CCNA 2 v3. 1 Module 7

1. CCNA 2 v3.1 Module 7

2. CCNA 2 Module 7 Distance Vector Routing Protocols RIP and IGRP

3. Distance vector routing updates • Routing table updates occur periodically or when topology change • Routers send their entire routing table to all adjacent neighbors • Routing table contains info about routing metrics • Hop count • Bandwidth • Cost • Load • Reliability • Delay

4. Distance vector routing loop • Loops occur when inconsistent routing tables are not updated due to slow convergence in a changing network

5. Count To Infinity • Count to infinity • Packets loop continuously around the network • Packet never reach their destination • May occur because • Network is down • Interface not configured correctly • To overcome count to infinity • Specify a maximum hop count • Split Horizon • Route Poisoning • Hold Down timers

6. Maximum hop count • Used to overcome count to infinity • Hop count • The number of routers a packet goes through • It increases each time a packet goes through a router • RIP maximum hop count is 15 • Packet is discarded if the hop count of 16 is reached • This is because the network 1 is unreachable

7. How does the Maximum Hop Count Work?

8. Split Horizon • Solution to count to infinity • Do not send routing updates to the router you learned the information about • Reduces incorrect routing information • Reduces routing overhead

9. Route poisoning

10. Poison reverse • When route poisoning is used with triggered updates it will speed up convergence time because neighboring routers do not have to wait 30 seconds before advertising the poisoned route. • Failed routes are advertised with infinite metrics.

11. Avoiding routing loops with triggered updates • Triggered updates • Sent immediately when topology changes • Used in conjunction with route poisoning • Ensure all routers know of failed routes before any holddown timers can expire • i.e., Don’t wait for the time limit to expire

13. Preventing routing loops with holddown timers • When a router receives an update saying a route is down, it marks the route as inaccessible and starts a hold-time timer • If at any time before the holddown timer expiresan update is received from: • the same neighbour indicating the network is accessible again • Router marks network as accessible + removes holddown timer • A different neighbour with a better metric • Router marks network as accessible + removes holddown timer • A different neighbour with a poorer metric • The update is ignored

14. RIP Routing Protocol • Open standard RFC 1058 and Internet Standard (STD) 56 • Documented by: • Request for Comments (RFC) 1058 • Internet Standard (STD) 56. • RIP Version 1 (RIP v1) • Classful Routing Protocol • RIP Version 2 (RIP v2) • Classless Routing Protocol • Ability to carry additional packet routing information. • Authentication mechanism to secure table updates. • Supports variable length subnet masking (VLSM)

15. RIP has slow convergence which results in • Routing loops • Count to infinity • To reduce these RIP uses • Count-to-infinity • Split horizon • Poison reverse • Holddown counters • Triggered updates

16. Configuring RIP • Enter global configuration mode • Router#config t • Specify RIP as the routing protocol • Router(config)#router rip • Specify directly connected networks • Router(config-router)#network 10.10.10.0 • A router running RIP can be configured to • Send triggered updates independently the scheduled updates (every 30 seconds) • Router(config-if)#ip rip triggered

17. RIP Configuration Issues • Count-to-infinity • Does not have to be configured with RIP • To disable Split Horizon • Router(config-if)#no ip split-horizon • Default holddown for RIP is 180 seconds • The default RIP update 30 seconds • Can be configured for longer intervals • To conserve bandwidth • Can be configured for for shorter intervals • To decrease convergence time • Command • Router(config)#router rip • Router(config-router)#update-timer seconds

18. To prevent RIP updates leaving an interface • Router(config-router)#passive-interface Fa0/0 • To configure RIP to exchange routing information in a non-broadcast network (Frame Relay) • RIP needs to be told of other neighboring RIP routers • Router(config-router)#neighbor <ip address>

19. Configure Router to Send/Receive Packets • To configure software to send and receive Rip V1 and V2 packets • Router(config-router)#version{1|2} • To configure an interface to send RIP V1 packets • Router(config-if)#ip rip send version 1 • To configure an interface to send RIP V2 packets • Router(config-if)#ip rip send version 2 • To configure an interface to send RIP V1 and V2 packets • Router(config-if)#ip rip send version 1 2 • To configure and interface to receive RIP V1 packets • Router(config-if)#ip rip receive version 1 • To configure and interface to receive RIP V2 packets • Router(config-if)#ip rip receive version 2 • To configure and interface to receive RIP V1 and V2 packets • Router(config-if)#ip rip receive version 1 2

20. Verifying RIP configuration • show ip protocols • Shows which routing protocols are carrying IP traffic on the router • Confirms • RIP routing is configured • The correct interfaces are sending and receiving RIP updates • The router is advertising the correct networks • show ip route • Displays the routing table • Verify that routes received by RIP neighbours are installed in the routing table • Additional commands to check RIP configuration • show interface interface • show ip interface interface • show running-config

22. Troubleshooting RIP update issues • debug ip rip • Displays RIP routing updates as they are sent and received • Problems such as discontiguous subnets or duplicate networks can be diagnosed with this command

23. Discontiguous subnet

24. Duplicate Subnets Duplicate Subnets

25. Other commands to troubleshoot RIP: • show ip rip database • show ip protocols {summary} • show ip route • debug ip rip {events} • show ip interface brief

26. Preventing routing updates through an interface • network command • If it is issued for a given network using RIP or IGRP • router immediately begin sending advertisements out all interfaces within the specified network address range • Router(config)#router RIP • Router(config-router)#network 192.101.21.0 • passive interface command • Stops the router from sending updates through a particular interface to a neighbor • Router continues to listen and use routing updates from that neighbor • Prevent other systems from learning about routes dynamically • Router(config)#router RIP • Router(config-router)#passive-interface fa0/0

27. Load balancing with RIP • Allows a router to take advantage of multiple best paths to a given destination • Calculated by dynamic routing protocol • Entered statically by administrator • RIP performs “round robin” load balancing • over 6 equal-cost paths, with 4 paths being default • takes turns forwarding packets over the parallel paths

28. Interface pointer cycles through the interfaces and routes in a deterministic fashion such as 1-2-3-4-1-2-3-4-1 • RIP metric is hop count • no regard is given to the speed of the links • 56 Kbps path will be given the same preference as the 155 Mbps path

30. Equal cost routes can be found by using the “show ip route “

31. When a router learns multiple routes to a specific network, the route with the lowest administrative distance is installed in the routing table

32. When a router has multiple routes to a network • route with lowest admin distance is installed in routing table • If the router has many routes, learned via the same routing process with the same admin distance • router chooses the path with the lowest cost or metric • If the router has multiple paths to a destination with the same admindistance and cost • load-balancing can occur • Cisco IOS allows up to 6 equal cost routes in a routing table • EIGRP allows up to 4equal cost routes • Static routes always install 6 routes • BGPby default allows only 1 route • IGRP can load balance up to 6unequal links

33. Router(config-router)#maximum-paths [no.] • Cisco IOS offers two methods of load balancing: • per-packet:If process switching is enabled, • Router will alternate paths on a per-packet basis • per-destination:If fast switching is enabled • Only one route will be used for that destination address • All packets bound for a specific host take the same path • Packets bound for a different host on the same network may use an alternate route

34. Integrating static routes with RIP • Static routes • User-defined routes • Force packets to take a specific path • Useful for specifying a “gateway of last resort”; a default route • Administrative Distance (AD) • Each dynamic routing protocol has a default AD • A static route can be defined as less desirable than a dynamically learned route, as long as the AD of the static route is higher than that of the dynamic route • floating static route • A route configured on a router to take the place of the RIP route in the event that the RIP routing process fails • RIP can advertise static routes using the redistribute static command

35. IGRP features • Distance vector routing protocol • Developed by Cisco • Sends routing updates every 90 second • Sends entire table • Advertising networks for an autonomous system • Metrics used are normally bandwidth and delay • Bandwidth • Delay – amount of time to get to the destination • Reliability – reliability of link determined by exchange of keepalives • Load • MTU – maximum transmission unit of the path

36. IGRP converges faster than RIP • thereby avoiding the routing loops • IGRP does not share the hop count limitation of RIP. • Useful Commands • show interface interface • show running-config • show running-config interfaceinterface • show running-config | begin interface interface • show running-config | begin igrp • show ip protocols

37. Metric K1 represents bandwidth by default is set to 1 Metric K3 represents delay and by default is set to 1 Metric K2, K4 and K5 are set to 0.

38. show ip route shows the IGRP metric values in brackets. A link with a higher bandwidth will have a lower metric A route with a lower cumulative delay will have a lower metric

39. IGRP Routes • GRP advertises three types of routes: • Interior • Routes between subnets of a network attached to a router interface • If the network attached to a router is not subnetted, IGRP does not advertise interior routes • System • Routes to networks within an autonomous system number • The IOS derives system routes from directly connected network interfaces • System routes information provided by other IGRProuters. • System routes do not include subnet information • Exterior • Routes to networks outside the autonomous system that are considered when identifying a gateway of last resort. • Gateway of last resort is used if • a better route is not found • The destination is not a connected network • Exterior routes are also referred to as "candidate default"

40. IGRP Routes

41. IGRP stability features Holddowns Prevent regular update messages from inappropriately reinstating a route that may not be up Split horizons • It is usually not useful to send information about a route back in the direction from which it came • Prevent routing loops Poison reverse • Poison reverse updates then are sent to remove the route and place it in holddown. • With IGRP, poison reverse updates are sent only if a route metric has increased by a factor of 1.1 or greater.

42. IGRP Timers • update timer specifies • how frequently routing update messages should be sent • Default is 90 sec • invalid timer specifies • how long a router should wait in the absence of routing-update messages about a route before declaring that route invalid • Default is 3 times the update period (270 sec) • holddown timer specifies • the amount of time for which information about poorer routes is ignored • Default is 3 times the update timer period + 10 sec (280 sec) • flush timer indicates • how much time should pass before a route is flushed from the routing table • Default is 7 times the routing update timer (after 630 sec)

44. Configuring IGRP • To set IGRP routing on • Router# config t • Router(config)# router igrp 101 • Router(config-router)#network 192.101.2.0 • To turn IGRP routing off • Router(config)# no router igrp 101

45. Troubleshooting IGRP • Problems to look out for • mistyped network statement • discontiguous subnets • Incorrect Autonomous System Number • Useful commands • show ip protocols • show ip route • debug ip igrp events • debug ip igrp transactions • ping • traceroute