis:
Data stores can be servers, SANs, NAS, tape bu units, or any other storage device or component.
Considers both client-server and server-server tfc.
Client-server tfc typically traverses multiple switches.
BW aggregation and switch forwarding rates can help eliminate bottlenecks for this type of tfc.
Some server apps generate high volumes btwn servers.
These server shouls be located close to each other (i.e. secured data centers).
Tfc across data center switches is typically very high.
Requires higher performing switches.
Topology Diagrams:
A graphical representation of a network infrastructure.
Shows how all switches are interconnected, including which ports interconnect devices.
It shows where and how many switches are in use.
Can also contain info on device densities and user grps.
Helps visually identify potential bottlenecks.
Very difficult to create after the fact.
Switch Features:
Switch Form Factors:
Fixed or modular config, and stackable or non-stackable.
Thickness is expressed in rack units. (i.e. 1U, 3U).
Fixed Config Switches - Cannot add hardware.
Modular Switches - chassis allows for multiple line cards which contain the ports.
The larger the chassis, the more modules it can support.
Stackable Switches:
Can be interconnected using a special backplane cable that provides high-bandwidth throughput btwn the switches.
Cisco's StackWise technology allows you to interconnect up to nine switches using fully redundant backplane connections.
Stacked switches effectively operate as a single larger switch.
Desirable where fault tolerance and BW availability are critical and a modular switch is too costly.
Performance:
Port Density:
Port density is the 3 of ports available per switch.
Fixed sitches typically 1,000 ports!
Large enterprise networks require high density, modular switches to make the best use of space and pwr.
Also prevent uplink bottlenecks.
A series of fixed swtches consume many additional ports for BW aggregation btwn switches.
Whith a modular switch, aggregation is less of an issue because the chassis backplane provides the BW.
Forwarding Rates:
The processing capabilities of a switch in bps.
Switch product lines are classified by forwarding rates.
If this rate is too low, it cannot accommodate wire-speed across all ports.
Wire speed = rate that each port is capable of (10Mbps etc).
E.G. 48-port GbE switch at wire speed = 48Gbps of tfc.
If the switch only supports 32 Gbps (internally), it cannot run at full wire speed across all posrts simultaneously.
Access switches typically do not need full wire speed because they are physically limited by their uplinks to the DL.
Link Aggregation:
Determine if there are enough ports to aggregate to support the required BW.
E.G. GbE 24-port switch could generate up to 24 Gbps.
If it is connected to the network by a single cable, it can only forward 1 Gbps to the rest of the network.
That results in 1/24th wire speed for each of the 24 devices.
Link aggregation helps to reduce these bottlenecks by allowing up to 8 ports to be grped, providing up to 8 Gbps.
With multiple 10GbE uplinks very high throughput rates can be achieved.
Cisco uses the term EtherChannel = aggregated ports.
Power over Ethernet (PoE):
PoE allows a switch to deliver power over existing Ethernet.
Can be used by IP phones and some wireless APs.
Allows more flexibility for equipment installations.
Adds considerable cost to the switch.
PoE switch marked with a `V`for volts.
Layer 3 Functions:
Typically, switches operate at L2 and deal primarily with MAC addresses.
L3 switches offer advanced functionality.
L3 switches = multilayer switches.
Switch features in a Hierarchical Network:
Access Layer switch Features:
Port security - first line of defense for a network.
How many or what devices are allowed to connect.
All Cisco switches support port layer security.
VLANs - component of converged networks.
Voice tfc is typically given a separate VLAN.
Port speed:
Fast Ethernet is adequate for VoIP and most data tfc.
PoE - much more expensive, so use only when required.
Link Aggregation - supported at all 3 lvls.
QoS - needed for VoIP.
Distribution Layer Switch features:
Collect all AL switch data and forward it to the CL switches.
Provides the inter-VLAN routing functions.
DL switches have higher processing capabilities than AL.
Need L3 to support inter-VLAN routing.
Security Policies:
Need L3 so advanced security policies can be applied.
ACLs control tfc flows through a network.
ACLs allow switches to filter tfc.
ACLs are CPU-intensive because they need to inspect every packet and match ACL rules.
Placing ACLs at the DL also reduces the 3 of switches that require the extra mgmt configuration.
Policy-based connectivity and departmentaléworkgroup access to the core layer.
Quality of Service:
DL switches need to maintain the priorities of tfc coming fm the AL switches that have implemented QoS.
If not all the devices support QoS, the benefits will be reduced -. poor performance and quality.
DL switches are under high demand.
They need redundancy for adequate availability.
DL switches are typically implemented in pairs.
Recommended that they support multiple, hot swappable pwr supplies.
Finally, they need to support link aggregation and high-bandwidth aggregated links back to the core.
Core Layer Switch Features:
The CL is the high-speed backbone.
The forwarding rate is dependent on the number of devices participating in the network.
If you choose an inadequate switch at the core, you face potential bottleneck issues slowing down all tfc.
CL switches should support aggregated 10GbE.
L3 redundancy has faster convergence than L2, so, ensure CL switches support L3 functions.
CL switches should support FULL redundancy features.
QoS is important at the core since high-speed WAN access is often prohibitivel expensive.
Switches for SMBs:
Identify the Cisco switches used in SMB applications.
The features of Cisco Catalyst Switches:
You cannot simply select a switch by the size of a business.
Businesses are often cross integrated with other entities.
A 6500 makes sense as an AL switch where there are hundreds of users in an area, such as a stock exchange.
Cat Express 500 - forwarding rates = 8.8 - 24 Gbps.
Cat 2960 - L3, QoS, no PoE, 16 - 32 Gbps.
Cat 3560 - enterprise-class PoE, QoS, 32 - 128 Gbps.
Cat 3750 - stackable high performance.
Cat 4500 - DL midrange modular - up to 136 Gbps.
Cat 4900 - data center.
Cat 6500 - DL and CL - up to 720 Gbps.
Miscellaneous:
MDF - Main Distribution Facility.
Gi 0é1 - abbreviation for Gigabit Ethernet ports.
Spanning Tree - protocols allows redundant paths, but shuts down some links to avoid switching loops.
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