CISCO – UNDERSTANDING SPLIT HORIZON CONFIGURATION:

BEFORE WE GO TO SPLIT HORIZON FIRST WE'LL SEE THE RECAP OF DISTANCE-VECTOR ROUTING PROTOCOL:

In Computer Communication Theory Relating To Packet-Switched Networks, A Distance-Vector Routing Protocol Is One Of The Two Major Classes Of Routing Protocols, The Other Major Class Being The Link-State Protocol. Distance-Vector Routing Protocols Use The Bellman-Ford Algorithm, Ford–Fulkerson Algorithm, Or DUAL FSM (In The Case Of Cisco Systems's Protocols) To Calculate Paths.

A Distance-Vector Routing Protocol Requires That A Router Informs Its Neighbors Of Topology Changes Periodically. Compared To Link-State Protocols, Which Require A Router To Inform All The Nodes In A Network Of Topology Changes, Distance-Vector Routing Protocols Have Less Computational Complexity And Message Overhead.

The Term Distance Vector Refers To The Fact That The Protocol Manipulates Vectors (Arrays) Of Distances To Other Nodes In The Network.

DISTANCE VECTOR CONCEPTS:

Advertise Entire Routing Table (Subnet Number And Metric) To Directly Connected Neighbors.

Key Points:

Updates Are Sent To All Neighboring Routers Through Active Interfaces, Once Every So Many Seconds.

If A Router Learns Multiple Routes To The Same Subnet, It Chooses The Best Route Based On The Metric (Number Of Hops).

Failure To Receive Updates After X Amount Of Time Results In Removal Of That Route .

Routers Add Directly Connected Subnets To Their Routing Tables, Even Without A Routing Protocol .

ROUTERS USING DISTANCE VECTOR PROTOCOL DO NOT HAVE KNOWLEDGE OF THE ENTIRE PATH TO A DESTINATION. INSTEAD DV USES TWO METHODS :

1. Direction In Which Or Interface To Which A Packet Should Be Forwarded.
2. Distance From Its Destination.

EXAMPLES OF: Distance-Vector Routing Protocols Include Ripv1 And Ripv2 And IGRP. EGP And BGP Are Not Pure Distance-Vector Routing Protocols Because A Distance-Vector Protocol Calculates Routes Based Only On Link Costs Whereas In BGP, For Example, The Local Route Preference Value Takes Priority Over The Link Cost.

TO CALCULATE THE BEST PATH :

The Methods Used To Calculate The Best Path For A Network Are Different Between Different Routing Protocols But The Fundamental Features Of Distance-Vector Algorithms Are The Same Across All DV Based Protocols.

Distance Vector Means That Routers Are Advertised As Vector Of Distance And Direction. Direction Is Simply Next Hop Address And Exit Interface And Distance Means Hop Count.

ROUTERS USING DISTANCE VECTOR PROTOCOL DO NOT HAVE KNOWLEDGE OF THE ENTIRE PATH TO A DESTINATION. INSTEAD DV USES TWO METHODS:

1.Direction In Which Router Or Exit Interface A Packet Should Be Forwarded.
2.Distance From Its Destination.

As The Name Suggests The DV Protocol Is Based On Calculating The Direction And Distance To Any Link In A Network. The Cost Of Reaching A Destination Is Calculated Using Various Route Metrics. RIP Uses The Hop Count Of The Destination Whereas IGRP Takes Into Account Other Information Such As Node Delay And Available Bandwidth.

Updates Are Performed Periodically In A Distance-Vector Protocol Where All Or Part Of A Router's Routing Table Is Sent To All Its Neighbors That Are Configured To Use The Same Distance-Vector Routing Protocol. RIP Supports Cross-Platform Distance Vector Routing Whereas IGRP Is A Cisco Systems Proprietary Distance Vector Routing Protocol. Once A Router Has This Information It Is Able To Amend Its Own Routing Table To Reflect The Changes And Then Inform Its Neighbors Of The Changes. This Process Has Been Described As ‘Routing By Rumor’ Because Routers Are Relying On The Information They Receive From Other Routers And Cannot Determine If The Information Is Actually Valid And True. There Are A Number Of Features Which Can Be Used To Help With Instability And Inaccurate Routing Information.

THE DBF (DISTRIBUTED BELLMAN-FORD PROTOCOL) :

The Distributed Bellman-Ford Protocol Is A Proactive Table-Driven Protocol On Basis Of The Bellman-Ford Algorithm. That Means That Every Node Maintains A Routing Table. There Are Three Different Steps For Catching Every Needed Entry. .

START CONDITIONS :

Each Router Starts With A Vector Of Distances To All Directly Attached Networks. Each Node Maintains A Routing Table Of Destination, Distance, and Successor.

SENDING STEP :

Every Node Sends Path Vector Tuples (Destination, Distance) To All Immediate Neighbors (No Broadcast). These Updates Are Send Periodically Every Second Or Minute. This Depends On The Size And The Dynamic Of The Network. Triggered Or Immediate Updates Are Sending Out Whenever Destination Vectors, Entries In The Routing Table Change.

RECEIVING STEP:

For Every Network Y, Router Finds Shortest Distance To X Considering Current Distance To X And It Takes Into Account The Distance To X From Its Neighbors. Router Updates Its Cost To X. After Doing This For All X Destination Routers (Nodes), The Router Goes To Send Step.

EXAMPLE :

Illustration 1 Shows The Adjacent Matrix From Our Thought Network After The Start Procedure Has Done. The Numbers Appearing On The Links Between Two Routers Are Called Link Costs. The Costs Of Links Can Signify The Hop Count, Bandwidth Or Even The Really Cost If There Are Two Different Operators Connected On One Link.

DV (DISTANCE-VECTOR) ALGORITHMS :

DV Algorithms Are Also Known As Bellman-Ford Routing Algorithms And Ford-Fulkerson Routing Algorithms. In These Algorithms, Every Router Has A Routing Table That Shows It The Best Route For Any Destination.

IN DV ALGORITHMS, EACH ROUTER HAS TO FOLLOW THESE STEPS:

1. It Counts The Weight Of The Links Directly Connected To It And Saves The Information To Its Table.
2. In A Specific Period Of Time, It Send Its Table To Its Neighbor Routers (Not To All Routers) And Receive The Routing Table Of Each Of Its Neighbors.
3. Based On The Information In Its Neighbors' Routing Tables, It Updates Its Own.

COUNT TO INFINITY :

NOTE : One Of The Most Important Problems With DV Algorithms Is Called "Count To Infinity”.(The “Counting To Infinity” Problem Is One Of The Most Serious Issues With The Basic RIP Algorithm). Counting To Infinity Is Just Another Name For A Routing Loop. In Distance Vector Routing, Routing Loops Usually Occur When An Interface Goes Down, Or When Two Routers Send Updates To Each Other At The Same Time.

EXAMPLE :

Router A----------Router B----------Router C----------Router D

Just Imagine That The Link Between A And B Is Cut. At This Time, B Corrects Its Table. After A Specific Amount Of Time, Routers Exchange Their Tables, And So B Receives C's Routing Table. Since C Doesn't Know What Has Happened To The Link Between A And B, It Says That It Has A Link To A With The Weight Of 2 (1 For C To B, And 1 For B To A -- It Doesn't Know B Has No Link To A). B Receives This Table And Thinks There Is A Separate Link Between C And A, So It Corrects Its Table And Changes Infinity To 3 (1 For B To C, And 2 For C To A, As C Said). Once Again, Routers Exchange Their Tables. When C Receives B's Routing Table, It Sees That B Has Changed The Weight Of Its Link To A From 1 To 3, So C Updates Its Table And Changes The Weight Of The Link To A To 4 (1 For C To B, And 3 For B To A, As B Said).

This Process Loops Until All Nodes Find Out That The Weight Of Link To A Is Infinity. This Situation Is Shown In The Table Below. In This Way, Experts Say DV Algorithms Have A Slow Convergence Rate. One Way To Solve This Problem Is For Routers To Send Information Only To The Neighbors That Are Not Exclusive Links To The Destination. For Example, In This Case, C Shouldn't Send Any Information To B About A, Because B Is The Only Way To A.

(Simple Example For Counting-To-Infinity: Just Imagine That you’re At Home During A Holiday Weekend And You've Got Your Father, Mother, And A Brother. So Let's Say You're Hungry. Imagine Yourself A Routed Protocol Packet. So You Go To Your Brother And Ask "When Are We Eating?" Well, Your Brother Says "Go Ask Mother." So You Move Yourself From Your Brother To Your Mother. Then You Ask Your Mother "When Are We Eating?" Your Mother Says, "Ask Your Father." So You Move Yourself From Mother To Father. Then You Ask "When Are We Eating?" Your Father Says, "Ask Your Brother." So You Move Yourself From Father To Brother. Then You Ask Your Brother Again, "When Are We Eating?" Your Brother Says The Same Thing He Said Before "Ask Your Mother." Then You Go To Your Mother, Ask The Question And You Get The Same Reply "Ask Your Father." Then You Go To Your Father, Ask The Question And You Get The Same Reply "Ask Your Brother." You Are Now In What They Call In Networking A "Routing Loop.

The Reason Why It's Called "Count To Infinity" Is Because The Counting Starts When You Went To Your Brother The First Time. Then The Count Goes To 2 When You Went To Your Mother. The Count Went To 3 When You Went To Your Father. The Count Went To 4 When You Went To Your Brother Again. Theoretically, You Could Remain In The Routing Loop Until Infinity Which Is A Looping Time).

Counting-To-Infinity Can Occur When One Router Has A Valid Metric That Points To An Address That Is Reachable Through An Intermediate Router While The Intermediate Router Has An Infinite-Distance Route To The Same Address.

The Bellman-Ford Algorithm Does Not Prevent Routing Loops From Happening And Suffers From The Count-To-Infinity Problem.

The Core Of The Count-To-Infinity Problem Is That If A Tells B That It Has A Path Somewhere, There Is No Way For B To Know If The Path Has B As A Part Of It.

If Routing Table Updates Are Sent By Both Routers At The Same In Time, The Intermediate Router Will Advertise That The Route To The Destination Address Is An Infinite-Distance Route While The Other Router Will Advertise That The Route Has A Valid Metric.

Because The Two Routers Use The Same Update Interval Between Updates, This Process Repeats Itself With The Next Routing Update, With The Difference That The Valid Metric Will Be Incremented By 1 Each Time Until An Infinite Metric Is Reached, Hence This Phenomenon Is Called Counting To Infinity.

NOTE: ROUTE POISONING DOES NOT SOLVE THE COUNTING-TO-INFINITY PROBLEM:

Route Poisoning Does Not Solve The Counting-To-Infinity Problem. Also Know: Route Poisoning Is A Feature That Distance Vector Protocols Use To Reduce The Chance Of Routing Loops. Route Poisoning Begins When A Router Notices That A Connected Route Is No Longer Valid. The Router Then Advertises That Route Out All Its Interfaces And With A Very Large Metric So That Other Routers Consider The Metric Infinite And The Route Invalid. However, Route Poisoning Does Not Solve The Counting-To-Infinity Problem.

COUNT -TO-INFINITY PROBLEM (SPLIT HORIZON SOLVES THE COUNTING-TO-INFINITY PROBLEM) :

Split Horizon Solves The Counting-To-Infinity Problem By Preventing A Router From Sending Routing Updates Out The Same Interface On Which It Learnt The Route. The Router Would Have Learnt The Route To The Destination Address Across The Link From The Intermediate Router. With Split Horizon, That Router Cannot Then Send Advertisements About The Route To The Destination Address Out Across The Same Link. Therefore The Intermediate Router Will Not Receive The Valid Metric From The Route To The Destination Address From The Other Router Ad The Count To Infinity Problem Will Not Occur, Solving The Count-To-Infinity Problem On A Single Link.

Because Of The “Count-To Infinity Problem” Arises Routing Update Messages Propagates Slowly Across The Network. This Problem Causes Inconsistencies In The Routing Tables Of Different Routers. “This Problem Is Also Called As Slow Convergence Problem”.

SOLUTION TO SLOW CONVERGENCE PROBLEM THAT ARE :

Some Of These Solutions Techniques Are :

Hold-Downs,

Split Horizons,

Poison Reverse Updates .

And Triggered Updates.

Now We Go For THE SPLIT HORIZON CONFIGURATION TECHNIQUE.

RFC 1058 Introduces Also Another Method For Routing Information Protocol: The Use Of Split Horizon. Split Horizon Means That If Node A Has Learned A Route To Node C Through Node B, Then Node A Does Not Send The Distance Vector Of C To Node B During A Routing Update.

Split Horizon Helps Reduce Convergence Time By Not Allowing Routers To Advertise Networks In The Direction From Which Those Networks Were Learned. The Split Horizon Rule States -Never Advertise A Route Out Of The Interface Through Which You Learned It. It Is Never Useful To Send Information About A Route Back In The Direction From Which It Came And Thus Split Horizons Is Used To Prevent Updates That Are Redundant To The Network. For This Purpose Router Records The Interface Over Which It Received A Particular Route.

LET’S SEE THE FOLLOWING EXAMPLE FIRST :

Distance-Vector Routing Protocol Like RIPv1 & RIPv2 Have Slow Convergence And Count-To-Infinity Issues. We Assume The Network Is:

Router A--------------Router B--------------Router C

A, B And C Are Three Routers Running Rip Routing Protocol. In The Convergence Status, B Knows There Is One Hop To C And A Knows There Are Two Hops To C Via B.

If The Link Is Broken Between B And C, Since B Get Advertisement From A That There Are 2 Hops Can Get To C From A. Then, B Updates Its Routing Table To Set Hop To 3.

A Get Advertisement From B. Then A Updates Its Routing Table To Set Hop To 4, So On And So On Until Count To Infinity. The Network Will Never Be Convergence Status.

WHY SPLIT HORIZON IS USEFUL IN THIS SITUATION: Following The Definition, Router A Will Not Send Advertisement To Router B Since A Learned Route To Router C From Router B. Its Reduces The COUNT-TO-INFINITY PROBLEM.

To Speed Up Convergent In RIP Network, It Introduce The Maximum Hop Number Is 15.

In The Case Above, When The Link Between Router B And Router C Is Broken, Router B Sends Advertisement To Router A That The Hop To Go To Router C Is 16, Which Is Also Called Split Horizon With Poison Reverse. Then, A Knows That C Is Unreachable And Updates Its Routing Table.

OVERVIEW OF SPLIT HORIZON :

SPLIT-HORIZON Deals With The "Routes" That Are Sent And Received Not The Hello Or Update Messages That Must Take Place Between All Routers. In Short, SPLIT HORIZON Is To Prohibit A Router From Advertising A Route Back Out The Interface From Which It Was Learned.

SPLIT HORIZON Is A Method Of Preventing A Routing Loop In A Network. The Basic Principle Is Simple: Information About The Routing For A Particular Packet Is Never Sent Back In The Direction From Which It Was Received.

SPLIT HORIZON Tries To Solve The “Counting To Infinity” Problem By Suppressing The Transmission Of Invalid Information About Routes That Fail.

INTRODUCTION :

In Computer Networking, SPLIT-HORIZON Route Advertisement Is A Method Of Preventing Routing Loops In Distance-Vector Routing Protocols By Prohibiting A Router From Advertising A Route Back Onto The Interface From Which It Was Learned.

NOTE:Split Horizon States That If A Neighboring Router Sends A Route To “A” Router, The Receiving Router Will Not Propagate This Route Back To The Advertising Router On The Same Interface.

With ROUTE POISONING, When A Router Detects That One Of Its Connected Routes Has Failed, The Router Will Poison The Route By Assigning An Infinite Metric To It And Advertising It To Neighbors. When A Router Advertises A Poisoned Route To Its Neighbors, Its Neighbors Break The Rule Of Split Horizon And Send Back To The Originator The Same Poisoned Route, Called A “Poison Reverse”.

In Order To Give The Router Enough Time To Propagate The Poisoned Route And To Ensure That No Routing Loops Occur While Propagation Occurs, The Routers Implement A Hold-Down Mechanism.

NOTE:Split Horizon Comes In Two Variations In The First Variation A Router Records The Interface Over Which It Received A Particular Route And Does Not Propagate The Information about That Route Back Over The Same Interface.Is A scheme For Avoiding Problems Caused by Including Routes in Updates Sent to the Router from Which They Were Learned.

In This Example: Network Node A Routes Packets To Node B Router In Order To Reach Node C Router. The Links Between The Nodes Are Distinct Point-To-Point Links.


According To The Split-Horizon Rule, Never Advertise A Route Out Of The Interface Through Which You Learned It). Node A Does Not Advertise Its Route For C (Namely A To B To C) Back To B. On The Surface, This Seems Redundant Since B Will Never Route Via Node A Because The Route Costs More Than The Direct Route From B To C. However, If The Link Between B And C Goes Down, And B Had Received A Route From A, B Could End Up Using That Route Via A. A Would Send The Packet Right Back To B, Creating A Loop.

With The Split-Horizon Rule In Place, This Particular Loop Scenario Cannot Happen, Improving Convergence Time In Complex, Highly-Redundant Environments.

Split Horizon Solves The Counting-To-Infinity Problem By Preventing A Router From Sending Routing Updates Out The Same Interface On Which It Learnt The Route. Thus, The Router Would Have Learn The Route To The Destination Address Across The Link From The Intermediate Router. With Split Horizon, That Router Cannot Then Send Advertisements About The Route To The Destination Address Out Across The Same Link. Therefore The Intermediate Router Will Not Receive The Valid Metric From The Route To The Destination Address From The Other Router Ad The Count To Infinity Problem Will Not Occur, Solving The Count-To-Infinity Problem On A Single Link.

Split Horizon Is Enabled By Default And Is A Rule That Says, "An Interface Will Not Advertise Route Information Back Out An Interface From Where It Originated." Split Horizon Is Used When Links Are Up To Prevent Routers Adjacent To Each Other From Having Routes Back And Forth To Each Other For A Network(S).

Split-Horizon: In A Single Path Network Split Horizon Will Stop The Countdown-To-Infinity By Not Advertising Routes Back Out The Interface On Which It Learned Them.

It Is Never Useful To Send Information About A Route Back In The Direction From Which It Came And Thus Split Horizons Is Used To Prevent Updates That Are Redundant To The Network. For This Purpose Router Records The Interface Over Which It Received A Particular Route And Does Not Propagates Its Information About That Route Back To The Same Interface. This Change Is Known As Split Horizon Technique.

The Split Horizon Rule States That A Router Should Not Advertise A Route Out Of An Interface Where The Route Was Learned.

For Example, When RIP Is The Designated Routing Protocol On The Network, The Split Horizon Rule Means That When The Router Sends A RIP Update Out A Particular Network Interface, It Should Never Include Routing Information Acquired For That Network Over That Same Interface. Sending Out A Route Update On An Interface Where The Same Update Was Learned Can Potentially Cause Routing Loops In A Network, Creating A Problem Commonly Known As The "Count To Infinity."

Split Horizon Helps To Prevent Instabilities In The Network By Suppressing The Propagation Of Bad Routing Information. Poison Reverse Is Another Technique Used To Prevent Route Instabilities In A Dynamic Routing Protocol.

With Poison Reverse, The Router Advertises A Route Update As Unreachable On An Interface Where The Same Route Update Was Learned.

SPLIT HORIZON WITH POISON REVERSE

This Is Yet Another Technique Used To Solve The “SLOW CONVERGENCE PROBLEM”. Larger Routing Loops Prevented Using Poison Reverse Updates. Once A Connection Disappears, The Router Advertising The Connection Retains The Entry For Several Update Periods, And Include An Infinite Cost In The Broadcast. The Updates Are Sent To Remove Downed Route And Place It In Hold-Down.

POISON REVERSE, OTHER SOLUTION TO SLOW CONVERGENCE PROBLEM :

To Make Poison Reverse More Efficient, It Must Be Combined With Triggered Updates. Triggered Updates Force A Router To Send An Immediate Broadcast When Receiving Bad News, Instead Of Waiting For The Next Periodic Broadcast. By Sending An Update Immediately, A Router Minimizes The Time It Is Vulnerable To Believing In Good News.

SPLIT-HORIZON ROUTING WITH POISON REVERSE ISOnce A Connection Disappears, The Router Advertising The Connection Retains The Entry For Several Update Periods, And Include An Infinite Cost In The Broadcast. The Updates Are Sent To Remove Downed Route And Place It In Hold-Down. This Sending Of Update Immediately Is Known As Poison Reverse.

Split Horizon With Poison Reverse Advertises Learned Routes As Unreachable In The Direction From Which They Are Learned.

Poison Reverse Is Another Way Of Avoiding Routing Loops. Its Rule States: Once You Learn Of A Route Through An Interface, Advertise It As Unreachable Back Through That Same Interface.

To Make Poison Reverse More Efficient, It Must Be Combined With Triggered Updates (Triggered Updates Allow A RIP Router To Announce Changes In Metric Values Almost Immediately Rather Than Waiting For The Next Periodic Announcement. The Trigger Is A Change To A Metric In An Entry In The Routing Table).

Triggered Updates Force A Router To Send An Immediate Broadcast When Receiving Bad News, Instead Of Waiting For The Next Periodic Broadcast. By Sending An Update Immediately, A Router Minimizes The Time It Is Vulnerable To Believing In Good News.

Split Horizon With Poison Reverse, Or Simply Poison Reverse Combines The Two Features. When A Route Fails The Router Uses Route Poisoning, I.E., The Router Advertises An Infinite-Metric Route About That Subnet Out All Interfaces, Including Interfaces Previously Prevented By Split Horizon. This Ensures That All Routers Know For Sure That The Route Has Failed, While Split Horizon Prevents Counting To Infinity.

A Variant Of Split-Horizon Route Advertising In Which A Router Actively Advertises Routes As Unreachable Over The Interface Over Which They Were Learned. The Effect Of Such An Announcement Is To Immediately Remove Most Looping Routes Before They Can Propagate Through The Network.

Split Horizon With Poisoned Reverse Includes Such Routes In Updates, But Sets The Corresponding Distance To Infinity If The Destination Is Routed On The Link. This Immediately Kills Two-Hop Loops. However, Poisoned Reverse Does Have The Disadvantage In That It Increases The Size Of The Routing Messages.

Considering A Large Network With Routers Connected To A Backbone. In This Case If Split Horizon With Poison Reverse Is Used, The Router Must Mention All Routes That It Learns From The Backbone, With Metrics Of 16. This Can Result In Large Update Messages, Almost All Of Whose Entries Indicate Unreachable Networks. In A Static Network, Advertising Reverse Routes With A Metric Of 16 Provides No Useful Additional Information.

If There Are Many Routers On One Broadcast Network, These Extra Entries Can Use Significant Bandwidth. The Reason They Are There Is To Improve Dynamic Behavior. When Topology Changes, Mentioning Routes That Should Not Go Through The Router As Well As Those That Should Can will Speed Up Convergence.

Finally We Say Split Horizon With Poison Reverse Differs From Simple Split Horizon .Because It Announces All Networks. However, Those Networks Learned In A Given Direction Are Announced With A Hop Count Of 16, Indicating That The Network Is Unreachable.

In A Single-Path Internetwork, Split Horizon With Poison Reverse Has No Benefit Beyond Split Horizon. However,

In A Multi path Internetwork, Split Horizon With Poison Reverse Greatly Reduces Count-To-Infinity And Routing Loops. Count-To-Infinity Can Still Occur In A Multi path Internetwork Because Routes To Networks Can Be Learned From Multiple Sources.

For Example :Split Horizon With Poison Reverse Advertises Learned Routes As Unreachable In The Direction From Which They Are Learned. Split Horizon With Poison Reverse Does Have The Disadvantage Of Additional RIP Message Overhead Because All Networks Are Advertised.

HOWEVER, SPLIT HORIZON WITH POISON REVERSE DOES HAVE DISADVANTAGES :

First, It Increases The Size Of The Routing Messages. In A Hub-Spoke Network, Hub Is As Backbone Router And Each Spoke Is As Gateway Router. “If Split Horizon With Poisoned Reverse Is Used, The Gateway Must Mention All Routes That It Learns From The Backbone, With Metrics Of 16 (In RIP). If The System Is Large, This Can Result In A Large Update Message, Almost All Of Whose Entries Indicate Unreachable Networks.”

Second, It Will Prevent Any Routing Loops That Involve Only Two Gateways Engaged In Mutual Deception. It Is Highly Possible That Three Or More Gateways In This Situation. So, Rfc 1058 Introduces “Triggered Updates” Approach. In Short, It Is Required To Send Update Messages Almost Immediately Whenever A Gateway Changes The Metric For A Route. Split Horizon Processing Is Done When Generating Triggered Updates As Well As Normal Updates.

Since This Is Nature Born Characters Of Distance-Vector Routing Protocol, More Sophisticated Routing Protocols Are Applied In The Industry Such As OSPF.

The Main Disadvantage Of Poison Reverse Is That It Can Significantly Increase The Size Of Routing Announcements In Certain Fairly Common Network Topologies

Split Horizon Eliminates Count-To-Infinity And Routing Loops During Convergence In Single-Path Internet works And Reduces The Chances Of Count-To-Infinity In Multi-Path Internet works.

THE ADVANTAGE AND DISADVANTAGE OF SPLIT HORIZON :

THE MAIN ADVANTAGE:

This Split Horizon Technique Is That A Router Won't Be Confused By Receiving Spurious Information About A Route Being Accessible When It Was Just Recently Told That The Route Was No Longer Valid. It Provides A Period Of Time For Out-Of-Date Information To Be Flushed From The System, Which Is Valuable Especially On Complex Internet works. The Addition Of Hold-Down To Split Horizon Can Also Help In Situations Where Split Horizon Alone Is Not Sufficient To Prevent Counting To Infinity.

SPLIT HORIZON HELPS REDUCE CONVERGENCE TIME : By Not Allowing Routers To Advertise Networks In The Direction From Which Those Networks Were Learned. The Only Information Sent In RIP Announcements Are For Those Networks That Are Beyond The Neighboring Router In The Opposite Direction. Networks Learned From The Neighboring Router Are Not Included.

THE DISADVANTAGES:

Split Horizon Is Useful In Preventing Routing Loops, But It Can Cause Problems On Hub-And-Spoke Frame Relay Networks. On The Hub Router, Multiple Frame Relay Virtual Circuits Are Usually Multiplexed And Terminated Onto One Physical Interface.

Under these conditions, the router is completely unaware that multiple virtual connections exist on the same physical interface. As such, the split horizon rule applies, and routes learned on one virtual circuit are never advertised to other virtual circuits on the same physical interface, even though they are transmitted on different virtual circuits



The Three Routers Are Now Configured To Exchange Route Information Via A Distance Vector Routing Protocol Such As RIP. Router A And Router B Routers Can Exchange Routing Updates Directly With Router C And Vice-Versa. However, Because Of The Split Horizon Issue At Router C, Router C Is Unable To Forward Routing Information Learned From Router A To Router B Or From B To A. Although Router C Is Logically Connected To The Two Remote Locations On Separate Virtual Circuits, Both Virtual Circuits Are Logically Multiplexed On The Same Physical Interface. The Split Horizon Rule Simply Forward Router C from Sending Route Information Out Of An Interface If The Same Route Information Was Learned From The Same Physical Interface.

A Workaround To This Problem Is To Have A Fully Meshed Topology By Adding A Virtual Circuit Directly Between Router B And Router A. In This Way, B And A Routers Can Exchange Route Information Directly With Each Other. However, A Fully Meshed Topology Increases The Operating Costs Of The Network. An Alternative Solution Is To Add A Separate Physical Interface On Vulture Router So That Each Remote Connection Is Terminated At The Hub Location On Different Hardware. This Is Not Viable Because Of The Added Hardware Cost. Moreover, This Solution Would Require A Different IP Subnet Address To Be Used For Each Virtual Connection. Another Solution Would Be To Use Advanced Dynamic Link-State Routing Protocols, Which Understand The NBMA Nature Of The Frame Relay Network. However, Advanced Link-State Routing Protocols, Such As Open Shortest Path First (OSPF), Place Greater Demands On The CPU And Memory Resources Of The Routers.

On Cisco Routers, Split Horizon Is Disabled By Default For Frame Relay So That Routing Updates Can Come In And Out Of The Same Interface. However, On Partially Meshed Frame Relay Networks, Some Protocols, Such As IPX, Appletalk, And Transparent Bridging, Require Split Horizon In Order To Work Properly.

USING SUB INTERFACES ON CISCO ROUTERS: Cisco Routers Support The Configuration Of Logical Sub Interfaces On A Physical Interface. Configuring Sub Interfaces Allows A Single Physical Interface To Be Treated As Multiple Virtual Interfaces. This Allows The Split Horizon Issues To Be Overcome. Packets Received On One Sub Interface Can Be Forwarded Out another Sub interface, Even Though They Are All Configured On The Same Physical Interface.

TWO DIFFERENT IMPLEMENTATIONS OF SUB INTERFACE TYPES ARE SUPPORTED BY CISCO ROUTERS: Point-To-Point And Multipoint Sub Interfaces. A Sub Interface Is A Logical Software Interface Managed Internally By The Router.

NOTE That A Sub Interface Uses Up Memory On The Router. The Number Of Sub Interfaces That Can Be Configured Largely Depends On The Amount Of Memory On The Router.

POINT-TO-POINT SUB INTERFACES :Point-To-Point Sub Interfaces Allow The Physical Frame Relay Interface To Be Partitioned Into A Number Of Virtual Point-To-Point Sub Networks. Each Point-To-Point Sub Network Can Be Assigned Its Own Network Number. To The Routed Protocol, Each Sub Interface Appears As If It Is Located On A Separate Interface. Routing Updates Received From One Logical Point-To-Point Sub Interface Can Be Forwarded Out To Another Logical Point-To-Point Sub Interface That Is Configured Under The Same Physical Interface Without Violating The Rule Of Split Horizon. On Partially Meshed Frame Relay Networks, A Point-To-Point Sub Interface Solves The Problem Introduced By Split Horizon.

MULTIPOINT SUB INTERFACES : The Second Implementation Of Frame Relay Sub Interfaces Is The Multipoint Sub Interface. A Multipoint Sub Interface Is Similar To The Physical Interface. On Cisco Routers, All Serial Interfaces Are Multipoint Interfaces By Default, And Multipoint Sub Interfaces Behave Exactly Like Physical Interfaces. Both Physical And Multipoint Sub Interfaces Are Subjected To The Rule Of Split Horizon. Compared With Point-To-Point Sub Interfaces Where Each Point-To-Point Connection Represents A Different Subnet, Multipoint Sub Interfaces Keep All Remote Sites On A Single Network. All Nodes Connected To A Multipoint Sub Interface Belong To The Same Subnet.

One Major Difference Between Point-To-Point Sub Interface And Multipoint Sub Interface Is That On A Point-To-Point Sub Interface, Only One DLCI Can Be Assigned To The Sub Interface. On A Multipoint Sub Interface, Multiple DLCI’s Can Be Assigned.

Consider An Example Of Multipoint Sub interfaces The Vulture Router Has A Multipoint Sub interface Configured Under Its Physical Interface. Three Virtual Circuits Are Terminated At The Multipoint Sub interface From Three Different Remote Locations. As Such, The Multipoint Interface Has Three Dlcis Assigned To Uniquely Identify The Virtual Connection Belonging To Each Location. All Nodes Are Placed On The Same Subnet Address. If Point-To-Point Sub interfaces Were Used, Each Point-To-Point Connection Would Require A Separate Network Layer Address.

On Cisco Devices, Split Horizon Is Enabled Or Disabled By Default, Depending On The Interface Type. Summarizes The Default Split Horizon Behavior On Cisco Frame Relay Interfaces.

IMPLEMENTATIONS

The Split-Horizon Method Is Effective And Simple To Implement, And Is Therefore Used By Most Distance-Vector Protocols. It Is Notably Used By:

RIP

IGRP

EIGRP

VPLS Uses The Split Horizon Technique To Avoid Loops In The Forwarding Plane.

Babel Applies Split-Horizon To Wired Links Only.

Split Horizon Exists For A Very Good Reason - Routing Loop Prevention. The Rule Of Split Horizon States That A Router Cannot Send An Advertisement For A Route Out The Same Interface That It Came In On. Split Horizon Is On By Default On All Interfaces Running RIP, IGRP, And EIGRP.

SPLIT HORIZON IN RIP :

RIP For IP, Like Most Distance Vector Routing Protocols, Announces Its Routes In An Unsynchronized And Unacknowledged Manner. This Can Lead To Convergence Problems. However, You Can Enable Modifications To The Announcement Algorithms To Reduce Convergence Time In Most Situations.

You Can Use Split Horizon To Ensure That Rip Never Advertises A Route Out Of The Interface Where It Was Learned. The “Counting To Infinity” Problem Is One Of The Most Serious Issues With The Basic RIP Algorithm.

COUNT-TO-INFINITY PROBLEM IN RIP :

The Classic Distance Vector Convergence Problem Is Known As The Count-To-Infinity Problem And Is A Direct Result Of The Asynchronous Announcement Scheme. When RIP For IP Routers Add Routes To Their Routing Table, Based On Routes Advertised By Other Routers, They Keep Only The Best Route In The Routing Table And They Update A Lower Cost Route With A Higher Cost Route Only If Is Being Announced By The Same Source As The Current Lower Cost Route.

Split Horizon Is A Method That Controls The Sending Of Rip Update And Query Packets. When You Enable Split Horizon On An Interface, Cisco Nx-Os Does Not Send Update Packets For Destinations That Were Learned From This Interface. Controlling Update Packets In This Manner Reduces The Possibility Of Routing Loops.

You Can Use Split Horizon With Poison Revers To Configure An Interface To Advertise Routes Learned By Rip As Unreachable Over The Interface That Learned The Routes.

RIP WITH SPLIT HORIZON POISON REVERSE

Router C Learns About Route X And Advertises That Route To Router B. Router B In Turn Advertises Route X To Router A, But Sends A Route X Unreachable Update Back To Router C.

By Default, Split Horizon Is Enabled On All Interfaces.

The Rule Of Split Horizon States That A Router Cannot Send An Advertisement For A Route Out The Same Interface That It Came In On. Split Horizon Is On By Default On All Interfaces Running RIP, IGRP, And EIGRP.

RIP - SPLIT HORIZON EXAMPLE :

SPLIT HORIZON Is A Technique Used To Avoid RIP Routing Loops. When Split Horizon Is Enabled On A Specific Interface, The Router Does Not Re-Advertise RIP Routes Received On That Interface.

Now, Let’s Analyze The Following Network :We Assume That RIP Auto Summarization Is Disabled On The Network.

R2 And R3 Will Send Routing Information About The Subnets 20.20.20.0./24 And 140.1.3.0/24, Respectively To R1. R1 Will Receive This Information Over The Interface Serial1/0.1 (Frame Relay Multipoint Interface). Because Split Horizon Is Enabled On R1′S Serial 1/0.1 Sub interface, Then R1 Will Not Advertise The Subnets 20.20.20.0./24 And 140.1.3.0/24 To R3 And R2, Respectively. To Avoid This, We Should Disable Split Horizon On The Interface Serial 1/0.1. Therefore, R1 Will Send The Subnets 20.20.20.0./24 And 140.1.3.0/24 To R3 And R2, Respectively.


The IP SPLIT-HORIZON COMMAND Has No Parameters Or Keywords, And It Was Introduced In Cisco IOS Software Release 10.0. By Default, This Command Is Enabled On All Types Of Interfaces, Except On Main Frame-Relay Interfaces And SDMS Interfaces.

In The Following Example: We’ll Configure R1 In Order To Enable IP Connectivity Between R2 And R3 By Disabling Split Horizon On The Interface Serial1/0.1.

R1(Config)#Router Rip
R1(Config-Router)#Version 2

R1(Config-Router)#Network 10.0.0.0
R1(Config-Router)#Network 140.1.0.0

R1(Config-Router)#No Auto-Summary
R1(Config-Router)#Exit

R1(Config)#!

R1(Config)#Interface Serial 1/0.1
R1(Config-Subif)# No Ip Split-Horizon

R1(Config-Subif)#^Z
R1#

To Verify Our Configuration, We’ll Use The Show Ip Interface Command On R1 And Show Ip Route Command On R2 And R3.

R1#Show IP Interface Serial1/0.1

Serial1/0.1 Is Up, Line Protocol Is Up
Internet Address Is 140.1.123.1/24
Broadcast Address Is 255.255.255.255
Address Determined By Setup Command
MTU Is 1500 Bytes
Helper Address Is Not Set
Directed Broadcast Forwarding Is Disabled
Multicast Reserved Groups Joined: 224.0.0.9
Outgoing Access List Is Not Set
Inbound Access List Is Not Set
Proxy ARP Is Enabled
Local Proxy ARP Is Disabled
Security Level Is Default

Split Horizon Is Disabled

/// Omitted Output

R2#Show Ip Route

/// Omitted Output

140.1.0.0/24 Is Subnetted, 2 Subnets
R 140.1.3.0 [120/2] Via 140.1.123.3, 00:00:09, Serial1/0
C 140.1.123.0 Is Directly Connected, Serial1/0
20.0.0.0/24 Is Subnetted, 1 Subnets
C 20.20.20.0 Is Directly Connected, Fastethernet0/1
10.0.0.0/24 Is Subnetted, 1 Subnets
R 10.10.10.0 [120/1] Via 140.1.123.1, 00:00:09, Serial1/0
R2#

R3#Show Ip Route

/// Omitted Output
140.1.0.0/24 Is Subnetted, 2 Subnets
C 140.1.3.0 Is Directly Connected, Fastethernet0/1
C 140.1.123.0 Is Directly Connected, Serial1/0
20.0.0.0/24 Is Subnetted, 1 Subnets
R 20.20.20.0 [120/2] Via 140.1.123.2, 00:00:20, Serial1/0
10.0.0.0/24 Is Subnetted, 1 Subnets
R 10.10.10.0 [120/1] Via 140.1.123.1, 00:00:20, Serial1/0
R3#

EIGRP - SPLIT HORIZON EXAMPLE:

Does EIGRP Use Split Horizon?

Split Horizon Is A Loop-Prevention Method. Essentially, When Using Splithorizon, A Routing Protocol Tries To Prevent A Routing Loop. It Does This Bynot Advertising A Route From An Interface From Which It Received An Advertisement For That Route.

EIGRP Uses Split Horizon, But You Can Disable It If Necessary. To Do So, Use The No IP SPLIT-HORIZON EIGRP {AS Number} Command. Keep In Mind That The No Ip Split-Horizon command Doesn't Affect EIGRP, As It Would RIP. Link-State Routing Protocols Such As OSPF And The Intermediate System-To-Intermediate System (IS-IS) Protocol Don't Use Split Horizon.

CONFIGURATION EXAMPLE :

In This Tutorial, R1 Will Serve As The Hub And R2 And R3 Will Be The Spokes. We'll First Configure EIGRP Over The 172.16.123.0 /24 Network, The Network Connecting The Three Routers.

R1#Conf T
R1(Config)#Router Eigrp 100
R1(Config-Router)#No Auto-Summary
R1(Config-Router)#Network 172.12.123.0 0.0.0.255

R2#Conf T
R2(Config)#Router Eigrp 100
R2(Config-Router)#No Auto-Summary
R2(Config-Router)#Network 172.12.123.0 0.0.0.255

R3#Conf T
R3(Config)#Router Eigrp 100
R3(Config-Router)#No Auto-Summary
R3(Config-Router)#Network 172.12.123.0 0.0.0.255
Running Show Ip Eigrp Neighbor On R1 Shows That Adjacencies To R2 And R3 Are Up.

R1#Show Ip Eigrp Neighbor

IP-EIGRP Neighbors For Process 100
H Address Interface Hold Uptime SRTT RTO Q Seq Type
(Sec) (Ms) Cnt Num
1 172.12.123.3 Se0/0 11 00:02:45 1 5000 0 1
0 172.12.123.2 Se0/0 161 00:03:01 1 5000 0 1

Each Router Will Now Advertise Its Loopback Address Via EIGRP.

R1#Conf T
R1(Config)#Router Eigrp 100
R1(Config-Router)#Network 1.1.1.0 0.0.0.255

R2#Conf T
R2(Config)#Router Eigrp 100
R2(Config-Router)#Network 2.2.2.0 0.0.0.255

R3#Conf T
R3(Config)#Router Eigrp 100
R3(Config-Router)#Network 3.3.3.0 0.0.0.255

Running Show Ip Eigrp Route On Each Router Shows That R1 Has A Route For Both R2’s And R3’s Loopback. R2 And R3 Will Only See R1’s Loopback Address, And Not Each Other’s. Why?

R1#Show Ip Route Eigrp

2.0.0.0/24 Is Subnetted, 1 Subnets
D 2.2.2.0 [90/2297856] Via 172.12.123.2, 00:03:19, Serial0/0
3.0.0.0/24 Is Subnetted, 1 Subnets
D 3.3.3.0 [90/2297856] Via 172.12.123.3, 00:03:04, Serial0/0

R2#Show Ip Route Eigrp

1.0.0.0/24 Is Subnetted, 1 Subnets
D 1.1.1.0 [90/2297856] Via 172.12.123.1, 00:03:40, Serial0/0.123

R3#Show Ip Route Eigrp

1.0.0.0/24 Is Subnetted, 1 Subnets
D 1.1.1.0 [90/2297856] Via 172.12.123.1, 00:05:17, Serial0/0.31

EIGRP Uses Split Horizon By Default To Prevent Routing Loops. In This Lab, Though, It Prevents Full Network Reachability. R2 And R3 Both Form Neighbor Relationships With R1’s Serial Physical Interface. R2 Advertises Its Loopback Address To R1’s Serial Interface, As Does R3. Split Horizon Does Not Allow A Route To Be Advertised Back Out The Same Interface It Was Received On. This Prevents R1 From Advertising R2’s Loopback To R3, Or R3’s Loopback To R2.

Split Horizon Must Be Disabled To Allow Full Network Reachability In This Lab. To Do So, Run No Ip Split-Horizon Eigrp 100 On R1’s Serial Interface. When Split Horizon Is Disabled, That Will Cause The Neighbor

Relationships To Fail, And Then Reestablish. Run Show Ip Route Eigrp 100 On Both R2 And R3. The Appropriate Route To The Remote Loopback Address Will Now Appear. From Each Router, Ping The Other Routers’ Loopbacks. All Pings Will Succeed.

R1#Conf T
R1(Config)#Int Serial0

R1(Config-If)#No Ip Split-Horizon Eigrp 100

10:02:23: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor 172.12.123.2 (Serial0/0) Down: Split Horizon Changed
10:02:23: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor 172.12.123.3 (Serial0/0) Down: Split Horizon Changed

10:02:27: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor 172.12.123.3 (Serial0/0) Ip: New Adjacency

10:02:54: %DUAL-5-NBRCHANGE: IP-EIGRP 100: Neighbor 172.12.123.2 (Serial0/0) Ip: New Adjacency

R2#Show Ip Route Eigrp

1.0.0.0/24 Is Subnetted, 1 Subnets
D 1.1.1.0 [90/2297856] Via 172.12.123.1, 00:00:06, Serial0/0.123
3.0.0.0/24 Is Subnetted, 1 Subnets
D 3.3.3.0 [90/2809856] Via 172.12.123.1, 00:00:06, Serial0/0.123

R3#Show Ip Route Eigrp

1.0.0.0/24 Is Subnetted, 1 Subnets
D 1.1.1.0 [90/2297856] Via 172.12.123.1, 00:00:12, Serial0/0.31
2.0.0.0/24 Is Subnetted, 1 Subnets
D 2.2.2.0 [90/2809856] Via 172.12.123.1, 00:00:12, Serial0/0.31

NOTE: Disabling Split Horizon Should Be Done With Care, But Knowing When And Where To Do So Shows That You Truly Understand How This Technology Works - And That's A Big Step On The Way To Earning Your CCNA!

That Disabling Split Horizon Resulted In The EIGRP Adjacencies Being Torn Down. They Came Back Up 20 - 35 Seconds After Being Torn Down According To The Timestamps, But That's A Good Detail To Keep In Mind!

BGP – SPLIT HORIZON :

FIRST UNDERSTAND THE BORDER GATEWAY PROTOCOL CONCEPTS:

Border Gateway Protocol (BGP) Is Responsible For Exchanging Routing Information Between All Of The Major Internet Service Providers (ISPS), As Well Between Larger Client Sites And Their Respective ISPS. And In Some Large Enterprise Networks, BGP Is Used To Interconnect Different Geographical Or Administrative Regions.

BGP Is An Exterior Gateway Protocol (EGP), Which Means That It Exchanges Routing Information Between Autonomous Systems (AS).

RIP Is A Distance Vector Protocol, OSPF Is A Link State Protocol, And EIGRP Is A Distance Vector Protocol That Incorporates Many Of The Advantages Of A Link State Protocol.

BGP, On The Other Hand, Uses A Path Vector Algorithm. This Means That Instead Of Reducing Each Route's Relative Importance In The Routing Table To A Single Metric Or Cost Value, BGP Keeps A List Of Every AS That The Path Passes Through. It Uses This List To Eliminate Loops Because A Router Can Check Whether A Route Has Already Passed Through A Particular AS By Simply Looking At The Path.

KEY POINTS OF BGP:

Note: A Detailed Discussion Of The BGP Protocol Plz Check The “Cisco - How BGP Established Neighbors Relationship:” Will Be Soon.

And also check - > http://premji-schoolofcisconetworking.blogspot.com/search/label/CISCO%20-%20BGP%20%28BASIC%29

BGP SPLIT-HORIZON RULE:

The BGP Split Horizon Rule Governs IBGP Behavior. This BGP Rule Specifies That Routes Learned Via IBGP Are Never Propagated To Other IBGP Peers.

There Is No Split Horizon Rule With BGP. Split Horizon Says It Can't Receive A Route In One Interface And Advertise It Back Out The Same Interface Seen Only In Distance Vector Protocols.

With IBGP That Concept Is Not The Same As It Could Be Peering With Another IBGP Neighbor On A Different Interface And That Route Would Not Be Advertised. So I Would Be Careful When Talking About Split Horizon With BGP As It Doesn't Exist.

This Is A Loop Prevention Mechanism. It States That IBGP Learned Routes Cannot Be Advertised To Another IBGP Neighbor That Is called IBGP Split Horizon Mechanism.

So The Split-Horizon Rule Says That Routes Learned Via IBPG Won't Be Sent Ot Other IBGP Peers. This Rule Is Used To Prevent Loop An Autonomous Systems (AS). The Rule Of BGP Split-Horizon States That Advertisements Received From One IBGP Peer Cannot Be Advertised To Another IBGP Peer.

When An IBGP Peer Is Configured As A Route-Reflector-Client, The Rule Of BGP Split-Horizon Is Effectively Disabled, Allowing Advertisements Received From One Ibgp Peer To Be Advertised To Another IBGP Peer. Using Route-Reflectors Is A Very Effective Way Of Mitigating The IBGP Full Mesh Requirement.

BGP – SPLIT HORIZON EXAMPLE :

BGP Confederations Help Us Divide A Larger BGP Autonomous System Into Smaller Autonomous Systems. Route-Reflectors On The Other Hand Are Employed To Suppress The Split Horizon Rule Of BGP (Split Horizon In BGP Stops A Router From Advertizing A Route It Learned From An IBGP Peer To Another Neighbor, Because IBGP Expects Full Mesh Connectivity Between All The Routers Running It).

CONCLUSION:

The Goal Of This Article Is To Give An Easy Way To Understand The “Cisco – Split Horizon Configuration.” Hope This Article Will Help Every Beginners Who Are Going To Start Cisco Lab Practice Without Any Doubts. 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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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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