Chapter 4 Distance Vector Routing Protocols

Chapter 4Distance VectorRouting ProtocolsCCNA2-1 Chapter 4Note for Instructors• These presentations are the result of a collaboration amongthe instructors at St. Clair College in Windsor, Ontario.• Thanks must go out to Rick Graziani of Cabrillo College. Hismaterial and additional information was used as a referencein their creation.• If anyone finds any errors or omissions, please let me knowat:• tdame@stclaircollege.ca.CCNA2-2 Chapter 4

Distance Vector Routing ProtocolsIntroduction toDistance Vector Routing ProtocolsCCNA2-3 Chapter 4Introduction to Distance Vector• There are advantages and disadvantages to using any typeof routing protocol.• Understanding the operation of distance vector routing iscritical to enabling, verifying, and troubleshooting theseprotocols.CCNA2-4 Chapter 4

Introduction to Distance VectorStatic Routes?CCNA2-5 Chapter 4Introduction to Distance Vector• Routing Information Protocol (RIP):• Metric: Hop count.• A hop count greater than 15 means that the network isunreachable.• Periodic routing updates.• Entire routing table is broadcast every 30 seconds.• Enhanced Interior Gateway Routing Protocol (EIGRP):• Cisco proprietary.• Composite metric: Bandwidth, delay, reliability and load.• It uses Diffusing Update Algorithm (DUAL) to calculatethe shortest path.• No periodic updates.• Multicast updates only on a change in topology.CCNA2-6 Chapter 4

Meaning of Distance Vector• The routing protocol does not know the entire topology of anetwork.• It only knows the routing information received from itsneighbors.• A Distance Vector routing protocol does not have theknowledge of the entire path to a destination network.CCNA2-7 Chapter 4Meaning of Distance Vector• A Distance Vector routing protocol does not have theknowledge of the entire path to a destination network.Network 172.16.3.0/24:• is 1 hop away (Distance)• through interface s0/0/0 (Vector)CCNA2-8 Chapter 4

Operation of Distance Vector• Periodic Updates:• Some distance vector routing protocols periodicallybroadcast the entire routing table to each of its neighbors(RIP – every 30 seconds).• Inefficient:Updates consume bandwidth androuter CPU resources.• Periodic updates are always sent even there havebeen no changes for weeks or months.• Router is only aware of the:• Network addresses of its own interfaces.• Network addresses the neighbors running thesame routing protocol.CCNA2-9 Chapter 4Operation of Distance Vector• Periodic Updates:R1 Update Timer expiresNeighbour of R1Updatessent.Broadcast!R1 is unaware ofR3 and its networksNeighbour of R1CCNA2-10 Chapter 4

Routing Protocol Algorithms• The algorithm used by aparticular routing protocolis responsible for buildingand maintaining therouter’s s routing table.• Defines the followingprocesses:• Mechanism for sending and receiving routing information.• Mechanism for calculating the best paths and installingroutes in the routing table.• Mechanism for detecting and reacting to topologychanges.CCNA2-11 Chapter 4Routing Protocol AlgorithmsCalculate Send Best UpdatePath and InstallRoutePeriodic UpdateUPDATE172.16.1.0/24 1172.16.1.0/24 S0/0/0 1CCNA2-12 Chapter 4

Routing Protocol AlgorithmsCalculate Send Best UpdatePath and InstallRoutePeriodic UpdateUPDATE172.16.3.0/24 S0/0/0 1172.16.3.0/24 1172.16.1.0/24 S0/0/0 1CCNA2-13 Chapter 4Routing Protocol AlgorithmsDetect and React to Topology ChangesTriggered UpdateUPDATEX172.16.3.0/24 (DOWN) 1172.16.3.0/24 (DOWN) 1172.16.3.0/24 S0/0/0 1 172.16.1.0/24 S0/0/0 1CCNA2-14 Chapter 4

Routing Protocol Characteristics• Other ways to compare routing protocols:• Time to convergence:• Faster the better.• Scalability:• How large a network the routing protocol can handle.• Classless or Classful:• Support VLSM and CIDR.• Resource usage:• Routing protocol usage of RAM, CPU utilization, andlink bandwidth utilization.• Implementation and maintenance:• Level of knowledge of a network administrator.CCNA2-15 Chapter 4Comparing Routing Protocol FeaturesFeatureDistance Vector Routing ProtocolsRIPv1RIPv2EIGRPSpeed of Convergence Slow Slow FastScalability Small Small LargeSupports VLSM No Yes YesResource Usage Low Low MediumImplementation Simple Simple ComplexCCNA2-16 Chapter 4

Distance Vector Routing ProtocolsNetwork DiscoveryCCNA2-17 Chapter 4Cold Start• Network Discovery:• Is part of the process of the routing protocol algorithmthat enables routers to learn about remote networks forthe first time.CCNA2-18 Chapter 4

Cold Start• When a router powers up:• Knows nothing about the network topology.• Knows only the information saved in NVRAM.• Sends updates about its known networks out all ports.CCNA2-19 Chapter 4Initial Exchange of Routing Information10.1.0.0• Sends an update about network 10.1.0.0 out the Serial 0/0/0interface with a metric of 1.• Sends an update about network 10.2.0.0 out the Fa0/0interface with a metric of 1.CCNA2-20 Chapter 4

Initial Exchange of Routing Information• First round of update exchanges: : Each router knows aboutthe connected networks of its directly connected neighbors.• When the update timers expire (Periodic Update), , the routersbegin the next exchange of information.CCNA2-25 Chapter 4Next Exchange of Routing Information• Sends an update about network 10.1.0.0 out the S0/0/0interface with a metric of 1 - AGAIN!• When R2 receives the update, , there is no change ininformation so the update is ignored.CCNA2-26 Chapter 4

Next Exchange of Routing InformationNEWNEW10.3.0.010.4.0.010.1.0.010.2.0.0• Sends an update about networks 10.3.0.0 with a metric of 1and 10.4.0.0 with a metric of 2 out the Serial 0/0/0 interface.• Sends an update about networks 10.1.0.0 with a metric of 2and 10.2.0.0 with a metric of 1 out the Serial 0/0/1 interface.CCNA2-27 Chapter 4Initial Exchange of Routing Information10.4.0.0• Sends an update about network 10.4.0.0 out the S0/0/0interface with a metric of 1 - AGAIN!• When R2 receives the update, , there is no change ininformation so the update is ignored.CCNA2-28 Chapter 4

Next Exchange of Routing Information10.3.0.010.4.0.0• R1 receives an update from R2 about network 10.3.0.0 andthere is no change – update ignored.• R1 receives an update from R2 about network 10.4.0.0 (new)and adds it to its routing table.CCNA2-29 Chapter 4Next Exchange of Routing Information10.1.0.010.2.0.0• R3 receives an update from R2 about network 10.2.0.0 andthere is no change – update ignored.• R3 receives an update from R2 about network 10.1.0.0 (new)and adds it to its routing table.CCNA2-30 Chapter 4

Next Exchange of Routing Information• The network has CONVERGED!• All routers now know about all of the networks attached toall of their neighbouring routers.CCNA2-31 Chapter 4Convergence• The amount of time ittakes for a network toconverge is directlyproportional to thesize of that network.• Routing protocols arecompared based onhow fast they canpropagate this information - their speed to convergence.• A network is not completely operable until it has converged.• Network administrators prefer routing protocols withshorter convergence times.CCNA2-32 Chapter 4

Distance Vector Routing ProtocolsRouting Table MaintenanceCCNA2-33 Chapter 4Routing Table Maintenance• Routing protocols must maintain the routing tables so thatthey have the most current information.• How it is maintained depends upon:• The type of routing protocol(distance vector, link state, path state)• The routing protocol itself (RIP, EIGRP, OSPF)CCNA2-34 Chapter 4

Routing Table Maintenance• Distance Vector Updates:• Periodic• Bounded• TriggeredCCNA2-35 Chapter 4Periodic Updates• A router sends the complete routing table to its neighbors ata predefined interval.• RIP – every 30 seconds.• Link failure, New Link, Router Failure, Link parameterchange.CCNA2-36 Chapter 4

Periodic Updates - RIP Timers• Invalid Timer:• If an update has notbeen received in180 seconds(the default), the route is marked as invalid by setting themetric to 16.• Route still is in routing table.• Flush Timer: 240 seconds (default)• When the flush timer expires, the route is removed fromthe routing table.• Hold-down down Timer:• Helps stabilize routing information and helps preventrouting loops. (Much more later!)CCNA2-37 Chapter 4Periodic Updates – Verifying RIP TimersElapsed time since lastupdate.CCNA2-38 Chapter 4

Triggered Updates• Speeds up convergence.• Sent when one of the following events occurs:• An interface changes state (up or down).• A route has entered (or exited) the unreachable state.• A route is installed in the routing table.CCNA2-41 Chapter 4Distance Vector Routing ProtocolsRouting LoopsCCNA2-42 Chapter 4

Defining a Routing Loop• A routing loop is a condition in which a packet is continuouslytransmitted within a series of routers without ever reaching itsintended destination network.• The loop can be a result of:• Incorrectly configured static routes.• Incorrectly configured route redistribution.• Inconsistent routing tables not being updated because ofslow convergence in a changing network.• Distance vector routing protocols are simple in theirimplementation and configuration, but this comes at a price.• Pure distance vector routing protocols suffer from possiblerouting loops.CCNA2-43 Chapter 4Implications of Routing Loops• A routing loop can have adevastating effect on a network,resulting in degraded networkperformance or even networkdowntime.• Link bandwidth will be usedfor traffic looping back andforth between the routers.• A router’s s CPU will beburdened with useless packet forwarding.• Routing updates might get lost or not be processed in atimely manner, making the situation even worse.• Packets might get lost in black holes, , never reaching theirintended destinations.CCNA2-44 Chapter 4

Routing Loop - Example• Network 1 Fails.• Router E sendsan update toXRouter A.X• Router A stopsrouting packetsto Network 1.• But Routers B, C, and D continue to do so because theyhave not yet been informed about the failure.• Router A sends out its update.• Routers B and D stop routing to network1, (via Router A).• However, Router C is still not updated. To router C,network 1 is still reachable via router B.CCNA2-45 Chapter 4XXRouting Loop - Example• Router C thinksnetwork 1 isstill 3 hops away.X• Sends a periodicupdate toRouter D.3• This update says:A path to network 1 exists by way of Router B andnetwork 1 is 4 hops away.CCNA2-46 Chapter 4

Routing Loop - Example• Router D routingtable informationfor Network 1.• Current path toNetwork 1= Unreachable3XX• Update from Router C:Network 1 is 4 hops by way of Router C• Normally, Router D ignores this routing information becauseit usually has a better route (2 hops via Router A) but thisroute is now down.CCNA2-47 Chapter 4Routing Loop - Example• Router Dchanges itsrouting table toreflect thisbetter, butincorrectinformation.34XNetwork 1 is available by way of Router C (4 hops)• Router D propagates the information to Router A.CCNA2-48 Chapter 4

Routing Loop - Example• Router Achanges itsrouting table.X354• Router A adds a new route to its routing table:Network 1 is available by way of Router D (5 hops).• Propagates the information to Routers B and E.CCNA2-49 Chapter 4Routing Loop - Example• Router B andRouter Echange theirrouting tables.365X4• Router B now believes:• Network 1 is available by way of Router A (6 hops).“Wow!I was about to tell Router C that Network 1was down, but now I have new information!”• Router B sends the incorrect information to Router C.CCNA2-50 Chapter 4

Routing Loop - Example• Router Cchanges itsrouting table.X37645X• Router C still believes:Network 1 is available by way of Router BBut now it believes its 7 hops instead of 3!• Propagates the incorrect information to Router D.CCNA2-51 Chapter 4Routing Loop - Example• Here we goagain!• The routerskeep sendingdata packetsand updates!• BUT……Router A thinks Network 1 is available via Router D.Router D thinks Network 1 is available via Router C.Router C thinks Network 1 is available via Router B.Router B thinks Network 1 is available via Router A.XCCNA2-52 Chapter 4

Routing Loop - ExampleXBy the way, Network 1is STILL down!• Data packets destined for Network 1 get caught in a routingloop, from Routers A to D to C to B to A to D etc.• As routing updates continue between the routers, the hopcount gets greater – to infinity? (Not quite – we will see in amoment.)CCNA2-53 Chapter 4Count-toto-Infinity ConditionX• Count to infinity is a condition that exists when inaccuraterouting updates increase the metric value to infinity for anetwork that is no longer reachable.• Each protocol defines infinity at a different value.• When the metric value exceeds the maximum value, andas each router receives this maximum metric, the networkis then considered unreachable.CCNA2-54 Chapter 4

Count-toto-Infinity ConditionX• RIP defines infinity as 16 hops.• When the routers “count to infinity,” they mark the routeas unreachable.CCNA2-55 Chapter 4Hold-Down Timers• Hold-Down timers are used to prevent regular updatemessages from inappropriately reinstating a route that mayhave gone bad.• They instruct routers to hold any changes that mightaffect routes for a specified period of time.XCCNA2-56 Chapter 4

Hold-Down Timers• If a route is identified as down or possibly down, any otherinformation for that route containing the same status, orworse, is ignored for a predetermined amount of time (thehold-down down period).• This means that routers will leave a route marked asunreachable in that state for a period of time that is longenough for updates to propagate the routing tables with themost current information.XCCNA2-57 Chapter 4Split Horizon Rule• Split Horizon:• Is another method used to prevent routing loops.• The split horizon rule says that a router should notadvertise a network through the interface from which theupdate came.CCNA2-58 Chapter 4

Split Horizon RuleBAe0s0X• Router 1 sends an update to Router 2 that Network A isavailable and Router 2 updates its routing table with theinformation.• Router 2 recognizes a change in topology.• This would normally trigger an update to neighbouring routersand cause a routing loop.• With split horizon enabled, Router 2 realizes it received theinformation from Router 1 and does not send the update.CCNA2-59 Chapter 4Split Horizon RuleWouldSplit Horizonavoid therouting loopin ourexample?X• When Network 1 went down, Router E sent an update toRouter A.• Router A then sent an update to Routers B and D that Network1 was no longer available.• Router C then sent an update to Router D that Network 1IS available.CCNA2-60 Chapter 4

Split Horizon RuleWouldSplit Horizonavoid therouting loopin ourexample?I need HOLD to IT!sendNO an update! WAY!X• Here is where split horizon comes in……• With Split Horizon disabled, , Router D would send anupdate to Router A about the status of Network 1 and setthe routing loop in motion.• With Split Horizon enabled, , Router D does not send theupdate to Router A because it already received an updateabout the status of Network 1 from Router A. ACCNA2-61 Chapter 4Split HorizonActually,that’s s notquitetrue!Hops = 16HOLD Well...IT!NO OK!WAY!X• Router D actually does send an update to Router A.• This update has a metric of 16 which means that the route isunreachable and Router A ignores the update.• When applied with split horizon, this deliberate “poisoning” ofthe route is called poison reverse.• Split Horizon with Poison Reverse is enabled by default.CCNA2-62 Chapter 4

IP and Time-ToTo-Live (TTL)• The Time to Live (TTL) is an 8-bit 8field in the IP header thatlimits the number of hops a packet can traverse through thenetwork before it is discarded.• The purpose of the TTL field is to avoid a situation in whichan undeliverable packet keeps circulating on the networkendlessly.• With TTL, the 8-bit 8field is set with a value by the sourcedevice of the packet.•CCNA2-63 Chapter 4IP and Time-ToTo-Live (TTL)• The TTL is decreased by 1 by every router on the route to itsdestination.• If the TTL field reaches 0 before the packet arrives at itsdestination, the packet is discarded and the router sends anInternet Control Message Protocol (ICMP) error messageback to the source of the IP packet.CCNA2-64 Chapter 4

Distance Vector Routing ProtocolsDistance Vector RoutingProtocols TodayCCNA2-65 Chapter 4RIP and EIGRP• For distance vectorrouting protocols, thereare really only twochoices: RIP or EIGRP.FeatureDistance Vector Routing ProtocolsSpeed ofConvergenceRIPRIPv2EIGRPSlow Slow FastScalability Small Small LargeSupports VLSM No Yes YesResource Usage Low Low MediumImplementation Simple Simple Complex• The decision about which routing protocol to use in a givensituation is influenced by a number of factors, including• Size of the network.• Compatibility between models of routers.• Administrative knowledge required.CCNA2-66 Chapter 4