Routing Loops

1. Routing Loops

2. Routing Loops A 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.

3. A packet arrives at Router 1 at time t1. Router 1 has already been updated and so 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 so 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:

4. Split Horizon Split Horizons – Disables the router from sending information about a ‘failed’ route in the routing table through the same interface that it learned about that route from. That is, it would prevent Router A from sending the updated information if received from Router B back 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

5. Poison Reverse • Poison Reverse – • A route that is not ‘good’ is sent a poison reverse which 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 a poison reverse, back to Router E. This makes sure all routes on the segment have received the poisoned route information.

6. A Solution to Count to Infinity • 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 a previously accessible network is not working & is inaccessible, the holddown timer will start. • If a new update arrives from a neighbor with a better metric than the original network entry, the holddown is removed & data is passed. • However, if an update is received from a neighbor router before the holddown timer expires & it has a lower metric than the previous route, the update is ignored & the holddown timer keeps ticking.

7. 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.

8. RIP Configuration Issues • RIP uses the following techniques to reduce routing loops and count to infinity. In some cases, configuration is required: • count-to-infinity • split horizon • poison reverse • holddown counters • triggered updates • To disable split horizon do: • RouterA(config-if)# no ip split-horizon

9. 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 • 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

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

11. RIP Configuration Issues NOTE for RIP: It’s metric used to determine the route to a destination is the hop count. As a packet goes from router to router, RIP increments a counter called hop count.

12. RIP Configuration Verification Use the following commands to make RIP verifications: • show ip route • The routing table statement will be proceeded by an “R” when the route is learned by the RIP • show ip protocols • This will verify: • Which protocol is configured – in this case RIP • Which interfaces are sending & receiving RIP updates • Which network the routing protocol is sending information to

13. Debugging Commands for RIP • Some RIP debugging commands are: • debug ip rip • show ip rip database • show ip interface brief

14. Classless Routing NOTE: Router Rip will not handle Classless Routing, but Rip ver2 will. A supernet route (classless route) is a route that covers a greater range of subnets with a single entry. As 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.

15. RIP – Load Balancing • Load-balancing describes 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 the lowest administrative distance is entered into the routing table. • To set maximum number of parallel paths: • RouterA(config-router)#maximum-paths [number]

16. Administrative Distance Route SourceDefault Distance Connected interface 0 Static route *(conditional) 1 or 0 EIGRP summary route 5 External BGP 20 Internal EIGRP 90 External EIGRP 170 IGRP 100 OSPF 110 IS-IS 115 RIP 120 EGP 140 Internal BGP 200 Unknown 255

17. 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 are fallback routes, or backup routes, that do not appear in the routing table until another route fails. As an example: RouterA(config)#ip route 200.10.10.0 255.255.255.0 192.16.10.1 130

18. RIP-Redistribute Static Routes RTA(config)# ip route 192.168.1.0 192.168.2.2 RTA(config)# router rip RTA(config-router)#default-information originate

19. 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, greater the load) • sends updates every 90 seconds • maximum hop count is 255 (default 100)

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

21. Router(config-router)# timers basic Update Invalid holdown flush Interval between updates route is invalid after receiving no updates in ‘x’ secs holddown time when route is flushed from table Setting IGPR Basic Timers Router# config term Router(config)# router igrp 100 Router(config-router)#timers basic 90 270 280 630

22. 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

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

24. Questions? Post them to “Chat” for further discussion End of presentation

25. Module 11 Access Lists Will Follow at 20:00