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Cisco 2 - Routers Chapter 7 Routing Loops Perrine modified by Brierley 5/25/2017 Page 1 Cisco 2 - Routers Routing Loops Chapter 7 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. Perrine modified by Brierley 5/25/2017 Page 2 Cisco 2 - Routers Chapter 7 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: Perrine modified by Brierley 5/25/2017 Page 3 Split Horizon Cisco 2 - Routers Chapter 7 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 Get to network 171.10.0.0 via B Perrine modified by Brierley 5/25/2017 171.10.0.0 Page 4 Cisco 2 - Routers Poison Reverse Chapter 7 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. Perrine modified by Brierley 5/25/2017 Page 5 Cisco 2 - Routers A Solution to Count to Infinity Chapter 7 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. Perrine modified by Brierley 5/25/2017 Page 6 Configure RIP Cisco 2 - Routers Chapter 7 210.45.20.0 net s0 s1 192.10.10.0 net 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 If topology changes, this command will ‘triggered’ those updates to the next router. Only applied to a serial interface. RouterA(config-router)# network 210.45.20.0 RouterA(config)#int s0 RouterA(config-if)# ip rip triggered Perrine modified by Brierley 5/25/2017 Page 7 Cisco 2 - Routers RIP Configuration Issues Chapter 7 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 Perrine modified by Brierley 5/25/2017 Page 8 Cisco 2 - Routers RIP Configuration Issues Chapter 7 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 Perrine modified by Brierley 5/25/2017 Page 9 RIP Configuration Issues Cisco 2 - Routers Chapter 7 Router#config term Router(config)# router rip 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 update – 30 seconds holddown - 180 seconds Administrative Distance - 120 Perrine modified by Brierley 5/25/2017 Page 10 Cisco 2 - Routers RIP Configuration Issues Chapter 7 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. Perrine modified by Brierley 5/25/2017 Page 11 Cisco 2 - Routers RIP Configuration Verification Chapter 7 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 Perrine modified by Brierley 5/25/2017 Page 12 Cisco 2 - Routers Debugging Commands for RIP Chapter 7 Some RIP debugging commands are: debug ip rip show ip rip database show ip interface brief Perrine modified by Brierley 5/25/2017 Page 13 Cisco 2 - Routers Classless Routing Chapter 7 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. Perrine modified by Brierley 5/25/2017 Page 14 Cisco 2 - Routers RIP – Load Balancing Chapter 7 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] Perrine modified by Brierley 5/25/2017 Page 15 Cisco 2 - Routers Administrative Distance Route Source Chapter 7 Default 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 Perrine modified by Brierley 5/25/2017 Page 16 Cisco 2 - Routers Floating Static Routes Chapter 7 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. As an example: RouterA(config)#ip route 200.10.10.0 255.255.255.0 192.16.10.1 130 Perrine modified by Brierley 5/25/2017 Page 17 Cisco 2 - Routers RIP-Redistribute Static Routes Chapter 7 RTA(config)# ip route 192.168.1.0 192.168.2.2 RTA(config)# router rip RTA(config-router)#default-information originate Perrine modified by Brierley 5/25/2017 Page 18 Cisco 2 - Routers IGRP Chapter 7 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) Perrine modified by Brierley 5/25/2017 Page 19 Cisco 2 - Routers IGRP Chapter 7 IGRP has number of features that are designed to enhance its stability: • holddowns • split horizons • poison reverse updates Perrine modified by Brierley 5/25/2017 Page 20 Setting IGPR Basic Timers Cisco 2 - Routers Chapter 7 Router# config term Router(config)# router igrp 100 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 Router(config-router)#timers basic 90 270 280 630 Perrine modified by Brierley 5/25/2017 Page 21 Configure IGRP Cisco 2 - Routers Chapter 7 210.45.20.0 network address s0 s1 192.10.10.0 network address 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 Perrine modified by Brierley 5/25/2017 Page 22 Cisco 2 - Routers Troubleshooting IGRP Chapter 7 Helpful commands for troubleshooting IGRP: • show ip protocols • show ip route • debug ip igrp events • debug ip igrp transactions • ping • traceroute Perrine modified by Brierley 5/25/2017 Page 23 Cisco 2 - Routers End of presentation Chapter 7 Questions? Post them to “Chat” for further discussion Perrine modified by Brierley 5/25/2017 Page 24 Cisco 2 - Routers Chapter 7 Module 11 Access Lists Will Follow at 20:00 Perrine modified by Brierley 5/25/2017 Page 25