2 Dynamic Routing Avoids configuration of static routes Routers react to changes in the networkRouters adjust their routing tables accordingly, without the intervention of the network administratorThere are problems associated with dynamic distance vector routing
3 Distance Vector Routing Protocols RIP is a distance vector routing protocol that is used in thousands of networks throughout the worldRIP is based on open standards and is easy to implement makes it attractive to some network administratorsRIP is a good basic protocol for networking studentsIGRP is another distance vector routing protocol.Unlike RIP, IGRP is a Cisco-proprietary protocol rather than a standards-based protocol.IGRP is simple to implementIGRP is a more complex routing protocol than RIP and can use many factors to determine the best route to a destination network.NOTE: for our PacketTracer labs, we’ll use EIGRP)
4 Distance vector routing protocols Require routers to forward their entire routing table when passing along updatesRouting table information is forwarded to neighbor routers, which continue to forward the information to their neighbors..These routing tables include information about the total cost of a route and the logical address of the first router on the path to each network contained in the table.Routers need to update the information in their routing tables to make good path determination decisions.Updates may be initiated when topology changes occurChanges in a network affect the decisions made by a router.A router may be taken off line for upgrades or repairs or an interface on a router may go down.If not aware of the changes that have occurred in a network, routers may switch packets to interfaces that are no longer connected to the best route.Distance vector routing protocols typically send out updates at certain time intervalsEvery 30 seconds for RIP..Every 90 seconds for IGRP
5 Routing LoopsA Network converges when all the routers in the network have the same routing information.If a link goes down, it is possible that invalid updates will continue to loop through out the network. This is called the count to infinity.RIP routing protocol counts the count to infinity by hop count. RIPs maximum hop count is 15.After 15 hops the packet is discarded by RIP.
6 A packet arrives at Router 1 at time t1 A packet arrives at Router 1 at time t1. Router 1 has already been updated and knows that the optimal route to the destination calls for Router 2 to be the next stopRouter 1 therefore forwards the packet to Router 2. Router 2 has not yet been updated and believes that the optimal next hop is Router 1. Router 2 therefore forwards the packet back to Router 1The packet will continue to bounce back and forth between the two routers until Router 2 receives its routing update or until the packet has been switched the maximum number of times allowedThis process illustrates the count to infinity problem - there are several solutions to this problem:
7 Split Horizon Split Horizon – Disables the router from sending information about a ‘failed’ route in the routing table. This is done by not sending the information through the same interface that it learned about the failed routeThat is, it would prevent Router A from sending the updated information if received from Router B back to Router BNetwork is downBAGet to network via BIs Down!
8 Poison Reverse Poison Reverse – A route that is not ‘good’ is sent a poison reverse which removes the routeNetwork 4Network 5CEWhen Network 5 goes down, Router E initiates route poisoning by entering a table entry for Network 5 as 16, for RIP, unreachable.By this poisoning of the route to Network 5, Router C is not susceptible to incorrect updates about the route to Network 5.When Router C receives a router poisoning from Router E, it sends an update, called a poison reverse, back to Router E.This makes sure all routes on the segment have received the poisoned route information.
9 One Solution to Count to Infinitive Holddown –Is used to prevent regular update messages from reinstating a route that may have gone badWhen a router receives an update from a neighbor indicating that a previously accessible network is not working - is inaccessible, the holddown timer will startIf a new update arrives from a different neighbor with a better metric than the original network entry, the holddown is removed and data is passedHowever, if an update is received from the same neighbor router before the holddown timer expires, and it has a lower metric than the previous route, the update is ignored and the holddown timer keeps ticking
10 Configure RIP s0 s1 e0 RouterA# config t RouterA(config)# router rip nets0nets1e0netRouterA# config tRouterA(config)# router ripRouterA(config-router)# networkRouterA(config-router)# networkRouterA(config-router)# networkRouterA(config)#int s0RouterA(config-if)# ip rip triggeredIf topology changes, this command will ‘triggered’ those updates to the next router. Only applied to a serial interface.
11 RIP Configuration Issues RIP uses the following techniques to reduce routing loops and count to infinity. In some cases, configuration is required:count-to-infinitysplit horizonpoison reverseholddown counterstriggered updatesTo disable split horizon do:RouterA(config-if)# no ip split-horizon
12 RIP Configuration Issues To change RIP’s update interval do:RouterA(config-router)# update-timer <seconds>To disable sending RIP updates do:RouterA(config-router)# passive-interface <interface>Command to receive either version of RIP, doRouterA(config-if)# ip rip receive version 1RouterA(config-if)# ip rip receive version 2RouterA(config-if)# ip rip receive version 1 2
13 RIP Configuration Issues Router# config termRouter(config)# router ripRouter(config-router)#timers basicupdateinvalidholddownflushIntervals between updatesroute is invalid after receiving no updates in secsholddown timewhen route is flushed from tableupdate – 30 secondsholddown secondsAdministrative Distance - 120
14 RIP Configuration Issues NOTE for RIP:Its metric to determine a route to a destination is the hop count.As a packet goes from router to router, RIP increments a counter called hop count.
15 RIP Configuration Verification Use the following commands to make RIP verifications:show ip routeThe routing table will have “R” by the routes determined by the RIP routing protocolshow ip protocolsThis will verify:RIP routing is configured (which protocol is configured)Which interfaces are sending & receiving RIP updatesWhich network it is sending information to
16 Debugging Commands for RIP Some RIP debugging commands are:debug ip ripshow ip rip databaseshow ip interface brief
17 Classless Routing NOTE: Router Rip cannot handle Classless Routing, but Rip ver2 can.A supernet route (classless route) is a route that covers a greater range of subnets with a single entry. An example a supernet of /16 could be /13.However, a router by default assumes that all subnets of a directly connected network should be present in the routing table.If a packet is received with an unknown destination address within an unknown subnet of a directly attached network, the router assumes that the subnet does not exist, and will drop this packet.To get around this problem, use a global command: ip classless.
18 RIP – Load BalancingLoad-balancing describes the ability of a router to transmit packets to a destination IP address over more than one pathWhen a router learns multiple routes to a specific network, the route with the lowest administrative distance is entered into the routing tableTo set maximum number of parallel paths:RouterA(config-router)#maximum-paths [number]
20 Floating Static Routes Floating static routes are static routes configured with an administrative distance value that is greater than that of the primary route (or routes).Essentially, floating static routes are fallback routes, or backup routes, that do not appear in the routing table until another route fails.Example:RouterA(config)#ip route
21 RIP – Redistribute Static Routes For RIP, if a static route is assigned to an interface that is not one of the networks defined in a network command, no dynamic routing protocols advertise the route. Use redistribute static command.To redistribute static default route, must use the default-information originate command.Example:RTA(config)# ip route s0RTA(config)# router ripRTA(config-router)# default-information originate
22 IGRPIGRP:must be assigned an “AS” (autonomous system # - 16 bit number)Cisco proprietarydistance-vectormetricsdelaybandwidth (1200 bps - 10 Gbps)reliability (1-224) (higher the number, more reliable)load (1-244) (higher the number, more it is under load)sends updates every 90 secondsmaximum hop count is 255 (default 100)
23 IGRPIGRP has number of features that are designed to enhance its stability:holddownssplit horizonspoison reverse updates
24 Setting IGRP Basic Timers Router# config termRouter(config)# router igrp 100Router(config-router)# timers basicupdateinvalidholddownflushIntervals between updatesroute is invalid after receiving no updates in secsholddown timewhen route is flushed from tableRouter(config-router)# timers basic [Default settings]