RIP TIMERS ON CISCO DEVICES INTERFACE:

This Article Goes Over The Basic Concepts Of Rip Timers And How It Operates As Well As How It Can Be Configured To Operate On Cisco Equipment.

FIRST UNDERSTANDING RIP ROUTING:

RIP Is One Of The Most Enduring Of All Routing Protocols. It’S A Very Simple Protocol, Based On Distance-Vector   (Or Bellman-Ford, As It's Also Known) Routing Algorithms That Predate ARPA Net   (The Advanced Research Projects Agency Network).

To Be Exact, R. E. Bellman, L. R. Ford, Jr., And D. R. Fulkerson Originally Described These Algorithms Academically Between 1957 And 1962. During The 1960s, These Algorithms Were Widely Deployed By Various Companies And Marketed Under Different Names.

The RIP Protocol In The Form That We Use Now Was Developed In The 1970s At Xerox Labs As Part Of The XNS (Xerox Network Systems) Routing Protocol Suite. The Most Popular Variants Are RIP Version 1, Described In RFC1058, And RIP Version 2, Described In RFC2453.

RIP (Routing Information Protocol) Is A Common Routing Protocol Used Within An Autonomous System. A De Facto Industry Standard, It Is Also Used By Routed, A Service Distributed By Berkeley.

Gated Supports All Version 1 Feature And Most Of The Features Of Version 2. The Following Version 2 Features Are Not Supported: Route Aggregation, Rip Management Information Base (MIB), Route Tag, And Authentication

RIP PROTOCOL OVERVIEW:

RIP Stands For Routing Information Protocol. RIP Is A Distance-Vector Routing Protocol, Which Employs The Hop Count As A Routing Metric. RIP Prevents Routing Loops By Implementing A Limit On The Number Of Hops Allowed In A Path From The Source To A Destination.

RIP Calculates The Best Route Based On Hop Count. The Maximum Number Of Hops Allowed For RIP Is 15. This Hop Limit, However, Also Limits The Size Of Networks That RIP Can Support. A Hop Count Of 16 Is Considered An Infinite Distance And Used To Deprecate Inaccessible, Inoperable, Or Otherwise Undesirable Routes In The Selection Process. The Main Purpose Of RIP Is To Prevent Routing Loops By Implementing A Limit On Number Of Hops Between A Source And Its Destination.

RIP IMPLEMENTS:

◙ - ►  SPLIT HORIZON:- 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.

◙ - ►  ROUTE POISONING:- Is 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  (Prevents Larger Loops. Rule : Once You Learn Of A Route Through An Interface, Advertise It As Unreachable Back Through That Same Interface!).

◙ - ►  HOLDDOWN MECHANISMS:- To Prevent Incorrect Routing Information From Being Propagated   (Prevents Incorrect Route Information From Entering Routing Tables. Rule: After A Route Is Advertised As Down, Do Not Listen To Routing Updates On That Route For A Specific Period Of Time!).

ALSO KNOW WHAT IS SPLIT HORIZON :

Split Horizon Is A Base Technique Used To Reduce The Chance Of Routing Loops. Split Horizon States That It Is Never Useful To Send Information About A Route Back In The Direction From Which The Information Came And Therefore Routing Information Should Not Be Sent Back To The Source From Which It Came. In Fact, Only The Interfaces Are Considered For The Direction, Not The Neighbors.

Note That This Rule Works Well Not Only For Routes Learned Via A Distance Vector Routing Protocol But Also For Routes Installed In A Routing Table As Directly Connected Networks. As They Reside On The Same Network, The Neighbors Do Not Need Any Advertisements On A Path To That Shared Network.

The Split Horizon Rule Helps Prevent Two-Node   (Two-Neighbor) Routing Loops And Also Improves Performance By Eliminating Unnecessary Updates.

ALSO KNOW WHAT IS POISON REVERSE :

Whereas Split Horizons Should Prevent Routing Loops Between Neighbor Routers, Poison Reverse Updates Are Intended To Defeat Larger Routing Loops. While The Simple Split Horizon Scheme Omits Routes Learned From One Neighbor In Updates Sent To That Neighbor, Split Horizon With Poison Reverse Includes Such Routes In Updates, But Sets Their Metrics To Infinity.

Poison Reverse Thus Establishes: A Single Direction Through Which Routes Can Be Reached Via A Particular Interface. Such An Interface Should Not Be Traversed In The Opposite Direction To Reach A Particular Destination. Poison Reverse Ensures This Single Direction By Blocking The Other Way (By Poisoning It With A High Cost, Such As Infinity In The Case Of RIP).

Its Effect Is Best Seen In The Following Situation: Once A Router Discovers It Has Lost Contact With A Neighboring Router, It Will Immediately Forward A Routing Update With The Inoperable Route Metric Set To Infinity. Additionally, The Router Will Broadcast The Route, With An Infinite Metric, For Several Regular Routing Update Periods To Ensure That All Other Routers On The Internetwork Have Received The Information And Gradually Converge.

Cisco Also Deploys So-Called Route Poisoning. This Technique Is Used, Upon Learning About The Unreachable Destination, To Advertise The Information On The Failed Route By Sending A Route Update With An Infinite Metric.

Poison Reverse Is Usually Used In Conjunction With Split Horizon, Thus The Mechanisms Work Together To Prevent Routing Loops   (A Potential Danger With Distance Vector Routing). Poison Reverse Is Also Used In Conjunction With Hold-down Timers.

THESE ARE SOME OF THE STABILITY FEATURES OF RIP: It Is Also Possible To Use The So Called RMTI &Nbsp; (Routing Information Protocol With Metric-Based Topology Investigation) Algorithm To Cope With The Count To Infinity Problem. With Its Help, It Is Possible To Detect Every Possible Loop With A Very Small Computation Effort.

RIP Is Probably The Most Widely Used. It Is A Distance-Vector Protocol Based On A 1970s Xerox Design. Ported To TCP/IP When LANs First Appeared In The Early 80s, RIP Has Changed Little In The Past Decade And Suffers From Several Limitations, Some Of Which Have Been Overcome With RIP-2, Which Is Not Discussed Here. RFC 1058 Documents RIP. RIP Router Transmitted Full Updates Every 30 Seconds.

◙ - ►  For More About - > Difference Between RIPv1, RIPv2 And RIPng:

◙ - ►  For More About - > RIP Notes And RIP Configuration :

◙ - ►  For More About - > RIP IP Split Horizon Notes:

◙ - ►  For More About - > RIP Message Digest 5 (MD5) Authentication:

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BASIC RIP CONFIGURATION

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Example For Basic RIP Configuration On Cisco Interface :

Router> Enable
Router B#Configure Terminal

Enter Configuration Commands, One Per Line. End With CNTL/Z.
Router B(Config)#Interface Ethernet0
Router B(Config-If)#Ip Address 192.168.30.1 255.255.255.0

Router B(Config-If)#Interface Serial0.1
Router B(Config-Subif)#Ip Address 172.25.2.2 255.255.255.0
Router B(Config-Subif)#Exit

Router B(Config)#Router Rip
Router B(Config-Router)#Network 172.25.0.0
Router B(Config-Router)#Network 192.168.30.0
Router B(Config-Router)#Exit

Router B(Config)#End
Router B#

You Enable RIP For An Interface By Associating Its IP Address With A Network. For Example, The Serial0.1 Subinterface In This Example Has An IP Address Of 172.25.2.2. So If You Want This Subinterface To Take Part In RIP Route Distribution, You Just Need To Include A Network Statement That Includes Its Address:

Router B(Config)#Router Rip
Router B(Config-Router)#Network 172.25.0.0

If You Have Other Interfaces That Are Also Part Of 172.25.0.0/16 On This Router, They Will Take Part In RIP As Well. It's Important To Note That RIP Network Statements Work With Classful Network Addresses. The Network Command May Appear To Accept Subnets Of Classful Addresses, But It Will Internally Rewrite These Subnet Addresses With The Classful Network Address:

Router B#Configure Terminal
Enter Configuration Commands, One Per Line. End With CNTL/Z.

Router B(Config)#Router Rip
Router B(Config-Router)#Network 172.25.1.0
Router B(Config-Router)#End

Router B#Show Run | Begin Router Rip
Router Rip
Network 172.25.0.0

Router B#

Since 172.25.0.0 Is A Class B Network, The Statement Network 172.25.0.0 Tells The Router To Include All Interfaces With IP Address In This Range. Note, However, That The Other Address On This Router, 192.168.30.1, Is Part Of The Class C Network, 192.168.30.0/24, Which Is Why We Need The Second Network Command:

Router B(Config)#Router Rip
Router B(Config-Router)#Network 192.168.30.0

If Your Router Has Several Addresses That Belong To Different Network Classes, As In, For Example, 192.168.X.0/24, You Have To Specify Them All Separately:

Router B(Config)#Router Rip
Router B(Config-Router)#Network 192.168.1.0
Router B(Config-Router)#Network 192.168.2.0
Router B(Config-Router)#Network 192.168.3.0

It Can Sometimes Be Confusing To Know Which Networks To Specify. You Use The Network Command To Specify Both The Routes That RIP Advertises And The Interfaces That It Uses.

If Your Router Receives A Route From Another Router, It Will Pass It Along To Other Routers, Whether You Have Specified A Network Statement For This Address Or Not. So The Network Command Selects Only The Routes That Are Local To This Router, Such As Directly Connected Networks. However, The Router Will Only Distribute Information About Directly Connected Routes If There Is A Network Statement For These Routes. And The Router Will Exchange RIP Information Only Through Interfaces Specified By Network Statements.

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RIP TIMERS

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RIP Timers – Default (Timers Basic, All In Seconds):

◙ - ►  Update Timer: 30 (How Often To Send Updates In Seconds).

◙ - ►  Default Invalid Route Timer: 180 (How Many Seconds, Since Seeing A Valid Update, To Consider The Route Invalid, And Placing The Route Into Hold Down) .

◙ - ►  Hold Down Timer: Default Holddown Period Is 180 Seconds. (Once In Hold Down, How Long (In Seconds) To “Not Believe” Any Equal Or Less Impressive (Worse) Route Updates For Routes That Are In Hold Down).

◙ - ►  Flush Timer: 240 (How Long To Wait To Delete A Route After It Has Timed Out).

RIP Has Several Timers That Control Things Like How Often It Sends Updates And How Long It Takes To Remove A Bad Route. You Can Adjust These Values With The Basic Configuration Command :

Router B#Configure Terminal
Enter Configuration Commands, One Per Line. End With CNTL/Z.

Router B(Config)#Router Rip
Router B(Config-Router)#Timers Basic 20 80 80 120

Router B(Config-Router)#Exit
Router B(Config)#End
Router B#

Discussion :

The Timers Basic Command Controls All Of The Adjustable Timers For RIP:

Router B(Config-Router)#Timers Basic 20 80 80 120

In Order:

◙ - ►  The Update Period,
◙ - ►  The Invalid Route Timer,
◙ - ►  The Holddown Timer,
◙ - ►  The Flush Timer. All Of These Times Are In Seconds.

The Update Period:

Controls How Often The Router Sends Updates To Its Neighbors. The Default Update Period Is 30 Seconds. Reducing This Period Can Help Improve Convergence Times. However, You Have To Remember That RIP Sends The Entire Routing Table In Each Update Cycle. So If The Routing Table Is Large, Reducing This Period Too Much Can Cause Serious Bandwidth Loading Problems On Slower Links.

The Invalid Route Timer :

Controls How Long The Router Will Wait Before A Particular Route Is Declared Invalid. If The Route Disappears From Routing Updates From Neighboring Routers For This Length Of Time, The Router Will Mark It Invalid.

The Router Will Set The RIP Metric For Invalid Routes To 16, Or Unreachable, When Distributing Them. This Technique Is Often Called "Route Poisoning." It Is Important To Remember That The Router Doesn't Remove Invalid Routes From Its Own Routing Table, And It Will Continue To Distribute Them To Other Routers.

The Default Invalid Route Timer:

Value Is 180 Seconds. Most References Advise Making This Value At Least Three Times The Update Period. Shorter Invalid Route Timers Can Cause Instability Problems. But If You Make It Too Much Longer, Then The Network Doesn't Respond Well To Topology Changes.

The Holddown Timer:

For A Particular Route Starts When The Router Gets A Routing Update That Says That A Particular Route Is Inaccessible.

This Could Happen, For Example, If Another Router's Invalid Route Timer For This Route Has Expired. But It Can Also Happen As A Result Of A Triggered Update, Indicating That A Particular Interface Pointing To This Network Has Gone Down.

The Router Will Keep The Unreachable Route In Its Table, And Will Distribute It To Other Routers With An Unreachable Metric. After The Holddown Timer Expires, The Router Will Delete The Unreachable Route And Start To Accept Other Routing Information For This Route.

The Default Holddown Period Is 180 Seconds. It Is Usually A Good Idea To Keep The Holddown Time The Same As The Invalid Route Timer To Help Ensure Network Stability.

The Flush Timer.

This Controls How Long The Router Will Keep A Route In Its Routing Table Before Purging It. The Default Flush Period Is 240 Seconds. It Must Be Greater Than The Holddown Time; Otherwise, Routes Can Be Flushed Before The Holddown Timer Expires, Which Makes The Holddown Time Meaningless.

We Recommend Using Extreme Caution When Adjusting RIP Timers. The Timers On Every Router In A RIP Network Must Be Equal, Or You Will See Terrible Instability Problems.

Note: That The RIP Timers Affect The Entire RIP Process, Meaning That You Can't Set The Timer Values Separately For Different Neighbors, Or Different Interfaces. So It Isn't Possible To Slow Down The Update Timers Over A Slow WAN Link And Make Them Shorter Over A Faster LAN Link.

In This Example, We Wanted To Make RIP Converge Faster After Topology Changes, So We Decreased All Of The Timers:

Router B(Config)#Router Rip
Router B(Config-Router)#Timers Basic 20 80 80 120

The Net Result Is That We Have Reduced The Time To Flush A Bad Route To 2 Minutes From The Default Value Of 4. But It Is Important To Notice That We Had To Decrease All Of The Timers To Achieve This Result Without Compromising Overall Network Stability.

As We Mentioned Earlier, Routers Will Send The Entire Routing Table On Every Update Cycle. So Making The Timers Too Short Can Cause Congestion Problems On Slower Links.

However, You Can Get Away With Shorter Update Periods If You Use Route Summarization Or Filtering. Usually People Are Interested In Reducing These Timers To Improve Convergence Times.

We Don't Recommend Increasing Them From The Default Values Because It Will Make The Network Respond Too Slowly To Topology Changes. If You Have A Problem With Older Or Slower Routers That Are Unable To Receive RIP Updates As Quickly As They Are Sent, A Better Solution Is To Adjust The Interpacket Delay.

You Can View The Values For All Of The Configured RIP Timers With The Show Ip Protocols Command:

Router B#Show Ip Protocols

Routing Protocol Is "Rip"
Sending Updates Every 20 Seconds, Next Due In 8 Seconds
Invalid After 80 Seconds, Hold Down 80, Flushed After 120
Outgoing Update Filter List For All Interfaces Is Not Set

Incoming Update Filter List For All Interfaces Is Not Set
Redistributing: Rip
Default Version Control: Send Version 1, Receive Any Version
Interface Send Recv Triggered RIP Key-Chain
Ethernet0 1 1 2
Serial0.1 1 1 2

Automatic Network Summarization Is In Effect
Maximum Path: 4
Routing For Networks:
172.25.0.0
192.168.30.0

Routing Information Sources:
Gateway Distance Last Update
172.25.2.1 120 00:00:14
Distance: (Default Is 120)
Router B#

Configuring Inter Packet Delay :

You Want To Slow Down The Rate That A Router Sends The Packets In A Single Update To Ensure That Slower Devices Aren't So Overwhelmed That They Lose Information

Router B#Configure Terminal
Enter Configuration Commands, One Per Line. End With CNTL/Z.

Router B(Config)#Router Rip
Router B(Config-Router)#Output-Delay 10
Router B(Config-Router)#Exit
Router B(Config)#End

Router B#

If The Routing Table Distributed In Each Update Is Small, It Will Fit Into A Single Packet. But The Maximum Size For A RIP Update Packet Is 512 Bytes. Each Packet Has An 8-Byte UDP Header, And 4 Bytes Of General RIP Information. The Remaining 500 Bytes Of The Packet Carry The Actual Routes. Each Individual Route Takes 20 Bytes, So There Can Be At Most 25 Routes In One Packet. If You Have More Than 25 Routes In Your Routing Table, RIP Must Send It As Multiple Packets.

The Problem With This Is That Some Devices Can't Process Large Amounts Of Incoming Routing Information Quickly, And They Wind Up Missing Some Of The Routing Information. This Is Particularly True For Older Legacy Equipment, Or Routers With Slow Processors Or Insufficient Memory. So If You Find That One Or More Of The Neighboring Routers Has A Mysteriously Incomplete Routing Table, It Might Help To Increase The Interpacket Spacing. The Other Reason For Increasing The Output-Delay Is To Prevent Bursts Of RIP Traffic From Causing Network Congestion.

For example: In Frame Relay Networks, If You Burst Above The Committed Information Rate (CIR), The Extra Packets Are Marked As Discard Eligible, Meaning That If There Is Congestion, These Are The First Packets That The Network Should Drop. Dropping Routing Protocol Packets Can Cause Serious Problems. Spreading Out This Burst Of RIP Traffic Might Be Sufficient To Prevent This Sort Of Problem.

The Output-Delay Command Allows You To Specify The Interpacket Delay In Milliseconds. The Default Is 0, Meaning That The Router Will Send Packets As Fast As It Can. You Can Specify Any Delay Value Between 8 And 50 Milliseconds With This Command. It Is Not Necessary To Specify A Corresponding Delay On Neighboring Routers.

The Output Of The Show Ip Protocols Command Includes The Output Delay Value If You Specify Any Nondefault Value:

Router B#show ip protocols

Routing Protocol is "rip"
Sending updates every 60 seconds, next due in 17 seconds
Invalid after 180 seconds, hold down 180, flushed after 240

Output delay 10 milliseconds between packets
Outgoing update filter list for all interfaces is not set

Incoming update filter list for all interfaces is not set

Redistributing: rip
Default version control: send version 1, receive any version

Interface Send Recv Triggered RIP
Key-chain
Ethernet0 1 1 2
Serial0.1 1 1 2
Automatic network summarization is in effect Maximum path: 4
Routing for Networks
: 172.25.0.0
192.168.30.0
Routing Information Sources:

Gateway Distance Last Update
172.25.2.1 120 00:00:23
Distance: (default is 120)
Router b# †

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CONCLUSION:

The Goal Of This Article Is To Give An Easy Way To Understand The “RIP Timers On Cisco Devices Interface:". 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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