FIRST UNDERSTAND IEEE 802 STANDARD
When We Talk About Local Area Network (LAN) Technology The IEEE 802 Standard May Be Heard. This Standard Defines Networking Connections For The Interface Card And The Physical Connections, Describing How They Are Done. The 802 Standards Were Published By The Institute Of Electrical And Electronics Engineers (IEEE). The 802.3 Standard Is Called Ethernet, But The IEEE Standards Do Not Define The Exact Original True Ethernet Standard That Is Common Today. There Is A Great Deal Of Confusion Caused By This. There Are Several Types Of Common Ethernet Frames. Many Network Cards Support More Than One Type.
The Ethernet Standard Data Encapsulation Method Is Defined By RFC 894. RFC 1042 Defines The IP To Link Layer Data Encapsulation For Networks Using The IEEE 802 Standards.
The 802 Standards Define The Two Lowest Levels Of The Seven Layer Network Model And Primarily Deal With The Control Of Access To The Network Media. The Network Media Is The Physical Means Of Carrying The Data Such As Network Cable. The Control Of Access To The Media Is Called Media Access Control (MAC).
CISCO ROUTERS SUPPORT TWO MAIN TRUNKING PROTOCOLS
There Are Two Major Trunking Protocols You Must Understand And Compare Successfully, Those Being ISL And IEEE 802.1Q. Let's Take A Look At The Details Of ISL.
ISL Is A Cisco-Proprietary Trunking Protocol, Making It Unsuitable For A Multivendor Environment. Also Cisco Routers Support Two Main Trunking Protocols, ISL AND 802.1Q. ISL Is A Cisco Proprietary Protocol, You Can Only Use It Between Cisco Devices.
802.1q Is An IEEE Open Standard That Is Supported By Most Manufacturers Of Network Hardware. Unfortunately, The 802.1q Open Standard Reached The Market Some Time After The Initial Demand For Trunking Protocols. So Most Manufacturers Of Layer 2 Switching Equipment Developed Their Own Proprietary Standards To Fill The Void. Cisco Developed ISL. All Newer Cisco Equipment Now Supports 802.1q,
But There Are Still Many Older Catalyst Switches That Cannot Support The Open Standard, So ISL Is Your Only Choice With This Equipment. In Any Case, Cisco's ISL Support Is Generally More Mature And Stable Than Its 802.1q Implementations. So While We Generally Recommend Working With Open Standards Where Possible, ISL Is Still Clearly The More Viable Option In Some Networks.
INTERSWITCH LINK (ISL)
INTER-SWITCH LINK (ISL) Protocol Is A Cisco Propriety Protocol And Inter-Switch Link (ISL) Is Available And Supported On Cisco Products Only (Only On Cisco's Equipment Through Fast & Gigabit Ethernet Links). If You Need A Non-Proprietary VLAN Protocol, Consider Using The IEEE 802.1Q Protocol. This Article Describes How To Configure Routing Between VLANS Using ISL Encapsulation.
ISL Is A Cisco Protocol For Interconnecting Multiple Switches And Maintaining VLAN Information As Traffic Goes Between Switches. ISL Provides VLAN Capabilities While Maintaining Full Wire Speed Performance On Fast Ethernet Links In Full- Or Half-Duplex Mode. ISL Operates In A Point-To-Point Environment And Will Support Up To 1000 VLANS. You Can Define Virtually As Many Logical Networks As Are Necessary For Your Environment.
INTER-SWITCH LINK (ISL) Protocol Primarily Is Used For Ethernet Media (Fast Ethernet Or Gigabit Ethernet). Cisco Has Also Included Provisions To Carry Token Ring, FDDI, And ATM Frames Over Ethernet ISL. Inter-Switch Link (ISL) Protocol Encapsulates The Entire Ethernet Frame (Fast Ethernet Or Gigabit Ethernet) With A 26-Byte Header And A 4-Byte Frame Check Sequence (FCS) For A Total Of 30 Bytes Of Overhead.
ISL Carries Data Hop-By-Hop Over Point-To-Point Links, Neither The Fast Ethernet Nor Gigabit Ethernet Constraints Of 1,500-Byte Data Frame Size Are Applicable. Therefore, ISL Can Be Used To Transport The Far-Larger 18K-Byte Token-Ring Frames (Or Alternatively, 100-Byte Packets).
Because ISL Is Based On Ethernet Technology, Fast Ethernet Category 5 Copper Cabling Or Fiber-Optic Cabling Can Be Used As A Transport Medium, Allowing Speeds Of 100M Bit/Sec Or Even Gigabit Speeds Between Switches And To Servers.
One Drawback, ISL Will Place Both A Header And Trailer Onto The Frame, Encapsulating It. This Increases The Overhead On The Trunk Line. We Know That The Default VLAN Is Also Known As The "Native VLAN", And Another Drawback To ISL Is That ISL Does Not Use The Concept Of The Native VLAN. This Means That Every Single Frame Transmitted Across The Trunk Will Be Encapsulated.
The 26-Byte Header That Is Added To The Frame By ISL Contains The VLAN ID; The 4-Byte Trailer Contains A Cyclical Redundancy Check (CRC) Value. The CRC Is A Frame Validity Scheme That Checks The Frame's Integrity.
This Encapsulation Leads To Another Potential Issue. ISL Encapsulation Adds 30 Bytes Total To The Size Of The Frame, Potentially Making Them Too Large For The Switch To Handle. (The Maximum Size For An Ethernet Frame Is 1518 Bytes.)
IEEE 802.1q Differs Substantially From ISL. In Contrast To ISL, Dot1q Does Not Encapsulate Frames. A 4-Byte Header Is Added To The Frame, Resulting In Less Overhead Than ISL. If The Frame Is Destined For Hosts Residing In The Native VLAN, That Header Isn't Added. Since The Header Is Only 4 Bytes In Size, And Isn't Even Placed On Every Frame, Using Dot1q Lessens The Chance Of Oversized Frames. When The Remote Port Receives An Untagged Frame, The Switch Knows That These Untagged Frames Are Destined For The Native VLAN.
FRAME TAGGING IN ISL:
With ISL, An Ethernet Frame Is Encapsulated With A Header That Transports VLAN Ids Between Switches And Routers. A 26-Byte Header That Contains A 10-Bit VLAN ID Is Prepended To The Ethernet Frame.
A VLAN ID Is Added To The Frame Only When The Frame Is Destined For A Nonlocal Network.
ISL STANDARD CONFIGURATION TASK
ISL ENCAPSULATION CONFIGURATION TASK LIST :
You Can Configure Routing Between Any Number Of VLANS In Your Network. This Section Documents The Configuration Tasks For Each Protocol Supported With ISL Encapsulation. The Basic Process Is The Same, Regardless Of The Protocol Being Routed. It Involves The Following Tasks:
• Enabling The Protocol On The Router
• Enabling The Protocol On The Interface
• Defining The Encapsulation Format As ISL OR TRISL.
• Customizing The Protocol According To The Requirements For Your Environment.
To Define The Encapsulation Format, Use The Following Commands In Interface Configuration Mode :
Step 1:
Router(Config-If)# Interface Type Slot/Port.Subinterface-Number - > Specifies The Subinterface On Which ISL Will Be Used.
Step 2:
Router(Config-If)# Encapsulation ISL Vlan-Identifier - > Defines The Encapsulation Format As ISL (Isl), And Specifies The VLAN Identifier.
Defining The IP Address :
Router(Config-If)# Ip Address Ip-Address Mask [Secondary] - > Specifies The IP Address For The Subnet On Which ISL Will Be Used.
INTERSWITCH LINK (ISL) CONFIGURATION EXAMPLE
THE FOLLOWING SET OF COMMANDS ALLOWS YOU TO CONNECT AN INTERSWITCH LINK (ISL) VLAN TRUNK TO YOUR ROUTER :
Router1#Configure Terminal
Enter Configuration Commands, One Per Line. End With Cntl/Z.
Router1(Config)#Interface Fastethernet0/0
Router1(Config-If)#No Ip Address
Router1(Config-If)#Speed 100
Router1(Config-If)#Full-Duplex
Router1(Config-If)#Exit
Router1(Config)#Interface Fastethernet0/0.1
Router1(Config-Subif)#Encapsulation Isl 1
Router1(Config-Subif)#Ip Address 172.25.1.5 255.255.255.0
Router1(Config-Subif)#Exit
Router1(Config)#Interface Fastethernet0/0.2
Router1(Config-Subif)#Encapsulation ISL 2
Router1(Config-Subif)#Ip Address 172.16.2.1 255.255.255.0
Router1(Config-Subif)#Exit
Router1(Config)#Interface Fastethernet0/0.3
Router1(Config-Subif)#Encapsulation ISL 574
Router1(Config-Subif)#Ip Address 10.22.1.2 255.255.255.0
Router1(Config-Subif)#Exit
Router1(Config)#End
Router1#
Discussion :
A Trunk Is A Point-To-Point Link Containing One Ore More Virtual Lans (VLANS). The Main Purpose Of A Trunk Is To Save Physical Interfaces. Without VLANS, If You Wanted To Connect Two Lan Segments Into A Router, You Would Need Two Ethernet Interfaces.
Unfortunately, This Does Not Scale Well. It Is Relatively Common For A Switch To Support Many VLANS. So There Is A Clear Advantage To Using Trunks To Bundle The VLANs Together Into A Single Link.
While Trunks Carry Traffic For Many Different Vlans, They Are Able To Keep Them Separate By Tagging Each Frame With The Unique Identification Number For The Appropriate VLAN.
This Allows Traffic From Multiple Lan Segments To Share The Same Physical Link Without Any Danger Of Frames Leaking Onto The Wrong Segment. When A Network Device Receives A Tagged Frame From A Trunk Link, It Removes The Tag, And Then Forwards The Frame To The Appropriate Lan Segment As A Normal Frame.
When You Connect A Router To A Trunk, It Can Route Layer 3 Packets Between The Various VLANs On The Trunk. Because Of The VLAN Tagging Scheme, Layer 2 Frames Cannot Pass From One VLAN To Another. So, Without A Router Device Of Some Kind, There Is No Way To Interconnect The VLANs.
The Configuration Where A Router Is Connected To A Trunk, To Allow Routing Between The Different VLANs, Is Often Called A "Router On A Stick" Because The Router Routes Its Packets Back Out Onto The Same Physical Interface That It Received Them Through.
The First Step When Configuring A Trunk On A Router Is To Select A Physical Lan Interface To Connect The Trunk To. In General, We Don't Recommend Using Anything Slower Than A Fast Ethernet Interface For This Purpose:
Router1(Config)#Interface Fastethernet0/0
Router1(Config-If)#No Ip Address
Router1(Config-If)#Speed 100
Router1(Config-If)#Full-Duplex
As You Can See, There Is No Special Configuration Necessary On The Physical Interface.
Then You Need To Create One Subinterface On This Physical Interface For Each Different VLAN. Because Each VLAN Represents A Different Layer 3 Network, You Need To Give Each Of The Subinterfaces Ip Addresses From The Corresponding Ip Subnets:
Router1(Config)#Interface Fastethernet0/0.1
Router1(Config-Subif)#Encapsulation ISL 1
Router1(Config-Subif)#Ip Address 172.25.1.5 255.255.255.0
The Encapsulation Command Associates This Subinterface With A Particular ISL VLAN Number. ISL VLAN Numbers Can Have Any Value Between 1 And 1,000. With This Subinterface Configured, The Router Is Now Able To Route Packets For Any Devices On This VLAN, Exactly As If It Were Directly Connected To The Physical Lan Segment.
The Show VLANS Command Displays Information About All Of The VLANs Configured On The Router :
Router1#Show Vlans
Virtual Lan Id: 1 (Inter Switch Link Encapsulation)
Vlan Trunk Interface: Fastethernet0/0.1
Protocols Configured: Address: Received: Transmitted:
Ip 172.25.1.5 203626 342261
Virtual Lan Id: 2 (Inter Switch Link Encapsulation)
Vlan Trunk Interface: Fastethernet0/0.2
Protocols Configured: Address: Received: Transmitted:
Ip 172.16.2.1 0 153807
Virtual Lan Id: 574 (Inter Switch Link Encapsulation)
Vlan Trunk Interface: Fastethernet0/0.3
Protocols Configured: Address: Received: Transmitted:
Ip 10.22.1.2 0 6
Router1#
We Have Configured This Router To Support Three Different VLANS, Each With Its Own Subinterface And Its Own Ip Address. The Subinterface Number Does Not Necessarily Need To Correspond To The VLAN ID, As We Have Assigned VLAN Number 574 To Subinterface Fastethernet0/0.3. But If You Make It A General Rule To Always Keep The Subinterface Number The Same As The VLAN Number, It Will Make Maintenance And Troubleshooting Considerably Simpler In A Large Network.
It Is Useful To Remember That You Don't Need To Create A Distinct Subinterface For Every VLAN On The Switch. There May Be Some VLANS On This Switch That You Don't Wish To Terminate On The Router. In This Case, The Router Will Simply Ignore Any Frames That Are Tagged With VLAN Numbers That It Doesn't Support.
You Can Use The Show Interfaces Command To See Information About The Trunking Configuration Of A Particular Subinterface:
Router1#Show Interfaces Fastethernet0/0.3
Fastethernet0/0.3 Is Up, Line Protocol Is Up
Hardware Is Amdfe, Address Is 0001.9670.B780 (Bia 0001.9670.B780)
Internet Address Is 10.22.1.2/24
Mtu 1500 Bytes, Bw 100000 Kbit, Dly 100 Usec,
Reliability 255/255, Txload 1/255, Rxload 1/255
Encapsulation Isl Virtual Lan, Color 574.
Arp Type: Arpa, Arp Timeout 04:00:00
Router1#
This Shows The Encapsulation Type (ISL) And The VLAN Number (574), Along With The Interface's Ip Address Information.
Cisco Added Support For Ip Unnumbered Addressing Of VLAN Subinterfaces In Version 12.2(4)T:
Router1(Config)#Interface Fastethernet0/0.1
Router1(Config-If)#Ip Unnumbered Loopback0
CONCLUSION:
The Goal Of This Article Is To Give An Easy Way To Understand The “CISCO - VLAN TRUNKS WITH INTERSWITCH LINK (ISL). 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+.
DISCLAIMER:
This Document Carries No Explicit Or Implied Warranty. Nor Is There Any Guarantee That The Information Contained In This Document Is Accurate. Every Effort Has Been Made To Make All Articles As Complete And As Accurate As Possible.
It Is Offered In The Hopes Of Helping Others, But You Use It At Your Own Risk. The Author Will Not Be Liable For Any Special, Incidental, Consequential Or Indirect Any Damages Due To Loss Of Data Or Any Other Reason That Occur As A Result Of Using This Document. But No Warranty Or Fitness Is Implied. The Information Provided Is On An "As Is" Basic. All Use Is Completely At Your Own Risk.
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