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CCNA2 Chapter 7

CCNA2 Chapter 7. Distance Vector Protocols. Avoids configuration of static routes Routers react to changes in the network Routers adjust their routing tables accordingly, without the intervention of the network administrator

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CCNA2 Chapter 7

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  1. CCNA2 Chapter 7 Distance Vector Protocols

  2. Avoids configuration of static routes Routers react to changes in the network Routers adjust their routing tables accordingly, without the intervention of the network administrator There are problems associated with dynamic distance vector routing Dynamic Routing

  3. RIP is a distance vector routing protocol that is used in thousands of networks throughout the world RIP is based on open standards and is easy to implement makes it attractive to some network administrators RIP is a good basic protocol for networking students IGRP is another distance vector routing protocol. Unlike RIP, IGRP is a Cisco-proprietary protocol rather than a standards-based protocol. IGRP is simple to implement IGRP 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) Distance Vector Routing Protocols

  4. Require routers to forward their entire routing table when passing along updates Routing 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 occur Changes 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 intervals Every 30 seconds for RIP.. Every 90 seconds for IGRP Distance vector routing protocols

  5. Routing Loops A Networkconvergeswhen all the routers in the network have thesame routing information. If a link goes down, it is possible that invalid updates will continue to loop through out the network. This is called thecount to infinity. RIP routing protocol counts the count to infinity byhop 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. Router 1 has already been updated and knows that the optimal route to the destination calls for Router 2 to be the next stop Router 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 1 The 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 allowed This 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 bynotsending the information through the same interface that it learned about the failed route • That is, it would prevent Router A from sending the updated information if received from Router Bback to Router B Network 171.10.0.0 is down B A 171.10.0.0 Get to network 171.10.0.0 via B Is Down!

  8. Poison Reverse • Poison Reverse – • A route that is not ‘good’ is sent apoison reversewhich removes the route Network 4 Network 5 C E When 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 apoison 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 bad • When a router receives an update from a neighbor indicating that apreviously accessible network is not working - is inaccessible, the holddown timer will start • If a new update arrives from a different neighbor with abetter metricthan the original network entry, the holddown is removed and data is passed • However, if an update is received from the same neighbor routerbefore 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 210.45.20.0 net s0 192.10.10.0 net s1 e0 172.120.0.0 net RouterA# config t RouterA(config)# router rip RouterA(config-router)# network 192.10.10.0 RouterA(config-router)# network 172.120.0.0 RouterA(config-router)# network 210.45.20.0 RouterA(config)#int s0 RouterA(config-if)# ip rip triggered If topology changes, this command will ‘triggered’ those updates to the next router.Only applied to a serial interface.

  11. RIP Configuration Issues • RIPuses the following techniques to reducerouting loopsandcount to infinity. In some cases, configuration is required: • count-to-infinity • split horizon • poison reverse • holddown counters • triggered updates • Todisablesplit 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, do • RouterA(config-if)# ip rip receive version 1 • RouterA(config-if)# ip rip receive version 2 • RouterA(config-if)# ip rip receive version 1 2

  13. RIP Configuration Issues Router# config term Router(config)# router rip Router(config-router)# timers basic update invalid holddown flush Intervals between updates route is invalid after receiving no updates in secs holddown time when route is flushed from table update – 30 seconds holddown - 180 seconds Administrative Distance - 120

  14. RIP Configuration Issues NOTE for RIP: Its metric to determine a route to a destination is thehop count. As a packet goes from router to router, RIPincrementsa counter called hop count.

  15. RIP Configuration Verification Use the following commands to make RIP verifications: • show ip route • The routing table will have “R” by the routes determined by the RIP routing protocol • show ip protocols • This will verify: • RIP routing is configured (which protocol is configured) • Which interfaces are sending & receiving RIP updates • Which network it is sending information to

  16. Debugging Commands for RIP • Some RIP debugging commands are: • debug ip rip • show ip rip database • show 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 172.16.0.0/16 could be 172.16.0.0/13. 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 Balancing • Load-balancingdescribes the ability of a router to transmit packets to a destination IP address over more than one path • When a router learns multiple routes to a specific network, the route with thelowest administrativedistance is entered into the routing table • To set maximum number of parallel paths: • RouterA(config-router)#maximum-paths [number]

  19. Administrative Distance Route SourceDefault Distance Connected interface 0 Static route 1 EIGRP summary route 5 External BGP 20 Internal EIGRP90 External EIGRP 170 IGRP100 OSPF 110 IS-IS 115 RIP120 EGP 140 Internal BGP 200 Unknown 255

  20. Floating Static Routes Floating static routes are static routes configured with an administrative distance value that is greaterthan that of the primary route (or routes). Essentially, floating static routes arefallback routes, or backup routes, that do not appear in the routing table until another route fails. Example: RouterA(config)#ip route 200.10.10.0 255.255.255.0 192.16.10.130

  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 0.0.0.0 0.0.0.0 s0 RTA(config)# router rip RTA(config-router)# default-information originate

  22. IGRP • IGRP: • must be assigned an “AS”(autonomous system # - 16 bit number) • Cisco proprietary • distance-vector • metrics • delay • bandwidth (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 seconds • maximum hop count is 255(default 100)

  23. IGRP • IGRP has number of features that are designed to enhance its stability: • holddowns • split horizons • poison reverse updates

  24. Setting IGRP Basic Timers Router# config term Router(config)# router igrp 100 Router(config-router)# timers basic update invalid holddown flush Intervals between updates route is invalid after receiving no updates in secs holddown time when route is flushed from table Router(config-router)# timers basic 90 270 280 630 [Default settings]

  25. Configure IGRP 210.45.20.0 network address s0 192.10.10.0 network address s1 e0 172.120.0.0 network address RouterA# config t RouterA(config)# router igrp 101 RouterA(config-router)# network 192.10.10.0 RouterA(config-router)# network 172.120.0.0 RouterA(config-router)# network 210.45.20.0

  26. Troubleshooting IGRP • Helpful commands for troubleshooting IGRP: • show ip protocols • show ip route • debug ip igrp events • debug ip igrp transactions • ping • traceroute

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