Some Topics That You Might Want To Pursue On Your Own That We Did Not Cover In This Article Are Listed Here: Ethernet Is Extremely Popular, And Many Fine Books And Whitepapers Have Been Written On It.
For These Reasons, We Assume That You Have Some Background In Ethernet Technology; Therefore, This Article Does Not Cover Ethernet Frames, Hubs, And Cables In Any Great Length.
This Chapter Instead Focuses More On:
Spanning Tree,
Fast Ethernet,
Gigabit Ethernet,
Ethernet And Token Ring Switching.
Ethernet Is The Most Widely Used Local Area Network (LAN) Technology. The Original And Most Popular Version Of Ethernet Supports A Data Transmission Rate Of 10 Mb/S. Newer Versions Of Ethernet Called "Fast Ethernet" And "Gigabit Ethernet" Support Data Rates Of 100 Mb/S And 1 Gb/S (1000 Mb/S). An Ethernet LAN May Use Coaxial Cable, Special Grades Of Twisted Pair Wiring, Or Fiber Optic Cable. "Bus" And "Star" Wiring Configurations Are Supported. Ethernet Devices Compete For Access To The Network Using A Protocol Called Carrier Sense Multiple Access With Collision Detection (CSMA/CD).
Ethernet Operates At The OSI Layer 2, The Data Link Layer.
The Data Link Layer Actually Is Divided Into Two SUBLAYERS :
LOGICAL LINK CONTROL (LLC) LAYER.
The LLC Layer :802.2, In This Case:
Is A Standardized Interface Between A Hardware-Specific MAC And A Layer 3 Protocol.
And THE MAC LAYER :
• Generating The Physical Source And Destination Address For A Frame. These Are 48-Bit Industry-Wide Unique Addresses; The First 3 Bytes Are Assigned By The IEEE, And The Last 3 Bytes Are Vendor-Unique.
• Ensuring Reliable Transmissions.
• Synchronizing Data Transmissions.
• Performing Error Recognition.
• Performing Flow Control.
Ethernet Technology Commonly Is Referred To As Carrier-Sense Multiple Access Collision Detect (CSMA/CD).
CARRIER SENSE : This Also Is Known As "Listen Before Talking." An Ethernet Station Wanting To Transmit A Frame Listens To The Medium Before It Transmits To Ensure That The Medium Is Available.
TALK IF QUIET : If The Channel Is Quiet For A Specific Amount Of Time, The Inter Frame Gap (IFG) Before The Station Might Begin A Transmission. If The Channel Is Busy, It Is Monitored Until It Becomes Free For The Length Of IFG Timer; After That Time, Transmission Might Begin.
COLLISION : A Collision Is Measured As An Excess Of Voltage On The "Cable" Or Medium. A Collision Usually Is Caused By Two Stations Transmitting Data At The Same Time. If A Collision Occurs, Both Frames Are Destroyed.
1. COLLISION DETECTION: If A Station Detects A Collision During Transmission, That Transmission Immediately Stops. A Signal Jam Also Is Sent On The Media To Destroy Any Fragmented Frames, Preventing Corrupted Data.
2. BACKOFF: After A Collision, A Stations Waits A Period Of Time Called The Backoff Period. The Backoff Timer Is A Random Timer Generated By A Backoff Algorithm. This Prevents All Stations From Trying To Transmit At The Exact Same Time After A Collision.
After The Backoff Timer Expires, The Station Attempts To Retransmit The Frame. If Another Collision Happens, The Station Keeps Trying To Retransmit The Frame For 16 Times. After 16 Unsuccessful Attempts, The Frame Is Dropped.
Ethernet Was Developed On Older Coax Type Cables Capable Of Transmitting Or Receiving A Signal At Any Given Time. This Is Precisely Why Ethernet Needed CSMA/CD Technology.
With The Advent Of Switches, Running Ethernet Over UTP And Fiber, Full-Duplex Ethernet Became Available.
Full-Duplex Ethernet Allows A Station To Simultaneously Transmit And Receive Data. Ethernet Frames Are Transmitted And Received Simultaneously On Two Pairs Of UTP Or Fiber At Any Given Time.
Full-Duplex Ethernet Is Essentially Ethernet Without CSMA/CD. Full-Duplex Mode Basically Doubles The Bandwidth Of Ethernet! To Run Full-Duplex Ethernet, The Station And Switch Both Must Be Capable And Configured For Full-Duplex Operation. A Hub With Multiple Stations Cannot Operate In Full-Duplex Mode.
NOTE : A Station Not Operating In The Correct Duplex Mode Will Generate An Enormous Number Of Collisions On The Port Where It Is Located. These Collisions Mostly Likely Will Be Registered As "Late Collisions." Be Sure That The Port On The Switch And The End Station Are Operating In The Same Duplex Mode.
In May 1995, The IEEE Adopted The Fast Ethernet Standard, 802.3u. Years Later, After Battling FDDI, 100VG Any Lan, And ATM, This Standard Became The Prominent Type Of LAN. As Costs Per Port Dropped Along With The Prices Of Network Interface Cards (Nics), Fast Ethernet Won Out Over FDDI And 100VG Any Lan And ATM For Many Reasons: • It Allows A Clean And Inexpensive Migration Path From Existing 10-Mbps Ethernet Networks. At First, It Could Run Only Fiber And Cat 5 UTP.
• Fast Ethernet Didn't Require Costly Fiber Connections And Didn't Require Complex Configurations.
• Fast Ethernet Addresses Qos With Enormous Amounts Of Bandwidth, While Relying On The Upper Layers Or Network Design To Provide QOS.
• Basically, Fast Ethernet Became A Plug-And-Play Tenfold Upgrade Of The LAN. 100-Mb Networks Evolved Out Of 10-Mb Networks Across Data Centers Everywhere.
• The 100Base MAC Uses The Original Ethernet MAC Operating At 10 Times The Speed. This Is Completely Backward Compatible With 10-Mbps Ethernet.
• 100Base-T Includes A Specification For An MII Interface. A MII Interface Is A 100-Mbps Version Of The AUI Adapter.
• Fast Ethernet Supports Full- And Half-Duplex Functionality.
• Fast Ethernet Operates Over A Wide Array Of Different Physical Layers: Cat 5, Cat 3, Fiber, And So On.
The IEEE Started To Work On The 802.3z, Or Gigabit Ethernet, Standard. Three Short Years Later, In June 1998, The 802.3z Standard Officially Was Adopted. For The Most Part, The Gigabit Standard Is The Fast Ethernet Standard Multiplied By 10. This Is Why 10-Gb Ethernet Products Are Right Around The Corner And 100-Gb Ethernet Is An Eventuality.
• Gigabit Ethernet Uses The 802.3 Frame Format, Identical To That Of 10-Mbps And 100-Mbps Ethernet.
• It Includes A Specification For A Gigabit MII (GMII). Unlike 10-Mbps Ethernet And 100-Mbps Ethernet, The GMII Is An Electrical Specification And Does Not Include A Physical Connector. Cisco's Physical Gigabit Interfaces Are Called Gbics. The Type Of GBIC Determines The Physical Gigabit Connection. There Are Currently Multimode Fiber (MMF), Single-Mode Fiber (SMF), And UTP Gbics, As Well As A Cisco Proprietary GBIC Called A Gigastack.
One Element That Makes Gigabit Ethernet One Of The Most Versatile Protocols In Years Is The Concept Of The GMII. Except For The Strictly 1000Base-TX Switches, Most Gigabit Ethernet Switches Come With Open Ports For Gbics. Depending On Your Network Needs, You Can Put Any Type Of GBIC In This Port. With The Click Of A GBIC, Your Network Can Change From A Limited 100-M Copper-Based Network To A 10,000-M Fiber-Based Network! The Sections That Follow Cover The More Common Gbics, Gigabit Standards, And Length Limitations.
1000Base-SX Gbics Use A Laser-Based Wavelength Of 850 Nms. Depending On The Cable Type, SX GBIC Operates At Distances Of 220 M To 550 M.
LX Gbics Use A Laser-Based Wavelength Of 1300 Nms. Depending On The Cable Type, LX GBIC Operate At Distances Of 550 M To 5000 M, Cisco Also Supports An LH And LX GBIC, Which Extends The IEEE 1000Base-LX Maximum Distance Of 5 Km.
The IEEE Standard For Gigabit Ethernet Transmission Over Cat 5 UTP Is 802.3ab. The Standard Defines The Maximum Distance To Be 100 Meters And The Copper To Be At Least Cat 5 Using Four Pairs Of Wires, Terminated With An RJ-45 Jack.
To Simplify The Configuration Of Ethernet Devices, The 802.3.U Committee Defined Fast Link Pulse (FLP). FLP Sends A Series Of Pulses On The Network That Can Deduce What Duplex And Speed The Link Is Operating At. The Station And The Hub/Switch Agree On The Highest Priority And Configure The Station In That Manner.
NOTE : Duplex Modes Are A Function Of The Hardware Built Into The Network Interface Card (NIC). Software Upgrades Will Not Allow You To Run Full-Duplex Mode. For Full-Duplex Mode To Work, The Station And The Switch Port Must Be Capable Of Full-Duplex Operation.
In The Early 1990s, Kalpana, Grand Junction, And Bay Networks Started To Ship Some Of The First Ethernet Switches. The Bay Networks 28115 Was One Of The First Switches To Introduce 10/100 Auto-Sensing Ports And Virtual Lans (VLANS).
More Importantly, All Switches Put An End To The Old Ethernet Repeater Rules, While Increasing Bandwidth. Until This Point, Many People Were Predicting That ATM Would Be The Only High-Speed Protocol In Use On The LAN—And If It Hadn't Been For The Ethernet Switch, They Might Have Been Right.
• Significant Bandwidth Improvement By Limiting A Collision Domain To A Single Port.
• Scalability. Repeater Rules Are Limited To A Single Port.
• VLAN Capability. Broadcast Domains Can Be Located Logically And Are Not Limited By Geographical Boundaries.
• Enhanced Security.
• Full-Duplex Capability.
A SWITCH FUNCTIONS MUCH LIKE A MULTIPORT BRIDGE: When VLANS Are Created, Virtual Bridges Are Created To Join The Ports In The VLAN. Broadcast, Unicast, And Multicast Traffic Is Forwarded To Each Member Of The VLAN. The Catalyst 5500 Series Switch Builds An Address Table By Recording The Source MAC Address Of Frames That It Received From Its Interfaces.
When A Frame Destined For An Address Not Yet In The Address Table Is Received, The Switch Floods The Frame Out All Ports And Trunks In The Same VLAN As The Frame Was Received. The Switch Does Not Forward The Frame Out The Interface That It Received It. When A Reply For That Frame Is Received, The Switch Records The New Address In The Address Table.
The Switch Forwards Subsequent Frames To A Single Port, Without Flooding It To All Ports. Traffic Can Leave The VLAN Only With The Aid Of A Router Or A Layer 3 Switch Providing Routing Functionality.
STORE-AND-FORWARD : The Port Adapter Reads The Entire Frame Into Memory And Then Determines Whether The Frame Should Be Forwarded. The Frame Is Forwarded Only If It Does Not Contain Any Errors. Store-And-Forward Mode Reduces The Amount Of Errors On The LAN, But There Is A Delay Associated With Reading And Verifying The Frame Before Forwarding It. In Modern ASIC-Based Switches, The Speed Of The ASIC Has Become So Great That The Latency Associated With Store-And-Forward Switches Is A Nonissue.
CUT-THROUGH : In This Mode, The Port Receives The First Few Bytes Of A Frame And Analyzes The Packet Header To Determine The Destination Of The Frame And Immediately Begins To Forward That Frame. The Frame Is Not Checked For Any Errors Before Forwarding, So This Mode Does Propagate Bad Frames On The Network.
ADAPTIVE CUT-THROUGH : This Mode Combines Both Aspects Of Cut-Through And Store-And-Forward Modes. In This Mode, The Port Operates In Cut-Through Mode Until A User-Defined Threshold Of Frame Errors Is Detected. When The Threshold Is Exceeded, The Port Switches To Store-And-Forward Mode.
Two Key Concepts In Switched Networks Are Broadcast Domains And Collision Domains. A Broadcast Domain Is The Area Of The Network That Forwards Broadcasts From One Portion Of Network To The Next. A Practical Example Of A Broadcast Domain Is An IP Or IPX Subnet. A Collision Domain Is A Function Of The Physical Properties Of A Device. Devices In The Same Collision Domain Reside On The Same "Wire" Or Hub/Repeater.
HUB/REPEATER :
All Ports Are In A Single Collision Domain.
All Ports Are In A Single Broadcast Domain.
BRIDGE :
Each Port Is A Separate Collision Domain.
All Ports Are In A Single Broadcast Domain.
ROUTER :
Each Port Is A Separate Collision Domain.
Each Port Is In A Separate Broadcast Domain.
SWITCH :
Each Port Is A Separate Collision Domain.
Each Port Is Configurable To Be In The Same Or A Separate Collision Domain.
The World Of Ethernet Switching Is Growing At A Tremendous Pace. There Are Many More Interesting And Useful Technologies That We Simply Did Not Have The Time To Discuss.
We Highly Recommend Reading Your Own, “Cisco LAN Switching Books” Which Is Loaded With Information On LAN Switching For Extended Reference.
UPLINKFAST, PORTFAST, BACKBONEFAST: These Are Ways To Help The STP Process Deal With User Traffic During Initialization Or Failure While STP Is Converging. These Technologies Are Simple To Configure And Can Avoid Lost Throughput During STP Convergence.
FAST ETHERCHANNEL/GIGABIT ETHERCHANNEL: Etherchannel Provides A Way For The Router To Aggregate Up To Four Fast Ethernet Ports In A Bundle. The Technology Also Applies To Gigabit Ethernet. We Like To Think Of Etherchannel As The PPP Multilink Of Ethernet. Etherchannel Treats The Bundle As One Large Physical Link And Can Distribute Traffic In Different Ways Across The Bundle. With Full-Duplex Mode, Etherchannel Bundles Can Reach Speeds Of 800 Mbps To 8,000 Mbps. Etherchannel Can Help Avoid Some STP Issues Because It Offers Resiliency Between Switches. When A Link Goes Down, The Bundle Simply Loses Bandwidth And Does Not Need To Wait For STP To Converge Before Sending User Traffic. There Are Rules To Etherchannel And How Ports Can Be Bundled, And They Are Different For The Various Families Of Switches.
There Is Also The Drawback That Etherchannel Can Be Used Only To Connect Two Switches
—For Example, Bundles Cannot Be Split Across Switches.
• Port Security— An Advanced Security Function Of All Cisco Switches Is Port Security. Port Security Allows You To Limit The Access On A Port To A Single MAC Address. When Another User Plugs Into The Port With A Different MAC Address, The Port Can Be Shut Down, Or Traps Can Be Sent To A Network-Management Station. This Is A Helpful Feature In The Field Because It Strictly Controls Physical Access To The Switch And Unwanted Moves Or Changes.
• Multicast (CGMP/IGMP)— This Does Not Mean That It Is Not Important, However. Multicast Traffic Is Playing An Ever-Increasing Role In The Modern Network.
Many Definitions For A VLAN Exist. The Definition That We Chose To Use Is Simple. Virtual LANS (VLANS) Can Be Easily Defined As Broadcast Domains That Can Extend Geographical Distances. When Configuring Ethernet Switching, Every Port Must Be Assigned To A VLAN. The Default VLAN Is Always VLAN 1. When Switches Ship From The Factory, They Are In Some Ways Plug-And-Play.
Every Port Is Assigned To VLAN 1, So Every Port Of The Switch Is In A Single Broadcast Domain. This Makes Migrating From Shared Ethernet Hubs To A Basic Switched Network Easy. VLANS Always Should Be Thought Of As Simply Broadcast Domains. Most VLANS Eventually Become IP/IPX Subnets Or Bridging Domains.
• A Single Subnet Is Used Per VLAN.
• Do Not Bridge Different VLANS Together.
• A Router/Layer 3 Switch Is Needed To Route Between VLANS.
• STP Must Run In Each VLAN To Prevent Loops. This Can Be Disabled, But It Is Not Recommend.
In 1998, The IEEE Approved The 802.3ac Standard That Defines Frame Format Extensions To Support Virtual Local Area Network (VLAN) Tagging On Ethernet Networks. The Vlan Protocol Permits Insertion Of An Identifier, Or "Tag", Into The Ethernet Frame Format To Identify The Vlan To Which The Frame Belongs. It Allows Frames From Stations To Be Assigned To Logical Groups.
This Provides Various Benefits Such As Easing Network Administration, Allowing Formation Of Work Groups, Enhancing Network Security, And Providing A Means Of Limiting Broadcast Domains, Refer To IEEE Standard 802.1q For Definition Of The VLAN Protocol. The 802.3ac Standard Defines Only The Implementation Details Of The VLAN Protocol That Are Specific To Ethernet.
If Present, The 4-Byte Vlan Tag Is Inserted Into The Ethernet Frame Between The Source Mac Address Field And The Length/Type Field. The First 2-Bytes Of The VLAN Tag Consist Of The "802.1q Tag Type" And Are Always Set To A Value Of 0x8100. The 0x8100 Value Is Actually A Reserved Length/Type Field Assignment That Indicates The Presence Of The VLAN Tag, And Signals That The Traditional Length/Type Field Can Be Found At An Offset Of 4-Bytes Further Into The Frame.
• The First 3-Bits Are A User Priority Field That May Be Used To Assign A Priority Level To The Ethernet Frame.
• The Next 1-Bit Is A Canonical Format Indicator (CFI) Used In Ethernet Frames To Indicate The Presence Of A Routing Information Field (RIF).
• The Last 12-Bits Are The VLAN Identifier (VID) Which Uniquely Identifies The VLAN To Which The Ethernet Frame Belongs.
With The Addition VLAN Tagging, The 802.3ac Standard Permitted The Maximum Length Of An Ethernet Frame To Be Extended From 1518-Bytes To 1522-Bytes. The Following Illustrates The Format Of An Ethernet Frame That Has Been "Tagged" With A VLAN Identifier Per The IEEE 802.3ac Standard:
A Powerful Function Of VLANS Is Their Capability To Span Distance. VLANS Are Communicated From Switch To Switch By The Means Of A VLAN Trunking Protocol (VTP). VTP Is Used To Maintain Global VLAN Information Between Switches. A VLAN Management Domain, Or VTP Domain, Consists Of One Or More Switches Interconnected And Sharing The Same Administrative Responsibility.
Anytime That You Want The VLANS On One Switch To Have Information About The Vlans On Another Switch (That Is, When You Want These Two Broadcast Domains To Communicate), You Need To Configure A VTP Domain And A Trunk. VTP Also Tracks All The VLANS In A VTP Domain And Propagates These In A Client/Server Manner From One Switch To Another. The Intent Of VTP Is To Ease Management And Provide A Common VLAN Database Across The VTP Domain.
VTP Advertisements Are Sent Out On All Trunk Connections In Inter-Switch Link (ISL) Frames, 802.1q Frames, IEEE 802.10, Or ATM LANE. VTP Frames Are Sent To The Destination MAC Address Of 0100.0CCC.CCC With An LLC Code Of SNAP (AAAA) And A Type Of 2003 In The SNAP Header. For VTP Messages To Successfully Be Transmitted, The Following Must Occur:
• VTP Will Accept Only Messages With The Same Domain Name. If Authentication Is Configured For That VTP Domain, That, Too, Must Match. The VTP Name Is Case-Sensitive.
• VTP Will Accept Only Messages With The Same Version, Type I Or Type II. This Setting Is Controlled, With The V2 Mode Being Either Enabled On Both Sides Of The Link Or Disabled. A Switch Might Be VTP Version II–Capable And Have V2 Mode Disabled. V2 Mode Is Primarily For Token Ring Switches.
• Catalyst Switches Must Be Adjacent, And Trunking Must Be Configured Between Them. For Ethernet Networks, The Trunking Protocol Is Dotq1 (802.1q) Or ISL. ATM Uses LANE, And FDDI Uses IEEE 802.10.
• VTP Servers Will Synchronize With VTP Clients Only If The VTP Client's Revision Number Is Less Than That Of The VTP Server. If The VTP Client's Revision Number Is Equal To Or Greater Than That Of The VTP Servers The Client VLAN Database Will Not Synchronize.
After A Trunk Is Established, VTP Sends Periodic Advertisement Out Each Trunk Port.
• VLAN IDs (ISL And 802q).
• Emulated LAN Names For ATM LANE.
• 802.10 SAID Values.
• VTP Domain Name And Configuration Revision Number. The Server With The Highest Revision Number Becomes The Primary Server. The Revision Number Is Incremented Every Time A VLAN Configuration Change Is Made.
• VLAN Configuration, VLAN ID, VLAN Name, And MTU Size For Each VLAN.
• Ethernet Frame Format.
VTP HAS TWO VERSIONS, Simply Called Version I And Version II. All The Switches In The VTP Domain Must Be On The Same Version. This Rule Does Not Apply To The Transparent-Mode Switches. Version II Offers The Following, The Most Important Being Support For Token Ring:
• TOKEN RING SUPPORT : VTP VII Supports Token Ring LAN Switching And VLANS (Token Ring Bridge Relay Function [TRBRF]). We Will Discuss TRBRF More In Upcoming Sections.
• UNRECOGNIZED TYPE : Length Value (TLV) Support Is Included. Unrecognized TLV Is Saved In NVRAM When The Switch Is In VTP Server Mode.
• VERSION-DEPENDENT TRANSPARENT MODE : VTP Will Forward VTP Messages That Do Not Match The Domain Name And Version To Switches Operating In VTP Transparent Mode.
• CONSISTENCY CHECKS : Consistency Checks Are Performed On VLAN Names, And Values Are Performed Only Upon New Entry Into The Switch.
VTP SERVER MODE : In VTP Server Mode, VLANS Can Be Created, Modified, And Deleted. VLAN Information Automatically Is Sent To All Adjacent VTP Servers And Clients In The Same VTP Domain. Caution Always Should Be Used When "Clearing" A VLAN From The VTP Server Because That VLAN Will Be Deleted On All VTP Servers And Clients In That DTP Domain. If Two Devices Are Configured As Servers, The Switch/Server With The Highest VTP Configuration Revision Will Be The Primary Server. VLAN Information Is Stored In The Switches NVRAM.
VTP CLIENT MODE : In VTP Client Mode, VLANS Cannot Be Created, Modified, Or Deleted. Only The Name And The VTP Mode And Pruning Can Be Changed. The Client Is At The Mercy Of The VTP Server For All VLAN Information. The Client Still Must Assign Ports To A VLAN, But The VLAN Will Not Be Active On The Switch Unless The VTP Server Sends Information To The Client On That VLAN. Furthermore, VLAN Information Is Stored Locally In The Switch's NVRAM When It Is Received From The Server On The Catalyst 2900XL/2500G Series Switches. The Catalyst 4000/5500/6500 Series Of Switches Do Not Store The VLAN Database If Configured Has A VTP Client.
VTP TRANSPARENT MODE : In VTP Transparent Mode, VTP Information That Is Local On The Switch Will Not Be Advertised, But VTP Information Received Form Other Switches Will Be Forwarded. VLANS Can Be Created, Modified, And Deleted On Transparent Switches. VLAN Information Also Is Stored In NVRAM.
DTP Is The Second Generation Of DISL (Dynamic ISL) And Exists To Ensure That The Different Parameters Involved In Sending ISL Or 802.1Q Frames, Like The Configured Encapsulation Type, Native VLAN, Hardware Capability, Etc.
Operational Overview: DTP Is A Layer-2 Protocol That Negotiates Configuration Parameters Between A Switch Port And It's Neighbor. It Uses Another Well-Known Multicast MAC Address Of 01-00-0c-Cc-Cc-Cc And A SNAP Protocol Type Of 0x2004. Here Is A Summary Of The Configuration Modes:
Note: ISL And 802.1Q Encapsulation Type Can Be Set Or Negotiated - ISL Will Be Preferred Over Dot1q, But Is Recommended To Be Set.
• DTP Assumes Point-To-Point Connection, And Cisco Devices Will Support 802.1Q Trunk Ports That Are Only Point-To-Point.
• During DTP Negotiation, The Ports Will Not Participate In STP. Only After The Port Type Becomes One Of The Three Types (Access, ISL Or 802.1Q), The Port Will Be Added To STP. (If PAGP Is Running That Is The Next Process To Run Prior To The Port Participating In STP).
• VLAN 1 Will Usually Be There On The Trunk Port. If The Port Is Trunking In ISL Mode, DTP Packets Are Sent Out On VLAN 1, Otherwise (For 802.1Q Trunking Or Non-Trunking Ports) On The Native VLAN.
• In Desirable Mode DTP Packets Transfer The VTP Domain Name, Which Must Match For A Negotiated Trunk To Come Up, Plus Trunk Configuration And Admin Status.
• Messages Are Sent Every 1s During Negotiation, And Every 30s After That.
• Be Careful That It Is Understood Modes (On, Nonegotiate, Off) Explicitly Specify In Which State The Port Will End Up. A Bad Configuration Can Lead To A Dangerous Inconsistent State Where One Side Is Trunking And The Other Is Not. A Port In On, Auto, Or Desirable Sends DTP Frames Periodically. If A Port In Auto Or Desirable Mode Doesn't See A DTP Packet In 5min It Will Be Set To Non-Trunk.
By Default, Trunking Ports Will Propagate Information About All VLANS, But AS Recommends Limiting That To The VLANS Defined On The Wiring Closet Switches. This Practice Has Many Advantages, Most Importantly, Is The Ability To Isolate Issues, Broadcasts, And Loops To One Wiring Closet Instead Of The Entire Network.
INTER-SWITCH LINK (ISL): ISL Is A Cisco Proprietary Trunking Encapsulation. ISL Is A Frame-Tagging Protocol;
The Frames On The Link Contain The Standard Ethernet, FDDI, Or Token Ring Frame And The VLAN Information Associated With That Frame.
ISL Is Supported On Links That Are 100 Mbps Or Greater In Speed. ISL Is An Extremely Efficient Protocol, And It Is The Protocol That Cisco Uses Internally For Catalyst To Communicate With The Route Switch Modules (RSMs) Or Other Layer 3 Switching Fabric.
SPANNING TREE Is Run On A Per-VLAN Basis (PVST) On ISL Trunks. This Means That Every VLAN Has A Root Bridge, And Trunks Go Into A Forward/Blocking Mode For Each VLAN On Each Trunk.
PVST Is Critical To Control On Large Networks, As Discussed In Upcoming Sections. • IEEE 802.1Q : 802.1q Is The Industry-Standard Trunking Protocol. 802.1q Operates Slightly Differently Than ISL. It Runs Mono Spanning Tree (MST) On The Default VLAN For All VLANS In The VTP Domain. In MST, One Root Bridge Is Elected For The Entire VTP Domain; This Is Called The Common Spanning Tree (CST). All VLAN Information Follows One Path In This Type Of Configuration. Cisco, Understanding The Need To Control Spanning Tree On Large Networks While Controlling Load, Implements PVST On All 802.1q VLANS.
- The Default VLAN Needs To Be The Same On Both Ends Of The Trunk. MST Will Run In This VLAN. It Is Critical That The Default VLAN Be The Same On Third-Party Switches Interacting With Cisco Switches.
- As Mentioned, 802.1q Uses MST. Cisco Overrides This, By Default, With PVST. Because The BPDUs Are Handled Differently Between Cisco And Third-Party Switches, Care Should Be Taken Whenever Integrating These Domains That Spanning Tree And The Default VLANS Are Consistent In Both Switches. The Entire Cisco VTP Domain Looks Like A Single Broadcast/ Spanning Tree Domain To The Third-Party Switches.
- BPDUs On The Native VLAN Of The Trunk Are Sent Untagged To The Reserved IEEE 802.1d Spanning Tree Multicast MAC Address (0180.C200.0000). The Bpdus On All Other VLANS On The Trunk Are Sent And Tagged On The Reserved Cisco Shared Spanning Tree (SSTP) Multicast MAC Address (0100.0ccc.Cccd).
• IEEE 802.10 : 802.10 Was Actually The First Protocol That The Industry Tried To Use For A VLAN Trunking Protocol. It Originally Was Developed For Extra Security On Defense Networks Or Large Mans. It Primarily Is Used On FDDI Networks Today Because Of Its Limitations.
Dynamic ISL Was Cisco's First Trunk-Negotiation Protocol. It Exists On All Catalyst 5500 Series Software 4.1 And Earlier. Originally, It Was Used To Negotiate Trunks For The ISL Protocol Only. Later, In 4.2, DISL Was Replaced With Dynamic Trunk Protocol (DTP). DTP Is Essentially DISL That Attempts To Automate ISL And 802.1q Trunk Configuration.
DTP Uses The Reserved Destination Multicast Address Of 0100.0CCC.CCCC For LAN Networks To Negotiate Trunks.
In The Default Auto State, DTP Messages Are Sent Out Every 30 Seconds On All Trunk Lines. Depending On the Mode of the Port, The Port Might Become An ISL Or 802.1q Trunk.
• ON— Puts The Port In A Permanent Trunking State. It Also Tries To Negotiate The Link To Be A Trunk.
• OFF— Disables The Port, And Thereby The Trunk.
• DESIRABLE— Makes The Port Attempt To Convert To A Trunk Link. The Port Becomes A Trunk If The Neighboring Port Is Set To On, Desirable, Or Auto Modes.
• AUTO— The Port Converts To A Trunk If The Neighboring Port Is Set To On Or Desirable Modes.
• NONEGOTIATE— Puts The Port Into Trunking Mode But Prevents The Port From Sending DTP Frames.
In Actuality, This Is Really Too Many Options For A Trunk. Network Administrators Either Configure A Port As A Trunk Or They Don't. It Could Be Argued That To Have A Network Be So Flexible The Trunks Can Or Should Be Added Dynamically Is A Security Risk.
Spanning Tree Protocol (STP) Is Designed To Prevent Problems Related To Bridging Loops. STP Solves The Problem By Blocking Redundant Paths And Allowing Only A Single Active Path.
Spanning Tree Works By Selecting A Root Switch Then Selecting A Loop-Free Path From The Root Switch To Every Other Switch. To Do That, Spanning Tree Must Choose A Single Root Bridge, One Root Port For Each Nonroot Switch, And A Single Designated Port For Each Network Segment.
When Ethernet Evolved From A Single Shared Cable To Networks With Multiple Bridges And Hubs, A Loop-Detection And Loop-Prevention Protocol Was Needed. The 802.1d Protocol, Developed By Radia Perlman, Provided This Loop Protection. It Did Such A Good Job That When Most Networks Went From Bridged Networks To Routed Networks, So The Importance Of Spanning Tree Was Almost Forgotten.
Because Of This, Spanning Tree Is Probably The Most Used But Least Understood Protocol In The Modern Internetwork. But With The Huge Success Of Ethernet Switching, Spanning Tree Again Becomes An Important Protocol To Control And, More Importantly, Understand. We Will Discuss Why Spanning Tree Has Become So Important In Switched Ethernet Networks In Upcoming Sections.
Spanning Tree's Purpose In Life Is To Elect A Root Bridge And Build Loop-Free Paths Leading Toward That Root Bridge For All Bridges In The Network. When Spanning Tree Is Converged, Every Bridge In The Network Has Its Bridged Interfaces In One Of Two States: Forwarding Or Blocking. If The Port Has The Best-Cost Path To The Root Bridge, It Is Forwarding And Thus Is The Shortest Path To Root. All Other Interfaces On The Bridge Are In A Blocking State.
• A Configuration BPDU, Used For Initial STP Configuration
• A Topology Change Notification (TCN) BPDU Used For Topology Changes
BPDUS Are Transmitted Using A Reserved Multicast Address Assigned To All Bridges. The BPDU Is Sent Out All Bridged LAN Ports And Is Received By All Bridges Residing On The LAN. The BPDU Is Not Forwarded Off The LAN By A Router.
ROOT ID: The ID Of The Bridge Assumed To Be Root. Upon Initialization, The Bridge Assumes That It Is Root.
TRANSMITTING BRIDGE ID AND PORT ID: The BID Of The Bridge Transmitting The BPDU, And What Port The BPDU Originated From.
COST TO ROOT: The Least-Cost Path To The Root Bridge From The Bridge Transmitting The BPDU. Upon Initialization, Because The Bridge Assumes Itself To Be Root, It Transmits A 0 For The Cost To Root.
THE BRIDGE ID (BID): Is An 8-Byte Field Composed From A 6-Byte MAC Address And A 2-Byte Bridge Priority. The MAC Address Used For The BID Is Generated From A Number Of Sources, Depending On The Hardware In Use For The Bridge. Routers Use A Physical Address, Whereas Switches Will Use An Address From The Backplane Or Supervisor Module.
The Priority Value Ranges From 0 To 65,535; The Default Value Is 32,768.
The Path Cost Is Used By Bridges To Determine The Best Possible Path To Root. Path Costs Recently Have Been Updated By The IEEE To Include Gigabit And Greater Links. The Lower The Path Cost Is, The More Preferable The Path Is.
That It Transitions Through During Its Operation. When STP Converges, It Is In One Of Two States, Forwarding Or Blocking.
This State Appears When A Bridge Is Having Problems Processing BPDUS, When A Trunk Is Improperly Configured, Or When The Port Is Administratively Down.
LISTENING : When A Bridge Port Initializes Or During The Absence Of BPDUS For A Certain Amount Of Time, STP Transitions To The Listening State. When STP Is In This State, The Port Is Actually Blocking And No User Data Is Sent On The Link. STP Follows A Three-Step Process For Convergence:
1. ELECT ONE ROOT BRIDGE : Upon Initialization, The Bridge Begins Sending BPDUS On All Interfaces. A Root Bridge Is Chosen Based On The Bridge With The Lowest BID. Recall That The BID Is A Combination Of A Priority And MAC Address. In The Event Of A Tie, The Bridge With The Lowest MAC Address Is Chosen As Root. All Ports Of The Root Bridge Are Put In The Forwarding State.
2. ELECT ONE ROOT PORT FOR EVERY NONROOT BRIDGE: After A Single Root Bridge Has Been Elected, STP Elects A Single Root Port On Each Bridge That Is Not Root. The Root Port Is The Bridge's Best Path To The Root Bridge. When A Root Port Is Elected, It Is Put Into The Forwarding State.
TO DETERMINE WHAT PORT SHOULD BE A ROOT PORT, STP FOLLOWS THIS DECISION PROCESS:
a. Lowest Root BID.
b. Lowest Path Cost To Root Bridge; The Cumulative Cost Of The All Paths To Root.
c. Lowest Sender BID.
d. Lowest Port ID.
When A Bridge Receives A BPDU, It Stores It In A Bridge Table For That Port. As New BPDUs Are Received On That Port, They Are Compared To Existing BPDUS. Using The Four-Step Process Listed Previously, BPDUs That Are More Attractive Or That Have Lower Costs Are Kept, And The Other Ones Are Discarded.
The Primary Variable That Influences The Root Port Election Is The Cost To The Root Bridge. This Is The Cumulative Path Cost Of All Links To The Root Bridge.
3. ELECT ONE DESIGNATED PORT/DESIGNATED BRIDGE FOR EVERY SEGMENT: For Every Segment, STP Elects One Port That Will Send And Receive All Information From That Segment To The Root Bridge. A Root Port Can Be Thought Of As The Port That Forwards Information To The Root, Whereas The Designated Port Can Be Thought Of As The Port That Sends Traffic Away From The Root. This Rule Applies Mostly To Shared-Media Bridges, Or Routers. Designated Ports On Back-To-Back Switched Trunk Lines Do Not Follow This Rule.
4. All Remaining Ports Become Nondesignated Ports And Are Put In Blocking Mode.
Ports That Remain Designated Or Root Ports For A Period Of 15 Seconds, The Default Forward Delay, Enter The Learning State. The Learning State Is Another 15 Seconds That The Bridge Waits While It Builds Its Bridge Table.
When The Bridge Reaches This Phase, Ports That Do Not Serve A Special Purpose, Such As A Root Port Or A Designated Port, Are Called Nondesignated Ports.
All Designated Ports Are Put In A Forwarding State, While All Nondesignated Ports Are Put Into A Blocking State. In The Blocking State, A Bridge Does Not Send Any Configuration Bpdus, But It Still Listens To Them.
A Blocking Port Also Does Not Forward Any User Data.
STP Has Three Basic Timers That Regulate And Age Bpdus: A Hello Timer, A Forward Delay Timer, And A Max Age Time.
• HELLO TIMER: The Default Hello Timer Is 2 Seconds. This Is The Amount Of Time Between Configuration Bpdus Sent By The Root Bridge.
• FORWARD DELAY TIMER: This Timer Is The Default 15 Seconds That The Routers Wait While Building Its Bridging Table. The Listening And Learning Stages Each Use This Single 15-Second Timer.
• MAX AGE TIMER: The Max Age Timer Is How Long A BPDU Is Stored Before It Is Flushed. If This Timer Expires Before The Interface Receives A New BPDU, The Interface Transitions To The Listening State. An Expired Max Ageparameter Usually Is Caused By A Link Failure.
The Default Value Is 20 Seconds.
STP Uses The Hello Timer To Space BPDUS And Has A Keepalive Mechanism.
The Hello Timer Always Should Prevent The MAX Age Value From Being Hit. When The Max Agetimer Expires, It Usually Indicates A Link Failure. When This Happens, The Bridge Re-Enters The Listening State.
For STP To Recover From A Link Failure, It Takes Approximately 50 Seconds; It Takes 20 Seconds For The BPDU To Age Out, The Max Age; And It Takes 15 Seconds For The Listening State And 15 Seconds For The Learning State.
NOTE : Two Other Forms Of STP Exist Besides IEEE 802.1d. DEC And IBM Are Two Other Forms Of Spanning Tree In Use. The Operation Of All Forms Of STP Is Similar, And Cisco Routers Support All Forms.
†
The Goal Of This Article Is To Give An Easy Way To Understand The “Ethernet Technical Overview ". Hope This Article Will Help Every Beginners Who Are Going To Start Cisco Lab Practice Without Any Doubts.
Some Topics That You Might Want To Pursue On Your Own That We Did Not Cover In This Article Are Listed Here, Thank You And Best Of Luck.
This Article Written Author By: Premakumar Thevathasan. CCNA, CCNP, CCIP, MCSE, MCSA, MCSA - MSG, CIW Security Analyst, CompTIA Certified A+.
For These Reasons, We Assume That You Have Some Background In Ethernet Technology; Therefore, This Article Does Not Cover Ethernet Frames, Hubs, And Cables In Any Great Length.
This Chapter Instead Focuses More On:
Spanning Tree,
Fast Ethernet,
Gigabit Ethernet,
Ethernet And Token Ring Switching.
ETHERNET
WHAT IS ETHERNET?
Ethernet Is The Most Widely Used Local Area Network (LAN) Technology. The Original And Most Popular Version Of Ethernet Supports A Data Transmission Rate Of 10 Mb/S. Newer Versions Of Ethernet Called "Fast Ethernet" And "Gigabit Ethernet" Support Data Rates Of 100 Mb/S And 1 Gb/S (1000 Mb/S). An Ethernet LAN May Use Coaxial Cable, Special Grades Of Twisted Pair Wiring, Or Fiber Optic Cable. "Bus" And "Star" Wiring Configurations Are Supported. Ethernet Devices Compete For Access To The Network Using A Protocol Called Carrier Sense Multiple Access With Collision Detection (CSMA/CD).
ETHERNET OPERATION :
Ethernet Operates At The OSI Layer 2, The Data Link Layer.
The Data Link Layer Actually Is Divided Into Two SUBLAYERS :
The LLC Layer :802.2, In This Case:
Is A Standardized Interface Between A Hardware-Specific MAC And A Layer 3 Protocol.
THE MAC LAYER HAS THE FOLLOWING RESPONSIBILITIES :
• Generating The Physical Source And Destination Address For A Frame. These Are 48-Bit Industry-Wide Unique Addresses; The First 3 Bytes Are Assigned By The IEEE, And The Last 3 Bytes Are Vendor-Unique.
• Ensuring Reliable Transmissions.
• Synchronizing Data Transmissions.
• Performing Error Recognition.
• Performing Flow Control.
CSMA/CD
ETHERNET CSMA/CD :
Ethernet Technology Commonly Is Referred To As Carrier-Sense Multiple Access Collision Detect (CSMA/CD).
ETHERNET TRANSMITS FRAMES IN THE FOLLOWING MANNER :
CARRIER SENSE : This Also Is Known As "Listen Before Talking." An Ethernet Station Wanting To Transmit A Frame Listens To The Medium Before It Transmits To Ensure That The Medium Is Available.
TALK IF QUIET : If The Channel Is Quiet For A Specific Amount Of Time, The Inter Frame Gap (IFG) Before The Station Might Begin A Transmission. If The Channel Is Busy, It Is Monitored Until It Becomes Free For The Length Of IFG Timer; After That Time, Transmission Might Begin.
COLLISION : A Collision Is Measured As An Excess Of Voltage On The "Cable" Or Medium. A Collision Usually Is Caused By Two Stations Transmitting Data At The Same Time. If A Collision Occurs, Both Frames Are Destroyed.
1. COLLISION DETECTION: If A Station Detects A Collision During Transmission, That Transmission Immediately Stops. A Signal Jam Also Is Sent On The Media To Destroy Any Fragmented Frames, Preventing Corrupted Data.
2. BACKOFF: After A Collision, A Stations Waits A Period Of Time Called The Backoff Period. The Backoff Timer Is A Random Timer Generated By A Backoff Algorithm. This Prevents All Stations From Trying To Transmit At The Exact Same Time After A Collision.
After The Backoff Timer Expires, The Station Attempts To Retransmit The Frame. If Another Collision Happens, The Station Keeps Trying To Retransmit The Frame For 16 Times. After 16 Unsuccessful Attempts, The Frame Is Dropped.
HALF- AND FULL-DUPLEX ETHERNET :
Ethernet Was Developed On Older Coax Type Cables Capable Of Transmitting Or Receiving A Signal At Any Given Time. This Is Precisely Why Ethernet Needed CSMA/CD Technology.
With The Advent Of Switches, Running Ethernet Over UTP And Fiber, Full-Duplex Ethernet Became Available.
Full-Duplex Ethernet Allows A Station To Simultaneously Transmit And Receive Data. Ethernet Frames Are Transmitted And Received Simultaneously On Two Pairs Of UTP Or Fiber At Any Given Time.
Full-Duplex Ethernet Is Essentially Ethernet Without CSMA/CD. Full-Duplex Mode Basically Doubles The Bandwidth Of Ethernet! To Run Full-Duplex Ethernet, The Station And Switch Both Must Be Capable And Configured For Full-Duplex Operation. A Hub With Multiple Stations Cannot Operate In Full-Duplex Mode.
NOTE : A Station Not Operating In The Correct Duplex Mode Will Generate An Enormous Number Of Collisions On The Port Where It Is Located. These Collisions Mostly Likely Will Be Registered As "Late Collisions." Be Sure That The Port On The Switch And The End Station Are Operating In The Same Duplex Mode.
FAST ETHERNET
FAST ETHERNET :
In May 1995, The IEEE Adopted The Fast Ethernet Standard, 802.3u. Years Later, After Battling FDDI, 100VG Any Lan, And ATM, This Standard Became The Prominent Type Of LAN. As Costs Per Port Dropped Along With The Prices Of Network Interface Cards (Nics), Fast Ethernet Won Out Over FDDI And 100VG Any Lan And ATM For Many Reasons: • It Allows A Clean And Inexpensive Migration Path From Existing 10-Mbps Ethernet Networks. At First, It Could Run Only Fiber And Cat 5 UTP.
• Fast Ethernet Didn't Require Costly Fiber Connections And Didn't Require Complex Configurations.
• Fast Ethernet Addresses Qos With Enormous Amounts Of Bandwidth, While Relying On The Upper Layers Or Network Design To Provide QOS.
• Basically, Fast Ethernet Became A Plug-And-Play Tenfold Upgrade Of The LAN. 100-Mb Networks Evolved Out Of 10-Mb Networks Across Data Centers Everywhere.
SOME IMPORTANT FEATURES AND SPECIFICATIONS OF FAST ETHERNET ARE AS FOLLOWS :
• The 100Base MAC Uses The Original Ethernet MAC Operating At 10 Times The Speed. This Is Completely Backward Compatible With 10-Mbps Ethernet.
• 100Base-T Includes A Specification For An MII Interface. A MII Interface Is A 100-Mbps Version Of The AUI Adapter.
• Fast Ethernet Supports Full- And Half-Duplex Functionality.
• Fast Ethernet Operates Over A Wide Array Of Different Physical Layers: Cat 5, Cat 3, Fiber, And So On.
GIGABIT ETHERNET
GIGABIT ETHERNET :
The IEEE Started To Work On The 802.3z, Or Gigabit Ethernet, Standard. Three Short Years Later, In June 1998, The 802.3z Standard Officially Was Adopted. For The Most Part, The Gigabit Standard Is The Fast Ethernet Standard Multiplied By 10. This Is Why 10-Gb Ethernet Products Are Right Around The Corner And 100-Gb Ethernet Is An Eventuality.
SOME IMPORTANT FEATURES AND SPECIFICATIONS OF GIGABIT ETHERNET ARE AS FOLLOWS :
• Gigabit Ethernet Uses The 802.3 Frame Format, Identical To That Of 10-Mbps And 100-Mbps Ethernet.
• It Includes A Specification For A Gigabit MII (GMII). Unlike 10-Mbps Ethernet And 100-Mbps Ethernet, The GMII Is An Electrical Specification And Does Not Include A Physical Connector. Cisco's Physical Gigabit Interfaces Are Called Gbics. The Type Of GBIC Determines The Physical Gigabit Connection. There Are Currently Multimode Fiber (MMF), Single-Mode Fiber (SMF), And UTP Gbics, As Well As A Cisco Proprietary GBIC Called A Gigastack.
One Element That Makes Gigabit Ethernet One Of The Most Versatile Protocols In Years Is The Concept Of The GMII. Except For The Strictly 1000Base-TX Switches, Most Gigabit Ethernet Switches Come With Open Ports For Gbics. Depending On Your Network Needs, You Can Put Any Type Of GBIC In This Port. With The Click Of A GBIC, Your Network Can Change From A Limited 100-M Copper-Based Network To A 10,000-M Fiber-Based Network! The Sections That Follow Cover The More Common Gbics, Gigabit Standards, And Length Limitations.
1000BASE-SX GIGABIT ETHERNET :
1000Base-SX Gbics Use A Laser-Based Wavelength Of 850 Nms. Depending On The Cable Type, SX GBIC Operates At Distances Of 220 M To 550 M.
1000 BASE-LX GIGABIT ETHERNET :
LX Gbics Use A Laser-Based Wavelength Of 1300 Nms. Depending On The Cable Type, LX GBIC Operate At Distances Of 550 M To 5000 M, Cisco Also Supports An LH And LX GBIC, Which Extends The IEEE 1000Base-LX Maximum Distance Of 5 Km.
1000BASE-T GIGABIT ETHERNET :
The IEEE Standard For Gigabit Ethernet Transmission Over Cat 5 UTP Is 802.3ab. The Standard Defines The Maximum Distance To Be 100 Meters And The Copper To Be At Least Cat 5 Using Four Pairs Of Wires, Terminated With An RJ-45 Jack.
ETHERNET AUTONEGOTIATION :
To Simplify The Configuration Of Ethernet Devices, The 802.3.U Committee Defined Fast Link Pulse (FLP). FLP Sends A Series Of Pulses On The Network That Can Deduce What Duplex And Speed The Link Is Operating At. The Station And The Hub/Switch Agree On The Highest Priority And Configure The Station In That Manner.
NOTE : Duplex Modes Are A Function Of The Hardware Built Into The Network Interface Card (NIC). Software Upgrades Will Not Allow You To Run Full-Duplex Mode. For Full-Duplex Mode To Work, The Station And The Switch Port Must Be Capable Of Full-Duplex Operation.
ETHERNET SWITCHING
ETHERNET SWITCHING :
In The Early 1990s, Kalpana, Grand Junction, And Bay Networks Started To Ship Some Of The First Ethernet Switches. The Bay Networks 28115 Was One Of The First Switches To Introduce 10/100 Auto-Sensing Ports And Virtual Lans (VLANS).
More Importantly, All Switches Put An End To The Old Ethernet Repeater Rules, While Increasing Bandwidth. Until This Point, Many People Were Predicting That ATM Would Be The Only High-Speed Protocol In Use On The LAN—And If It Hadn't Been For The Ethernet Switch, They Might Have Been Right.
ETHERNET SWITCHES PROVIDE SEVERAL KEY ADVANTAGES OVER TRADITIONAL SHARED MEDIA LANS:
• Significant Bandwidth Improvement By Limiting A Collision Domain To A Single Port.
• Scalability. Repeater Rules Are Limited To A Single Port.
• VLAN Capability. Broadcast Domains Can Be Located Logically And Are Not Limited By Geographical Boundaries.
• Enhanced Security.
• Full-Duplex Capability.
A SWITCH FUNCTIONS MUCH LIKE A MULTIPORT BRIDGE: When VLANS Are Created, Virtual Bridges Are Created To Join The Ports In The VLAN. Broadcast, Unicast, And Multicast Traffic Is Forwarded To Each Member Of The VLAN. The Catalyst 5500 Series Switch Builds An Address Table By Recording The Source MAC Address Of Frames That It Received From Its Interfaces.
When A Frame Destined For An Address Not Yet In The Address Table Is Received, The Switch Floods The Frame Out All Ports And Trunks In The Same VLAN As The Frame Was Received. The Switch Does Not Forward The Frame Out The Interface That It Received It. When A Reply For That Frame Is Received, The Switch Records The New Address In The Address Table.
The Switch Forwards Subsequent Frames To A Single Port, Without Flooding It To All Ports. Traffic Can Leave The VLAN Only With The Aid Of A Router Or A Layer 3 Switch Providing Routing Functionality.
SWITCHES FORWARD TRAFFIC IN THREE PRIMARY MODES :
BROADCAST DOMAINS VS COLLISION DOMAINS
BROADCAST DOMAINS AND COLLISION DOMAINS :
Two Key Concepts In Switched Networks Are Broadcast Domains And Collision Domains. A Broadcast Domain Is The Area Of The Network That Forwards Broadcasts From One Portion Of Network To The Next. A Practical Example Of A Broadcast Domain Is An IP Or IPX Subnet. A Collision Domain Is A Function Of The Physical Properties Of A Device. Devices In The Same Collision Domain Reside On The Same "Wire" Or Hub/Repeater.
HOW VARIOUS NETWORK DEVICES SEGMENT COLLISION AND BROADCAST DOMAINS :
All Ports Are In A Single Collision Domain.
All Ports Are In A Single Broadcast Domain.
Each Port Is A Separate Collision Domain.
All Ports Are In A Single Broadcast Domain.
Each Port Is A Separate Collision Domain.
Each Port Is In A Separate Broadcast Domain.
Each Port Is A Separate Collision Domain.
Each Port Is Configurable To Be In The Same Or A Separate Collision Domain.
LAN SWITCHING FOR EXTENDED REFERENCE
The World Of Ethernet Switching Is Growing At A Tremendous Pace. There Are Many More Interesting And Useful Technologies That We Simply Did Not Have The Time To Discuss.
We Highly Recommend Reading Your Own, “Cisco LAN Switching Books” Which Is Loaded With Information On LAN Switching For Extended Reference.
INFORMATION ON LAN SWITCHING FOR EXTENDED REFERENCE :
UPLINKFAST, PORTFAST, BACKBONEFAST: These Are Ways To Help The STP Process Deal With User Traffic During Initialization Or Failure While STP Is Converging. These Technologies Are Simple To Configure And Can Avoid Lost Throughput During STP Convergence.
FAST ETHERCHANNEL/GIGABIT ETHERCHANNEL: Etherchannel Provides A Way For The Router To Aggregate Up To Four Fast Ethernet Ports In A Bundle. The Technology Also Applies To Gigabit Ethernet. We Like To Think Of Etherchannel As The PPP Multilink Of Ethernet. Etherchannel Treats The Bundle As One Large Physical Link And Can Distribute Traffic In Different Ways Across The Bundle. With Full-Duplex Mode, Etherchannel Bundles Can Reach Speeds Of 800 Mbps To 8,000 Mbps. Etherchannel Can Help Avoid Some STP Issues Because It Offers Resiliency Between Switches. When A Link Goes Down, The Bundle Simply Loses Bandwidth And Does Not Need To Wait For STP To Converge Before Sending User Traffic. There Are Rules To Etherchannel And How Ports Can Be Bundled, And They Are Different For The Various Families Of Switches.
There Is Also The Drawback That Etherchannel Can Be Used Only To Connect Two Switches
—For Example, Bundles Cannot Be Split Across Switches.
• Port Security— An Advanced Security Function Of All Cisco Switches Is Port Security. Port Security Allows You To Limit The Access On A Port To A Single MAC Address. When Another User Plugs Into The Port With A Different MAC Address, The Port Can Be Shut Down, Or Traps Can Be Sent To A Network-Management Station. This Is A Helpful Feature In The Field Because It Strictly Controls Physical Access To The Switch And Unwanted Moves Or Changes.
• Multicast (CGMP/IGMP)— This Does Not Mean That It Is Not Important, However. Multicast Traffic Is Playing An Ever-Increasing Role In The Modern Network.
VLAN
VIRTUAL LANS (VLANS):
Many Definitions For A VLAN Exist. The Definition That We Chose To Use Is Simple. Virtual LANS (VLANS) Can Be Easily Defined As Broadcast Domains That Can Extend Geographical Distances. When Configuring Ethernet Switching, Every Port Must Be Assigned To A VLAN. The Default VLAN Is Always VLAN 1. When Switches Ship From The Factory, They Are In Some Ways Plug-And-Play.
Every Port Is Assigned To VLAN 1, So Every Port Of The Switch Is In A Single Broadcast Domain. This Makes Migrating From Shared Ethernet Hubs To A Basic Switched Network Easy. VLANS Always Should Be Thought Of As Simply Broadcast Domains. Most VLANS Eventually Become IP/IPX Subnets Or Bridging Domains.
THE BASIC DESIGN RULES THAT APPLY TO BROADCAST DOMAINS ALSO APPLY TO VLANS:
• A Single Subnet Is Used Per VLAN.
• Do Not Bridge Different VLANS Together.
• A Router/Layer 3 Switch Is Needed To Route Between VLANS.
• STP Must Run In Each VLAN To Prevent Loops. This Can Be Disabled, But It Is Not Recommend.
VLAN TAGGING :
In 1998, The IEEE Approved The 802.3ac Standard That Defines Frame Format Extensions To Support Virtual Local Area Network (VLAN) Tagging On Ethernet Networks. The Vlan Protocol Permits Insertion Of An Identifier, Or "Tag", Into The Ethernet Frame Format To Identify The Vlan To Which The Frame Belongs. It Allows Frames From Stations To Be Assigned To Logical Groups.
This Provides Various Benefits Such As Easing Network Administration, Allowing Formation Of Work Groups, Enhancing Network Security, And Providing A Means Of Limiting Broadcast Domains, Refer To IEEE Standard 802.1q For Definition Of The VLAN Protocol. The 802.3ac Standard Defines Only The Implementation Details Of The VLAN Protocol That Are Specific To Ethernet.
If Present, The 4-Byte Vlan Tag Is Inserted Into The Ethernet Frame Between The Source Mac Address Field And The Length/Type Field. The First 2-Bytes Of The VLAN Tag Consist Of The "802.1q Tag Type" And Are Always Set To A Value Of 0x8100. The 0x8100 Value Is Actually A Reserved Length/Type Field Assignment That Indicates The Presence Of The VLAN Tag, And Signals That The Traditional Length/Type Field Can Be Found At An Offset Of 4-Bytes Further Into The Frame.
THE LAST 2-BYTES OF THE VLAN TAG CONTAIN THE FOLLOWING INFORMATION :
• The First 3-Bits Are A User Priority Field That May Be Used To Assign A Priority Level To The Ethernet Frame.
• The Next 1-Bit Is A Canonical Format Indicator (CFI) Used In Ethernet Frames To Indicate The Presence Of A Routing Information Field (RIF).
• The Last 12-Bits Are The VLAN Identifier (VID) Which Uniquely Identifies The VLAN To Which The Ethernet Frame Belongs.
With The Addition VLAN Tagging, The 802.3ac Standard Permitted The Maximum Length Of An Ethernet Frame To Be Extended From 1518-Bytes To 1522-Bytes. The Following Illustrates The Format Of An Ethernet Frame That Has Been "Tagged" With A VLAN Identifier Per The IEEE 802.3ac Standard:
VTP AND TRUNKING PROTOCOLS
VLAN TRUNKING PROTOCOL (VTP) :
A Powerful Function Of VLANS Is Their Capability To Span Distance. VLANS Are Communicated From Switch To Switch By The Means Of A VLAN Trunking Protocol (VTP). VTP Is Used To Maintain Global VLAN Information Between Switches. A VLAN Management Domain, Or VTP Domain, Consists Of One Or More Switches Interconnected And Sharing The Same Administrative Responsibility.
Anytime That You Want The VLANS On One Switch To Have Information About The Vlans On Another Switch (That Is, When You Want These Two Broadcast Domains To Communicate), You Need To Configure A VTP Domain And A Trunk. VTP Also Tracks All The VLANS In A VTP Domain And Propagates These In A Client/Server Manner From One Switch To Another. The Intent Of VTP Is To Ease Management And Provide A Common VLAN Database Across The VTP Domain.
VTP Advertisements Are Sent Out On All Trunk Connections In Inter-Switch Link (ISL) Frames, 802.1q Frames, IEEE 802.10, Or ATM LANE. VTP Frames Are Sent To The Destination MAC Address Of 0100.0CCC.CCC With An LLC Code Of SNAP (AAAA) And A Type Of 2003 In The SNAP Header. For VTP Messages To Successfully Be Transmitted, The Following Must Occur:
• VTP Will Accept Only Messages With The Same Domain Name. If Authentication Is Configured For That VTP Domain, That, Too, Must Match. The VTP Name Is Case-Sensitive.
• VTP Will Accept Only Messages With The Same Version, Type I Or Type II. This Setting Is Controlled, With The V2 Mode Being Either Enabled On Both Sides Of The Link Or Disabled. A Switch Might Be VTP Version II–Capable And Have V2 Mode Disabled. V2 Mode Is Primarily For Token Ring Switches.
• Catalyst Switches Must Be Adjacent, And Trunking Must Be Configured Between Them. For Ethernet Networks, The Trunking Protocol Is Dotq1 (802.1q) Or ISL. ATM Uses LANE, And FDDI Uses IEEE 802.10.
• VTP Servers Will Synchronize With VTP Clients Only If The VTP Client's Revision Number Is Less Than That Of The VTP Server. If The VTP Client's Revision Number Is Equal To Or Greater Than That Of The VTP Servers The Client VLAN Database Will Not Synchronize.
After A Trunk Is Established, VTP Sends Periodic Advertisement Out Each Trunk Port.
THE VTP ADVERTISEMENTS CONTAIN THE FOLLOWING :
• VLAN IDs (ISL And 802q).
• Emulated LAN Names For ATM LANE.
• 802.10 SAID Values.
• VTP Domain Name And Configuration Revision Number. The Server With The Highest Revision Number Becomes The Primary Server. The Revision Number Is Incremented Every Time A VLAN Configuration Change Is Made.
• VLAN Configuration, VLAN ID, VLAN Name, And MTU Size For Each VLAN.
• Ethernet Frame Format.
VTP HAS TWO VERSIONS, Simply Called Version I And Version II. All The Switches In The VTP Domain Must Be On The Same Version. This Rule Does Not Apply To The Transparent-Mode Switches. Version II Offers The Following, The Most Important Being Support For Token Ring:
• TOKEN RING SUPPORT : VTP VII Supports Token Ring LAN Switching And VLANS (Token Ring Bridge Relay Function [TRBRF]). We Will Discuss TRBRF More In Upcoming Sections.
• UNRECOGNIZED TYPE : Length Value (TLV) Support Is Included. Unrecognized TLV Is Saved In NVRAM When The Switch Is In VTP Server Mode.
• VERSION-DEPENDENT TRANSPARENT MODE : VTP Will Forward VTP Messages That Do Not Match The Domain Name And Version To Switches Operating In VTP Transparent Mode.
• CONSISTENCY CHECKS : Consistency Checks Are Performed On VLAN Names, And Values Are Performed Only Upon New Entry Into The Switch.
VTP MODES
VTP OPERATES IN ONE OF THREE MODES :
VTP SERVER MODE : In VTP Server Mode, VLANS Can Be Created, Modified, And Deleted. VLAN Information Automatically Is Sent To All Adjacent VTP Servers And Clients In The Same VTP Domain. Caution Always Should Be Used When "Clearing" A VLAN From The VTP Server Because That VLAN Will Be Deleted On All VTP Servers And Clients In That DTP Domain. If Two Devices Are Configured As Servers, The Switch/Server With The Highest VTP Configuration Revision Will Be The Primary Server. VLAN Information Is Stored In The Switches NVRAM.
VTP CLIENT MODE : In VTP Client Mode, VLANS Cannot Be Created, Modified, Or Deleted. Only The Name And The VTP Mode And Pruning Can Be Changed. The Client Is At The Mercy Of The VTP Server For All VLAN Information. The Client Still Must Assign Ports To A VLAN, But The VLAN Will Not Be Active On The Switch Unless The VTP Server Sends Information To The Client On That VLAN. Furthermore, VLAN Information Is Stored Locally In The Switch's NVRAM When It Is Received From The Server On The Catalyst 2900XL/2500G Series Switches. The Catalyst 4000/5500/6500 Series Of Switches Do Not Store The VLAN Database If Configured Has A VTP Client.
VTP TRANSPARENT MODE : In VTP Transparent Mode, VTP Information That Is Local On The Switch Will Not Be Advertised, But VTP Information Received Form Other Switches Will Be Forwarded. VLANS Can Be Created, Modified, And Deleted On Transparent Switches. VLAN Information Also Is Stored In NVRAM.
TRUNKING MODE :
DTP Is The Second Generation Of DISL (Dynamic ISL) And Exists To Ensure That The Different Parameters Involved In Sending ISL Or 802.1Q Frames, Like The Configured Encapsulation Type, Native VLAN, Hardware Capability, Etc.
Operational Overview: DTP Is A Layer-2 Protocol That Negotiates Configuration Parameters Between A Switch Port And It's Neighbor. It Uses Another Well-Known Multicast MAC Address Of 01-00-0c-Cc-Cc-Cc And A SNAP Protocol Type Of 0x2004. Here Is A Summary Of The Configuration Modes:
Note: ISL And 802.1Q Encapsulation Type Can Be Set Or Negotiated - ISL Will Be Preferred Over Dot1q, But Is Recommended To Be Set.
• DTP Assumes Point-To-Point Connection, And Cisco Devices Will Support 802.1Q Trunk Ports That Are Only Point-To-Point.
• During DTP Negotiation, The Ports Will Not Participate In STP. Only After The Port Type Becomes One Of The Three Types (Access, ISL Or 802.1Q), The Port Will Be Added To STP. (If PAGP Is Running That Is The Next Process To Run Prior To The Port Participating In STP).
• VLAN 1 Will Usually Be There On The Trunk Port. If The Port Is Trunking In ISL Mode, DTP Packets Are Sent Out On VLAN 1, Otherwise (For 802.1Q Trunking Or Non-Trunking Ports) On The Native VLAN.
HIERARCHICAL (MULTILAYER) NETWORK DESIGN
HIERARCHICAL (MULTILAYER) NETWORK DESIGN:
• In Desirable Mode DTP Packets Transfer The VTP Domain Name, Which Must Match For A Negotiated Trunk To Come Up, Plus Trunk Configuration And Admin Status.
• Messages Are Sent Every 1s During Negotiation, And Every 30s After That.
• Be Careful That It Is Understood Modes (On, Nonegotiate, Off) Explicitly Specify In Which State The Port Will End Up. A Bad Configuration Can Lead To A Dangerous Inconsistent State Where One Side Is Trunking And The Other Is Not. A Port In On, Auto, Or Desirable Sends DTP Frames Periodically. If A Port In Auto Or Desirable Mode Doesn't See A DTP Packet In 5min It Will Be Set To Non-Trunk.
By Default, Trunking Ports Will Propagate Information About All VLANS, But AS Recommends Limiting That To The VLANS Defined On The Wiring Closet Switches. This Practice Has Many Advantages, Most Importantly, Is The Ability To Isolate Issues, Broadcasts, And Loops To One Wiring Closet Instead Of The Entire Network.
TRUNKING ENCAPSULATIONS
THREE PRIMARY TRUNKING ENCAPSULATIONS ARE AVAILABLE FOR ETHERNET :
INTER-SWITCH LINK (ISL): ISL Is A Cisco Proprietary Trunking Encapsulation. ISL Is A Frame-Tagging Protocol;
The Frames On The Link Contain The Standard Ethernet, FDDI, Or Token Ring Frame And The VLAN Information Associated With That Frame.
ISL Is Supported On Links That Are 100 Mbps Or Greater In Speed. ISL Is An Extremely Efficient Protocol, And It Is The Protocol That Cisco Uses Internally For Catalyst To Communicate With The Route Switch Modules (RSMs) Or Other Layer 3 Switching Fabric.
SPANNING TREE Is Run On A Per-VLAN Basis (PVST) On ISL Trunks. This Means That Every VLAN Has A Root Bridge, And Trunks Go Into A Forward/Blocking Mode For Each VLAN On Each Trunk.
PVST Is Critical To Control On Large Networks, As Discussed In Upcoming Sections. • IEEE 802.1Q : 802.1q Is The Industry-Standard Trunking Protocol. 802.1q Operates Slightly Differently Than ISL. It Runs Mono Spanning Tree (MST) On The Default VLAN For All VLANS In The VTP Domain. In MST, One Root Bridge Is Elected For The Entire VTP Domain; This Is Called The Common Spanning Tree (CST). All VLAN Information Follows One Path In This Type Of Configuration. Cisco, Understanding The Need To Control Spanning Tree On Large Networks While Controlling Load, Implements PVST On All 802.1q VLANS.
THE FOLLOWING IS A LIST OF OTHER RESTRICTIONS ON 802.1Q TRUNKS :
- The Default VLAN Needs To Be The Same On Both Ends Of The Trunk. MST Will Run In This VLAN. It Is Critical That The Default VLAN Be The Same On Third-Party Switches Interacting With Cisco Switches.
- As Mentioned, 802.1q Uses MST. Cisco Overrides This, By Default, With PVST. Because The BPDUs Are Handled Differently Between Cisco And Third-Party Switches, Care Should Be Taken Whenever Integrating These Domains That Spanning Tree And The Default VLANS Are Consistent In Both Switches. The Entire Cisco VTP Domain Looks Like A Single Broadcast/ Spanning Tree Domain To The Third-Party Switches.
- BPDUs On The Native VLAN Of The Trunk Are Sent Untagged To The Reserved IEEE 802.1d Spanning Tree Multicast MAC Address (0180.C200.0000). The Bpdus On All Other VLANS On The Trunk Are Sent And Tagged On The Reserved Cisco Shared Spanning Tree (SSTP) Multicast MAC Address (0100.0ccc.Cccd).
• IEEE 802.10 : 802.10 Was Actually The First Protocol That The Industry Tried To Use For A VLAN Trunking Protocol. It Originally Was Developed For Extra Security On Defense Networks Or Large Mans. It Primarily Is Used On FDDI Networks Today Because Of Its Limitations.
DYNAMIC ISL (DISL) AND DYNAMIC TRUNK PROTOCOL (DTP):
Dynamic ISL Was Cisco's First Trunk-Negotiation Protocol. It Exists On All Catalyst 5500 Series Software 4.1 And Earlier. Originally, It Was Used To Negotiate Trunks For The ISL Protocol Only. Later, In 4.2, DISL Was Replaced With Dynamic Trunk Protocol (DTP). DTP Is Essentially DISL That Attempts To Automate ISL And 802.1q Trunk Configuration.
DTP Uses The Reserved Destination Multicast Address Of 0100.0CCC.CCCC For LAN Networks To Negotiate Trunks.
In The Default Auto State, DTP Messages Are Sent Out Every 30 Seconds On All Trunk Lines. Depending On the Mode of the Port, The Port Might Become An ISL Or 802.1q Trunk.
DTP OPERATES IN THE FOLLOWING MODES :
• ON— Puts The Port In A Permanent Trunking State. It Also Tries To Negotiate The Link To Be A Trunk.
• OFF— Disables The Port, And Thereby The Trunk.
• DESIRABLE— Makes The Port Attempt To Convert To A Trunk Link. The Port Becomes A Trunk If The Neighboring Port Is Set To On, Desirable, Or Auto Modes.
• AUTO— The Port Converts To A Trunk If The Neighboring Port Is Set To On Or Desirable Modes.
• NONEGOTIATE— Puts The Port Into Trunking Mode But Prevents The Port From Sending DTP Frames.
In Actuality, This Is Really Too Many Options For A Trunk. Network Administrators Either Configure A Port As A Trunk Or They Don't. It Could Be Argued That To Have A Network Be So Flexible The Trunks Can Or Should Be Added Dynamically Is A Security Risk.
STP
Spanning Tree Protocol (STP) Is Designed To Prevent Problems Related To Bridging Loops. STP Solves The Problem By Blocking Redundant Paths And Allowing Only A Single Active Path.
Spanning Tree Works By Selecting A Root Switch Then Selecting A Loop-Free Path From The Root Switch To Every Other Switch. To Do That, Spanning Tree Must Choose A Single Root Bridge, One Root Port For Each Nonroot Switch, And A Single Designated Port For Each Network Segment.
802.1D SPANNING TREE PROTOCOL (STP):
When Ethernet Evolved From A Single Shared Cable To Networks With Multiple Bridges And Hubs, A Loop-Detection And Loop-Prevention Protocol Was Needed. The 802.1d Protocol, Developed By Radia Perlman, Provided This Loop Protection. It Did Such A Good Job That When Most Networks Went From Bridged Networks To Routed Networks, So The Importance Of Spanning Tree Was Almost Forgotten.
Because Of This, Spanning Tree Is Probably The Most Used But Least Understood Protocol In The Modern Internetwork. But With The Huge Success Of Ethernet Switching, Spanning Tree Again Becomes An Important Protocol To Control And, More Importantly, Understand. We Will Discuss Why Spanning Tree Has Become So Important In Switched Ethernet Networks In Upcoming Sections.
SPANNING TREE OPERATION :
Spanning Tree's Purpose In Life Is To Elect A Root Bridge And Build Loop-Free Paths Leading Toward That Root Bridge For All Bridges In The Network. When Spanning Tree Is Converged, Every Bridge In The Network Has Its Bridged Interfaces In One Of Two States: Forwarding Or Blocking. If The Port Has The Best-Cost Path To The Root Bridge, It Is Forwarding And Thus Is The Shortest Path To Root. All Other Interfaces On The Bridge Are In A Blocking State.
STP ACCOMPLISHES THIS BY TRANSMITTING SPECIAL MESSAGES CALLED BRIDGE PROTOCOL DATA UNITS (BPDUS). BPDUS EXIST IN TWO FORMS:
• A Configuration BPDU, Used For Initial STP Configuration
• A Topology Change Notification (TCN) BPDU Used For Topology Changes
BPDUS Are Transmitted Using A Reserved Multicast Address Assigned To All Bridges. The BPDU Is Sent Out All Bridged LAN Ports And Is Received By All Bridges Residing On The LAN. The BPDU Is Not Forwarded Off The LAN By A Router.
THE BPDU CONTAINS THE FOLLOWING RELEVANT INFORMATION:
ROOT ID: The ID Of The Bridge Assumed To Be Root. Upon Initialization, The Bridge Assumes That It Is Root.
TRANSMITTING BRIDGE ID AND PORT ID: The BID Of The Bridge Transmitting The BPDU, And What Port The BPDU Originated From.
COST TO ROOT: The Least-Cost Path To The Root Bridge From The Bridge Transmitting The BPDU. Upon Initialization, Because The Bridge Assumes Itself To Be Root, It Transmits A 0 For The Cost To Root.
THE BRIDGE ID (BID): Is An 8-Byte Field Composed From A 6-Byte MAC Address And A 2-Byte Bridge Priority. The MAC Address Used For The BID Is Generated From A Number Of Sources, Depending On The Hardware In Use For The Bridge. Routers Use A Physical Address, Whereas Switches Will Use An Address From The Backplane Or Supervisor Module.
The Priority Value Ranges From 0 To 65,535; The Default Value Is 32,768.
The Path Cost Is Used By Bridges To Determine The Best Possible Path To Root. Path Costs Recently Have Been Updated By The IEEE To Include Gigabit And Greater Links. The Lower The Path Cost Is, The More Preferable The Path Is.
STP STATES
STP HAS FIVE PRIMARY STATES (STATES OF STP):
That It Transitions Through During Its Operation. When STP Converges, It Is In One Of Two States, Forwarding Or Blocking.
DISABLED :
This State Appears When A Bridge Is Having Problems Processing BPDUS, When A Trunk Is Improperly Configured, Or When The Port Is Administratively Down.
LISTENING : When A Bridge Port Initializes Or During The Absence Of BPDUS For A Certain Amount Of Time, STP Transitions To The Listening State. When STP Is In This State, The Port Is Actually Blocking And No User Data Is Sent On The Link. STP Follows A Three-Step Process For Convergence:
1. ELECT ONE ROOT BRIDGE : Upon Initialization, The Bridge Begins Sending BPDUS On All Interfaces. A Root Bridge Is Chosen Based On The Bridge With The Lowest BID. Recall That The BID Is A Combination Of A Priority And MAC Address. In The Event Of A Tie, The Bridge With The Lowest MAC Address Is Chosen As Root. All Ports Of The Root Bridge Are Put In The Forwarding State.
2. ELECT ONE ROOT PORT FOR EVERY NONROOT BRIDGE: After A Single Root Bridge Has Been Elected, STP Elects A Single Root Port On Each Bridge That Is Not Root. The Root Port Is The Bridge's Best Path To The Root Bridge. When A Root Port Is Elected, It Is Put Into The Forwarding State.
TO DETERMINE WHAT PORT SHOULD BE A ROOT PORT, STP FOLLOWS THIS DECISION PROCESS:
a. Lowest Root BID.
b. Lowest Path Cost To Root Bridge; The Cumulative Cost Of The All Paths To Root.
c. Lowest Sender BID.
d. Lowest Port ID.
When A Bridge Receives A BPDU, It Stores It In A Bridge Table For That Port. As New BPDUs Are Received On That Port, They Are Compared To Existing BPDUS. Using The Four-Step Process Listed Previously, BPDUs That Are More Attractive Or That Have Lower Costs Are Kept, And The Other Ones Are Discarded.
The Primary Variable That Influences The Root Port Election Is The Cost To The Root Bridge. This Is The Cumulative Path Cost Of All Links To The Root Bridge.
3. ELECT ONE DESIGNATED PORT/DESIGNATED BRIDGE FOR EVERY SEGMENT: For Every Segment, STP Elects One Port That Will Send And Receive All Information From That Segment To The Root Bridge. A Root Port Can Be Thought Of As The Port That Forwards Information To The Root, Whereas The Designated Port Can Be Thought Of As The Port That Sends Traffic Away From The Root. This Rule Applies Mostly To Shared-Media Bridges, Or Routers. Designated Ports On Back-To-Back Switched Trunk Lines Do Not Follow This Rule.
4. All Remaining Ports Become Nondesignated Ports And Are Put In Blocking Mode.
LEARNING :
Ports That Remain Designated Or Root Ports For A Period Of 15 Seconds, The Default Forward Delay, Enter The Learning State. The Learning State Is Another 15 Seconds That The Bridge Waits While It Builds Its Bridge Table.
FORWARDING AND BLOCKING :
When The Bridge Reaches This Phase, Ports That Do Not Serve A Special Purpose, Such As A Root Port Or A Designated Port, Are Called Nondesignated Ports.
All Designated Ports Are Put In A Forwarding State, While All Nondesignated Ports Are Put Into A Blocking State. In The Blocking State, A Bridge Does Not Send Any Configuration Bpdus, But It Still Listens To Them.
A Blocking Port Also Does Not Forward Any User Data.
STP TIMERS:
STP Has Three Basic Timers That Regulate And Age Bpdus: A Hello Timer, A Forward Delay Timer, And A Max Age Time.
THE TIMERS ACCOMPLISH THE FOLLOWING FOR STP:
• HELLO TIMER: The Default Hello Timer Is 2 Seconds. This Is The Amount Of Time Between Configuration Bpdus Sent By The Root Bridge.
• FORWARD DELAY TIMER: This Timer Is The Default 15 Seconds That The Routers Wait While Building Its Bridging Table. The Listening And Learning Stages Each Use This Single 15-Second Timer.
• MAX AGE TIMER: The Max Age Timer Is How Long A BPDU Is Stored Before It Is Flushed. If This Timer Expires Before The Interface Receives A New BPDU, The Interface Transitions To The Listening State. An Expired Max Ageparameter Usually Is Caused By A Link Failure.
The Default Value Is 20 Seconds.
STP Uses The Hello Timer To Space BPDUS And Has A Keepalive Mechanism.
The Hello Timer Always Should Prevent The MAX Age Value From Being Hit. When The Max Agetimer Expires, It Usually Indicates A Link Failure. When This Happens, The Bridge Re-Enters The Listening State.
For STP To Recover From A Link Failure, It Takes Approximately 50 Seconds; It Takes 20 Seconds For The BPDU To Age Out, The Max Age; And It Takes 15 Seconds For The Listening State And 15 Seconds For The Learning State.
NOTE : Two Other Forms Of STP Exist Besides IEEE 802.1d. DEC And IBM Are Two Other Forms Of Spanning Tree In Use. The Operation Of All Forms Of STP Is Similar, And Cisco Routers Support All Forms.
ETHERNET STANDARD TABLE
CONCLUSION:
The Goal Of This Article Is To Give An Easy Way To Understand The “Ethernet Technical Overview ". Hope This Article Will Help Every Beginners Who Are Going To Start Cisco Lab Practice Without Any Doubts.
Some Topics That You Might Want To Pursue On Your Own That We Did Not Cover In This Article Are Listed Here, Thank You And Best Of Luck.
This Article Written Author By: Premakumar Thevathasan. CCNA, CCNP, CCIP, MCSE, MCSA, MCSA - MSG, CIW Security Analyst, CompTIA Certified A+.
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