1. CCNA 2 v3.1 Module 7

2. Module 7 Distance Vector Routing Protocols RIP and IGRP
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 To overcome 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
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 expires an 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
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
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
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)#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
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 admin distance and cost
load-balancing can occur
Cisco IOS allows up to 6 equal cost routes in a routing table
EIGRP allows up to 4 equal cost routes
Static routes always install 6 routes
BGP by default allows only 1 route
IGRP can load balance up to 6 unequal 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
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 interface interface
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 IGRP routers.
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 invalid 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 To turn IGRP routing off
Router# config t
Router(config)# router igrp 101
Router(config-router)#network
To turn IGRP routing off
Router(config)# no router igrp 101

45. Troubleshooting IGRP Problems to look out for Useful commands
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