- •Table of Contents
- •Cisco Switching Black Book
- •Introduction
- •Overview
- •Is This Book for You?
- •How to Use This Book
- •The Black Book Philosophy
- •Chapter 1: Network Switching Fundamentals
- •In Depth
- •Physical Media and Switching Types
- •A Bit of History
- •Networking Architectures
- •The Pieces of Technology
- •Repeaters
- •Hubs
- •Bridges
- •Routers
- •Switches
- •Network Design
- •Collision Domains
- •Broadcast Domains
- •Why Upgrade to Switches?
- •Switched Forwarding
- •Switched Network Bottlenecks
- •The Rule of the Network Road
- •Switched Ethernet Innovations
- •Fast Ethernet
- •Gigabit Ethernet
- •The Cisco IOS
- •Connecting to the Switch
- •Powering Up the Switch
- •The Challenges
- •Entering and Exiting Privileged EXEC Mode
- •Entering and Exiting Global Configuration Mode
- •Entering and Exiting Interface Configuration Mode
- •Entering and Exiting Subinterface Configuration Mode
- •Saving Configuration Changes
- •Chapter 2: Basic Switch Configuration
- •In Depth
- •Campus Hierarchical Switching Model
- •Access Layer
- •Distribution Layer
- •Core Layer
- •Remote Network Monitoring
- •Connecting to the Console Port
- •Console Cable Pinouts
- •Console Connectors
- •Switch IOSs
- •The IOS Configuration Modes
- •Limiting Telnet Access
- •Implementing Privilege Levels
- •Setting the Login Passwords
- •Setting Privilege Levels
- •Assigning Allowable Commands
- •Configuring the Hostname
- •Configuring the Date and Time
- •Configuring an IP Address and Netmask
- •Configuring a Default Route and Gateway
- •Configuring Port Speed and Duplex
- •Enabling SNMP Contact
- •Logging On to a Switch
- •Setting the Login and Enable Passwords
- •Changing the Console Prompt
- •Entering a Contact Name and Location Information
- •Configuring System and Time Information
- •Configuring an IP Address and Netmask
- •Configuring a Default Route and Gateway
- •Viewing the Default Routes
- •Configuring Port Speed and Duplex
- •Enabling SNMP
- •Configuring Trap Message Targets
- •Configuring the Console Port
- •Configuring Telnet
- •Configuring the Password
- •Configuring an IP Address and Default Gateway
- •Configuring SNMP
- •Configuring ROM
- •Entering ROM Configuration Mode
- •Booting ROM Mode from a Flash Device
- •Configuring SNMP
- •Configuring RMON
- •Using Set/Clear Command Set Recall Key Sequences
- •Chapter 3: WAN Switching
- •In Depth
- •WAN Transmission Media
- •Synchronous Transport Signal (STS)
- •Cisco WAN Switches
- •MGX 8200 Series
- •IGX 8400 Series
- •WAN Switch Hardware Overview
- •Cisco WAN Switch Network Topologies
- •Network Management
- •WAN Manager
- •Accessing and Setting Up IGX and BPX Switches
- •Adding New Users
- •Using the History Command
- •Displaying a Summary of All Card Modules
- •Displaying Detailed Information for a Card Module
- •Displaying the Power and Temperature of a Switch
- •Displaying the ASM Statistics for BPX
- •Configuring the ASM Setting for BPX
- •Logging Out
- •Resetting the Switch
- •Displaying Other Switches
- •Setting the Switch Name
- •Setting the Time Zone
- •Configuring the Time and Date
- •Configuring the Control and Auxiliary Ports
- •Modifying the Functions of the Control and Auxiliary Ports
- •Configuring the Printing Function
- •Configuring the LAN Interface
- •Accessing the MGX 8850 and 8220
- •Adding New Users
- •Changing Passwords
- •Assigning a Switch Hostname
- •Displaying a Summary of All Modules
- •Displaying Detailed Information for the Current Card
- •Changing the Time and Date
- •Displaying the Configuration of the Maintenance and Control Ports
- •Displaying the IP Address
- •Configuring the IP Interface
- •Displaying the Alarm Level of the Switch
- •Chapter 4: LAN Switch Architectures
- •In Depth
- •The Catalyst Crescendo Architecture
- •ASICs
- •The Crescendo Processors
- •Crescendo Logic Units
- •Other Cisco Switch Processors, Buses, ASICs, and Logic Units
- •AXIS Bus
- •CEF ASIC
- •Phoenix ASIC
- •SAGE ASIC
- •QTP ASIC
- •QMAC
- •Bridging Types
- •Source Route Bridging
- •Source Route Transparent Bridging
- •Source Route Translational Bridging
- •Transparent Bridging
- •Source Route Switching
- •Switching Paths
- •Process Switching
- •Fast Switching
- •Autonomous Switching
- •Silicon Switching
- •Optimum Switching
- •Distributed Switching
- •NetFlow Switching
- •System Message Logging
- •Loading an Image on the Supervisor Engine III
- •Booting the Supervisor Engine III from Flash
- •Setting the Boot Configuration Register
- •Configuring Cisco Express Forwarding
- •Enabling CEF
- •Disabling CEF
- •Enabling dCEF
- •Disabling dCEF
- •Disabling CEF on an Individual Interface
- •Configuring CEF Load Balancing
- •Disabling CEF Load Balancing
- •Enabling Network Accounting for CEF
- •Setting Network Accounting for CEF to Collect Packet Numbers
- •Viewing Network Accounting for CEF Statistics
- •Viewing the Adjacency Table on the 8500 GSR
- •Clearing the Adjacency Table on the 8500 GSR
- •Clearing the Server Logging Table
- •Disabling Server Logging
- •Displaying the Logging Configuration
- •Displaying System Logging Messages
- •Chapter 5: Virtual Local Area Networks
- •In Depth
- •The Flat Network of Yesterday
- •Why Use VLANs?
- •VLAN Basics
- •A Properly Switched Network
- •Switched Internetwork Security
- •Scaling with VLANs
- •VLAN Boundaries
- •VLAN Membership Types
- •Traffic Patterns Flowing through the Network
- •VLAN Trunking
- •Trunk Types
- •LAN Emulation (LANE)
- •VLAN Trunking Protocol (VTP)
- •VTP Versions
- •VTP Advertisements
- •VTP Switch Modes
- •Methods for VLAN Identification
- •Dynamic Trunking Protocol
- •InterVLAN Routing
- •Internal Route Processors
- •How InterVLAN Routing Works
- •Configuring a Static VLAN on a Catalyst 5000 Series Switch
- •Configuring Multiple VLANs on a Catalyst 5000 Series Switch
- •Creating VLANs on a Catalyst 1900EN Series
- •Assigning a Static VLAN to an Interface on a 1900EN Series
- •Viewing the VLAN Configuration on a 1900 Series
- •Viewing an Individual VLAN Configuration on a 1900 Series
- •Configuring a Trunk Port on a Cisco 5000 Series
- •Mapping VLANs to a Trunk Port
- •Configuring a Trunk Port on a Cisco 1900EN Series
- •Clearing VLANs from Trunk Links on a Cisco 5000 Series
- •Clearing VLANs from Trunk Links on a Cisco 1900EN Series
- •Verifying a Trunk Link Configuration on a 5000 Series
- •Verifying a Trunk Link Configuration on a 1900EN Series
- •Configuring the VTP Version on a Catalyst 5000 Switch
- •Configuring a VTP Domain on a Catalyst 1900 Switch
- •Setting a VTP Domain Password on a Catalyst Switch
- •Configuring a Catalyst 1900 Switch as a VTP Server
- •Configuring a Catalyst 1900 Switch as a VTP Client
- •Configuring a Catalyst 1900 Switch for Transparent Mode
- •Configuring VTP Pruning on a Catalyst 1900 Switch
- •Configuring VTP on a Set/Clear CLI Switch
- •Configuring VTP on a 1900 Cisco IOS CLI Switch
- •Verifying the VTP Configuration on a Set/Clear CLI
- •Displaying VTP Statistics
- •Configuring VTP Pruning on a Set/Clear CLI Switch
- •Disabling Pruning for Unwanted VLANs
- •Configuring IP InterVLAN Routing on an External Cisco Router
- •Configuring IPX InterVLAN Routing on an External Router
- •In Depth
- •Internal Route Processors
- •Available Route Processors
- •Routing Protocol Assignment
- •Supervisor Engine Modules
- •Supervisor Engines I and II
- •Supervisor Engine III
- •Using the Supervisor Engine
- •Etherport Modules
- •Port Security
- •Manually Configured MAC Addresses
- •Determining the Slot Number in Which a Module Resides
- •Accessing the Internal Route Processor from the Switch
- •Configuring a Hostname on the RSM
- •Assigning an IP Address and Encapsulation Type to an Ethernet Interface
- •Setting the Port Speed and Port Name on an Ethernet Interface
- •Configuring a Default Gateway on a Catalyst 5000
- •Verifying the IP Configuration on a Catalyst 5000
- •Enabling RIP on an RSM
- •Configuring InterVLAN Routing on an RSM
- •Configuring IPX InterVLAN Routing on the RSM
- •Configuring AppleTalk InterVLAN Routing on an RSM
- •Viewing the RSM Configuration
- •Assigning a MAC Address to a VLAN
- •Viewing the MAC Addresses
- •Configuring Filtering on an Ethernet Interface
- •Configuring Port Security on an Ethernet Module
- •Clearing MAC Addresses
- •Configuring the Catalyst 5000 Supervisor Engine Module
- •Changing the Management VLAN on a Supervisor Engine
- •Viewing the Supervisor Engine Configuration
- •Configuring the Cisco 2621 External Router for ISL Trunking
- •Configuring Redundancy Using HSRP
- •Chapter 7: IP Multicast
- •In Depth
- •IP Multicasting Overview
- •Broadcast
- •Unicast
- •Multicast
- •IP Multicasting Addresses
- •The Multicast IP Structure
- •Delivery of Multicast Datagrams
- •Multicast Distribution Tree
- •Multicast Forwarding
- •IGMP Protocols
- •Internet Group Management Protocol (IGMP)
- •IGMPv1
- •IGMPv2
- •Time to Live
- •Multicast at Layer 2
- •IGMP Snooping
- •Cisco Group Management Protocol
- •Router Group Management Protocol
- •GARP Multicast Registration Protocol
- •Configuring IP Multicast Routing
- •Disabling IP Multicast Routing
- •Enabling PIM on an Interface
- •Disabling PIM on an Interface
- •Configuring the Rendezvous Point
- •Adding a Router to a Multicast Group
- •Configuring a Router to Be a Static Multicast Group Member
- •Restricting Access to a Multicast Group
- •Changing the IGMP Version
- •Configuring Multicast Groups
- •Removing Multicast Groups
- •Configuring Multicast Router Ports
- •Displaying Multicast Routers
- •Removing the Multicast Router
- •Configuring IGMP Snooping
- •Disabling IGMP Snooping
- •Displaying IGMP Statistics
- •Displaying Multicast Routers Learned from IGMP
- •Displaying IGMP Multicast Groups
- •Configuring CGMP
- •Disabling CGMP
- •Displaying CGMP Statistics
- •Configuring RGMP on the Switch
- •Disabling RGMP on the Switch
- •Configuring RGMP on the Router
- •Disabling RGMP on the Router
- •Displaying RGMP Groups
- •Displaying RGMP VLAN Statistics
- •Configuring GMRP
- •Disabling GMRP
- •Enabling GMRP on Individual Ports
- •Disabling GMRP on Individual Ports
- •Configuring GMRP Registration
- •Displaying the GMRP Configuration
- •Setting GMRP Timers
- •Displaying GMRP Timers
- •Disabling Multicast Suppression
- •Chapter 8: WAN Cell Switching
- •In Depth
- •ATM Overview
- •LANE
- •ATM Protocols
- •ATM Circuit Switching
- •ATM Cells
- •The ATM Switch and ATM Endpoints
- •The ATM Reference Model
- •Specifying ATM Connections
- •ATM Addressing
- •Local Area Network Emulation (LANE)
- •LANE Components
- •Integrated Local Management Interface (ILMI)
- •LANE Communication
- •LANE Configuration Guidelines
- •How LANE Works
- •Implementing LANE
- •Configuring ATM on the 5000 Switch
- •Connecting in an ATM Network
- •Monitoring and Maintaining LANE
- •Accessing the ATM LANE Module
- •Displaying the Selector Field
- •Configuring the LES/BUS
- •Verifying the LES/BUS Configuration
- •Configuring a LEC for an ELAN
- •Verifying a LEC Configuration on an ELAN
- •Configuring the LECS
- •Viewing the LANE Database
- •Binding the LECS Address to an Interface
- •Verifying the LECS Configuration
- •Chapter 9: LightStream Switches
- •In Depth
- •LightStream 100
- •LightStream 1010
- •LightStream 2020
- •Neighborhood Discovery Function
- •Virtual Path Connections
- •LightStream Troubleshooting Tools
- •LightStream Boot Process
- •Supported Troubleshooting Protocols
- •Snooping Mechanisms
- •Multiprotocol Over ATM
- •Configuring the Hostname
- •Configuring an Enable Password
- •Configuring the Processor Card Ethernet Interface
- •Configuring Virtual Private Tunnels
- •Verifying an ATM Interface Connection Status
- •Viewing the Configured Virtual Connections
- •Configuring the LECS ATM Address on a LightStream 1010 Switch
- •Configuring the Advertised LECS Address
- •Viewing the LANE Configuration
- •Viewing the Installed Modules
- •Configuring the MPC
- •Configuring the MPS
- •Changing the MPS Variables
- •Monitoring the MPS
- •Enabling ILMI Autoconfiguration
- •Configuring LANE on a LightStream 1010
- •Powering on the LightStream 100 ATM Switch
- •Configuring the LS100 Switch
- •Recovering a Lost Password
- •Chapter 10: Layer 2 Redundant Links
- •In Depth
- •Layer 2 Switching Overview
- •Frames
- •Broadcast and Multicast Frames
- •Unknown Unicasts
- •Layer 2 Network Loops
- •Danger! Data Loops!
- •STP Root Bridges
- •Bridge Protocol Data Units
- •Root Bridge Selection
- •Spanning Tree Convergence Time
- •STP Port States
- •EtherChannel
- •Link Failure
- •Port Aggregation Protocol
- •Fast Convergence Components of STP
- •PortFast
- •UplinkFast
- •BackboneFast
- •Viewing the STP Configuration on a Command Line Switch
- •Configuring the STP Root Switch
- •Configuring the STP Secondary Root Switch
- •Verifying the VLAN Priority Settings
- •Preparing to Enable EtherChannel
- •Verifying the EtherChannel Configuration
- •Defining an EtherChannel Administrative Group
- •Viewing an EtherChannel Administrative Group
- •Identifying the Template Port
- •Verifying the EtherChannel Configuration on a Command Line Interface IOS
- •Verifying the PortFast Configuration
- •Verifying the UplinkFast Configuration
- •Viewing the BackboneFast Configuration
- •Chapter 11: Multilayer Switching
- •In Depth
- •How MLS Works
- •MLS Components
- •MLS Flows
- •Access List Flow Masks
- •MLS Troubleshooting Notes
- •Configuring MLS
- •MLS Cache
- •Aging Timers
- •VLAN ID
- •VTP Domain
- •Management Interfaces
- •Configuring an External MLS Route Processor
- •Assigning a VLAN ID
- •Adding an MLS Interface to a VTP Domain
- •Enabling MLS on an Individual Interface
- •Disabling MLS on an External Router Interface
- •Configuring the MLS Switch Engine
- •Disabling MLS on a Catalyst 6000
- •Disabling MLS on a Catalyst 5000
- •Configuring the MLS Cache on the Catalyst 5000
- •Configuring Fast Aging on a Catalyst 5000
- •Configuring Fast Aging on a Catalyst 6000
- •Disabling Fast Aging on a Catalyst 6000
- •Configuring Long Aging on the Catalyst 6000
- •Disabling Long Aging on the Catalyst 6000
- •Configuring Normal Aging on the Catalyst 6000
- •Disabling Normal Aging on the Catalyst 6000
- •Assigning MLS Management to an Interface on the Catalyst 5000
- •Disabling MLS Management on an Interface on the Catalyst 5000
- •Monitoring and Viewing the MLS Configuration
- •Viewing the MLS Aging Configuration on a Catalyst 6000
- •Displaying the IP MLS Configuration
- •Displaying MLS VTP Domain Information
- •Viewing the MLS VLAN Interface Information
- •Viewing MLS Statistics on the Catalyst 5000
- •Viewing MLS Statistics on the Catalyst 6000
- •Viewing MLS Entries
- •Chapter 12: Hot Standby Routing Protocol
- •In Depth
- •Routing Problems
- •Routing Information Protocol
- •Proxy ARP
- •ICMP Router Discovery Protocol
- •The Solution
- •HSRP Message Format
- •The HSRP States
- •HSRP Configuration
- •HSRP Interface Tracking
- •Opening a Session on an Internal Route Processor
- •Entering Configuration Mode on an RSM
- •Enabling HSRP and Assigning an IP Address to a Standby Group
- •Assigning an HSRP Interface Priority
- •Assigning a Preempt Delay to a Standby Group
- •Removing a Preempt Delay from a Standby Group
- •Setting the HSRP Hello and Hold Timers
- •Removing the HSRP Hello and Hold Timers
- •Configuring Two RSFC Interfaces as One HSRP Group
- •Enabling Interface Tracking
- •Using the show standby Command
- •Using the debug Command
- •Chapter 13: Policy Networking
- •In Depth
- •Access Security Policies
- •Core Layer Policies
- •Distribution Layer Policies
- •Security at the Access Layer
- •Configuring Passwords
- •Limiting Telnet Access
- •Implementing Privilege Levels
- •Configuring Banner Messages
- •Physical Device Security
- •Port Security
- •VLAN Management
- •Creating a Standard Access List
- •Creating an Extended Access List
- •Implementing Privilege Levels on a 1900EN
- •Configuring Banner Messages
- •Enabling HTTP Access
- •Enabling Port Security
- •Displaying the MAC Address Table
- •Chapter 14: Web Management
- •In Depth
- •Standard and Enterprise Edition CVSM
- •CVSM Client Requirements
- •CVSM Access Levels
- •CVSM Default Home Page
- •The Switch Image
- •Configuring the Switch with an IP Address and Setting the Default Web Administration Port
- •Connecting to the Web Management Console
- •Configuring the Switch Port Analyzer
- •Chapter 15: The Standard Edition IOS
- •In Depth
- •The 1900 and 2820 Series Switches
- •Main Menu Choices
- •[C] Console Settings
- •[A] Port Addressing
- •[R] Multicast Registration
- •Configuring Network Settings on the 1900 and 2820 Series
- •Configuring Broadcast Storm Control on Switch Ports
- •Configuring SNMP on the 1900 Series
- •Configuring Port Monitoring on the Standard Edition IOS
- •Configuring VLANs on the Standard Edition IOS
- •Configuring Spanning Tree Protocol
- •Chapter 16: Switch Troubleshooting
- •In Depth
- •Hardware Troubleshooting
- •No Power
- •POST
- •Indicator Lights
- •Switch Cabling
- •Cable Problems
- •Switch Troubleshooting Tools
- •CiscoWorks for Switched Internetworks
- •IOS Software Troubleshooting Commands
- •Viewing the Set/Clear IOS Configuration
- •Viewing the VTP Domain Configuration on a Set/Clear IOS
- •Viewing Port Statistics on a Set/Clear IOS
- •Launching the Diagnostic Console on a Cisco 1900 or 2820 Series Switch
- •Using the Diagnostic Console to Upgrade the Firmware on a Cisco 1900 or 2820 Series Switch
- •Using the Diagnostic Console for Debugging the Firmware and Hardware
- •Appendix A: Study Resources
- •Books
- •Cisco Group Study and Users Groups
- •Online Resources
- •Asynchronous Transfer Mode
- •Cisco IOS
- •Hot Standby Router Protocol
- •IP Multicast
- •Multilayer Switching
- •Quality of Service
- •Spanning Tree Protocol
- •TACACS+
- •VLANs
- •Standards Organizations
- •Cisco Job Search Sites
- •Overview
- •Appendix C: The Cisco Consultant
- •Overview
- •Establishing Credibility
- •Come Off As an Expert
- •Designing a Solution
- •Estimating the Cost
- •Presenting the Final Proposal and Creating Expectations
- •Contracting
- •Document, Document, Document
- •The Way to Fail
- •Failing to Be There When Promised, or Rushing through the Job
- •Failing to Manage Your Time
- •Assuming You Know What the Customer Needs
- •Failing to Take Responsibility
- •Conclusion
- •Required Equipment
- •Lab Objectives
- •Possible Solution
- •The 1912 Basic Configuration
- •The Catalyst 5000 Basic Configuration
- •Configuring the Cisco 2621 Interface for ISL Trunking
- •Appendix E: Switch Features
- •Access Layer Switches
- •Cisco Catalyst 1900
- •Cisco Catalyst 2820
- •Cisco Catalyst 2900
- •Cisco Catalyst 3000
- •Cisco Catalyst 3500 Series XL
- •Cisco Catalyst 3900 Series
- •Distribution Layer Switches
- •Cisco Catalyst 4000 Series
- •Catalyst 5000 Series
- •Catalyst 6000 Series
- •Core Layer/WAN Switches
- •Cisco Catalyst 8400 Series
- •Cisco Catalyst 8500 Series
- •BPX 8600 Series
- •MGX 8800 Series
- •12000 Series Gigabit Switch Routers
Switched Forwarding
Switches route data based on the destination MAC address contained in the frame’s header. This approach allows switches to replace Layer 2 devices such as hubs and bridges.
After a frame is received and the MAC address is read, the switch forwards data based on the switching mode the switch is using. This strategy tends to create very low latency times and very high forwarding rates. Switches use three switching modes to forward information through the switching fabric:
∙Store−and−forward
∙Cut−through
∙FragmentFree
Tip |
Switching fabric is the route data takes to get from the input port on the switch to the output port |
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on the switch. The data may pass through wires, processors, buffers, ASICs, and many other |
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components. |
Store−and−Forward Switching
Pulls the entire packet received into its onboard buffers, reads the entire packet, and calculates its cyclic redundancy check (CRC). It then determines if the packet is good or bad. If the CRC calculated on the packet matches the CRC calculated by the switch, the destination address is read and the packet is forwarded out the correct port on the switch. If the CRC does not match the packet, the packet is discarded. Because this type of switching waits for the entire packet before forwarding, latency times can become quite high, which can result in some delay of network traffic.
Cut−Through Switching
Sometimes referred to as realtime switching or FastForward switching, cut−through switching was developed to reduce the latency involved in processing frames as they arrive at the switch and are forwarded on to the destination port. The switch begins by pulling the frame header into its network interface card buffer. As soon as the destination MAC address is known (usually within the first 13 bytes), the switch forwards the frame out the correct port.
This type of switching reduces latency inside the switch; however, if the frame is corrupt because of a late collision or wire interference, the switch will still forward the bad frame. The destination receives the bad frame, checks its CRC, and discards it, forcing the source to resend the frame. This process will certainly waste bandwidth; and if it occurs too often, major impacts can occur on the network.
In addition, cut−through switching is limited by its inability to bridge different media speeds. In particular, some network protocols (including NetWare 4.1 and some Internet Protocol [IP] networks) use windowing technology, in which multiple frames may be sent without a response. In this situation, the latency across a switch is much less noticeable, so the on−the−fly switch loses its main competitive edge. In addition, the lack of error checking poses a problem for large networks. That said, there is still a place for the fast cut−through switch for smaller parts of large networks.
FragmentFree Switching
Also known as runtless switching, FragmentFree switching was developed to solve the late−collision problem. These switches perform a modified version of cut−through switching. Because most corruption in a packet occurs within the first 64 bytes, the switch looks at the entire first 64 bytes to get the destination MAC address, instead of just reading the first 13 bytes. The minimum valid size for an Ethernet frame is 64 bytes. By verifying the first 64 bytes of the frame, the switch then determines if the frame is good or if a collision occurred during transit.
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Combining Switching Methods
To resolve the problems associated with the switching methods discussed so far, a new method was developed. Some switches, such as the Cisco Catalyst 1900, 2820, and 3000 series, begin with either cut−through or FragmentFree switching. Then, as frames are received and forwarded, the switch also checks the frame’s CRC. Although the CRC may not match the frame itself, the frame is still forwarded before the CRC check and after the MAC address is reached. The switch performs this task so that if too many bad frames are forwarded, the switch can take a proactive role, changing from cut−through mode to store−and−forward mode. This method, in addition to the development of high−speed processors, has reduced many of the problems associated with switching.
Only the Catalyst 1900, 2820, and 3000 series switches support cut−through and FragmentFree switching. You might ponder the reasoning behind the faster Catalyst series switches not supporting this seemingly faster method of switching. Well, store−and−forward switching is not necessarily slower than cut−through switching—when switches were first introduced, the two modes were quite different. With better processors and integrated−circuit technology, store−and−forward switching can perform at the physical wire limitations. This method allows the end user to see no difference in the switching methods.
Switched Network Bottlenecks
This section will take you step by step through how bottlenecks affect performance, some of the causes of bottlenecks, and things to watch out for when designing your network. A bottleneck is a point in the network at which data slows due to collisions and too much traffic directed to one resource node (such as a server). In these examples, I will use fairly small, simple networks so that you will get the basic strategies that you can apply to larger, more complex networks.
Let’s start small and slowly increase the network size. We’ll take a look at a simple way of understanding how switching technology increases the speed and efficiency of your network. Bear in mind, however, that increasing the speed of your physical network increases the throughput to your resource nodes and doesn’t always increase the speed of your network. This increase in traffic to your resource nodes may create a bottleneck.
Figure 1.6 shows a network that has been upgraded to 100Mbps links to and from the switch for all the nodes. Because all the devices can send data at 100Mbps or wire−speed to and from the switch, a link that receives data from multiple nodes will need to be upgraded to a faster link than all the other nodes in order to process and fulfill the data requests without creating a bottleneck. However, because all the nodes—including the file servers—are sending data at 100Mbps, the link between the file servers that is the target for the data transfers for all the devices becomes a bottleneck in the network.
Figure 1.6: A switched network with only two servers. Notice that the sheer number of clients sending data to the servers can overwhelm the cable and slow the data traffic.
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Many types of physical media topologies can be applied to this concept. In this demonstration, we will utilize Ethernet 100BaseT. Ethernet 10BaseT and 100BaseT are most commonly found in the networks of today.
We’ll make an upgrade to the network and alleviate our bottleneck on the physical link from the switch to each resource node or server. By upgrading this particular link to a Gigabit Ethernet link, as shown in Figure 1.7, you can successfully eliminate this bottleneck.
Figure 1.7: The addition of a Gigabit Ethernet link on the physical link between the switch and the server.
It would be nice if all network bottleneck problems were so easy to solve. Let’s take a look at a more complex model. In this situation, the demand nodes are connected to one switch and the resource nodes are connected to another switch. As you add additional users to switch A, you’ll find out where our bottleneck is. As you can see from Figure 1.8, the bottleneck is now on the trunk link between the two switches. Even if all the switches have a VLAN assigned to each port, a trunk link without VTP pruning enabled will send all the VLANs to the next switch.
Figure 1.8: : A new bottleneck on the trunk link between the two switches.
To resolve this issue, you could implement the same solution as the previous example and upgrade the trunk between the two switches to a Gigabit Ethernet. Doing so would eliminate the bottleneck. You want to put switches in place whose throughput is never blocked by the number of ports. This solution is referred to as using non−blocking switches.
Non−Blocking Switch vs. Blocking Switch
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We call a switch a blocking switch when the switch bus or components cannot handle the theoretical maximum throughput of all the input ports combined. There is a lot of debate over whether every switch should be designed as a non−blocking switch; but for now this situation is only a dream, considering the current pricing of non−blocking switches.
Let’s get even more complicated and introduce another solution by implementing two physical links between the two switches and using full−duplexing technology. Full duplex essentially means that you have two physical wires from each port—data is sent on one link and received on another. This setup not only virtually guarantees a collision−free connection, but also can increase your network traffic to almost 100 percent on each link.
You now have 200 percent throughput by utilizing both links. If you had 10Mbps on the wire at half duplex, by implementing full duplex you now have 20Mbps flowing through the wires. The same thing goes with a 100BaseT network: Instead of 100Mbps, you now have a 200Mbps link.
Tip If the interfaces on your resource nodes can implement full duplex, it can also be a secondary solution for your servers.
Almost every Cisco switch has an acceptable throughput level and will work well in its own layer of the Cisco hierarchical switching model or its designed specification. Implementing VLANs has become a popular solution for breaking down a segment into smaller collision domains.
Internal Route Processor vs. External Route Processor
Routing between VLANs has been a challenging problem to overcome. In order to route between VLANs, you must use a Layer 3 route processor or router. There are two different types of route processors: an external route processor and an internal route processor. An external route processor uses an external router to route data from one VLAN to another VLAN. An internal route processor uses internal modules and cards located on the same device to implement the routing between VLANs.
Now that you have a pretty good idea how a network should be designed and how to monitor and control bottlenecks, let’s take a look at the general traffic rule and how it has changed over time.
The Rule of the Network Road
Network administrators and designers have traditionally strived to design networks using the 80/20 rule. Using this rule, a network designer would try to design a network in which 80 percent of the traffic stayed on local segments and 20 percent of the traffic went on the network backbone.
This was an effective design during the early days of networking, when the majority of LANs were departmental and most traffic was destined for data that resided on the local servers. However, it is not a good design in today’s environment, where the majority of traffic is destined for enterprise servers or the Internet.
A switch’s ability to create multiple data paths and provide swift, low−latency connections allows network administrators to permit up to 80 percent of the traffic on the backbone without causing a massive overload of the network. This ability allows for the introduction of many bandwidth−intensive uses, such as network video, video conferencing, and voice communications.
Multimedia and video applications can demand as much as 1.5Mbps or more of continuous bandwidth. In a typical environment, users can rarely obtain this bandwidth if they share an average 10Mbps network with dozens of other people. The video will also look jerky if the data rate is not sustained. In order to support this application, a means of providing greater throughput is needed. The ability of switches to provide dedicated bandwidth at wire−speed meets this need.
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