1. CCNA 2 v3.1 Module 7 Distance Vector Routing Protocols

2. Objectives

3. Distance Vector Updates

4. Problem: Routing Loops
Routing loops can occur when inconsistent routing tables are not updated due to slow convergence in a changing network.

5. Problem: Counting to Infinity
A sends update to B and D
E sends update to A
C still unaware of failure, advertises route via B X
Network fails
D updates its routing table to include route from C, and forwards this incorrect information to A.

6. Solution: Defining a Maximum for Infinity
The routing protocol permits the routing update to propagate (loop) until the metric exceeds its maximum allowed value. 
In RIP, if the hop count exceeds the maximum of 15 hops the packet is discarded and the network is considered unreachable.

7. Solution: Split Horizon
Split Horizon is another mechanism used to avoid routing loops.
Information about routes is prevented from being advertised out the router interface through which the information was received.

8. Solution: Route Poisoning
Poison Reverse updates are used to overcome large routing loops by sending explicit information when a subnet or network is not accessible.
Sets the hop count for that network to infinity (one more than the maximum).
Advertise it with

9. Solution: Triggered Updates
Router detects topology change, immediately sends update to adjacent routers – doesn’t wait for the update timer to expire.
Wave of updates (single route not routing tables) propagates throughout the network.
Ensure all routers know of failed routes before holddown timers expire.

10. RIP Triggered Updates Triggered updates can be configured for RIP
int s0
ip rip triggered
Periodic broadcasts are suppressed on the interface where triggered updates are enabled

11. Solution: Holddown Timers
The count to infinity problem can be avoided by using holddown timers.
When the router marks a route inaccessible it starts a holddown timer.
if a route with a worse metric is received for a route that is marked as inaccessible ignore it until holddown timer expires (see next slide)

12. Wait until holddown timer expires
Holddown Timers Is
update from same neighbor?
Update received: network inaccessible, start holddown timer
Update received: network accessible again
Yes
Network accessible,
remove holddown timer No
Different
neighbor,
better metric? No
Ignore update,
Wait until holddown timer expires
Yes

13. Key Characteristics of RIP

14. RIP Two versions of RIP: RIP v1: Classful routing protocol
Does not include subnet masks in updates
RIP v2: Classless routing protocol
Carry additional packet routing information.
Authentication mechanism to secure table updates.
Supports variable length subnet masking (VLSM).
RIP updates occur every ________
The maximum number of hops in a path is ____
RIP implements split horizon and holddown mechanisms.
30 secs.
15.

15. RIP Commands We will now cover the following in more detail
Using router rip to enable and start RIP
Use the network commands to seed the routing table
Monitoring IP packet flow using the show ip protocol command
Displaying the routing table using the show ip route command

16. Configuring RIP

17. Using the ip classless Command
I know some /24 subnets, but not /24
So packet is dropped

18. IP Classless
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. So the router will drop the packet even if there is a default route.
Configuring ip classless on the router resolves this problem by allowing the router to ignore the classful boundaries of the networks in its routing table and simply route to the default route.

19. RIP Configuration Issues
To reduce routing loops and counting to infinity, RIP uses the following:
Defining infinity
Split horizon
Route poisoning & poison reverse
Triggered updates
Holddown timers

20. Preventing Routing Updates through an Interface

21. The show ip protocols Command

22. The show ip route Command

23. Typical RIP configuration errors:
incorrect network statement
discontiguous subnets (bad planning)
can result in wrong route being advertised
split horizons

24. Troubleshooting RIP Update Issues
debug ip rip
show ip rip database
Displays summary address entries in the RIP routing database
show ip protocols {summary}
show ip route
shows routing table
debug ip rip {events}
send and receive info
show ip interface brief
summarized information on protocols per interface

25. Debug Router RIP

26. Load Balancing with RIP
Here each path is considered equal by RIP metric (2 hops)

27. RIP Load Balancing Router(config-router)# maximum-paths 5
Load balancing allows a router to simultaneously use multiple paths to a destination. RIP can load balance over 6 equal-cost paths, (default 4 paths).
Router(config-router)#
maximum-paths 5
RIP performs what is referred to as “round robin” load balancing:
If process switching is enabled, paths alternate on a ___________ basis.
If fast switching is enabled, paths alternate on a _______________ basis.
per-packet
per-destination

28. Administrative Distance

29. Integrating Static Route with RIP
Static routes are user-defined routes
A router running RIP can receive a default route via an update from another router running RIP.
Another option is for the router to generate the default route itself.
The administrator can override a static route with dynamic routing information by adjusting the administrative distance values.
The default administrative distance for RIP is 120
By default the administrative distance of a static route is 1
and that of a directly connected link is 0 (most desirable).
Static routes are not normally distributed but distribution can be enabled
Router(config) router rip
Router(config-router)# redistribute static

30. Floating Static Route
A backup route that takes over if the original route fails
A route that has an AD of 130 will not be used until the RIP route with an AD of 120 is not available.

31. IGRP
Interior Gateway Routing Protocol is a DV protocol proprietary to Cisco.
IGRP sends routing updates at 90 second intervals, advertising networks for a particular AS.
Key design characteristics of IGRP are a follows:
The versatility to automatically handle indefinite, complex topologies
The flexibility as it has multiple metric which can handle different bandwidth and delay characteristics
Scalable to large networks (max TTL 255)

32. IGRP Features

33. IGRP Commands Configure using the router igrp and network commands
The show ip protocol command
For monitoring IP packet flow
– Displays parameters including metric values K1 to K5.
K1= bandwidth, K3= delay.
The show ip interfaces command
The show ip route command
Shows the actual metric values for each specific route
The debug ip igrp command

34. IGRP Metrics Bandwidth – The lowest bandwidth value in the path
Delay – The cumulative interface delay along the path
Reliability – The reliability on the link towards the destination as determined by the exchange of keepalives
Load – The load on a link towards the destination based on bits per second
MTU – Maximum Transmission Unit for the path

35. IGRP Routes: Interior, System, & Exterior

36. Interior Exterior System
Routes between subnets of a network attached to a router interface.
If the network is not subnetted, IGRP does not advertise them.
Exterior
Routes to networks outside AS.
Used to identify default gateway.
Different routers may choose different routes as the gateway of last resort.
System
Routes to networks within the AS.
Derived from directly connected interfaces and information from other IGRP-speaking devices.
Do not include subnet information.

37. IGRP Stability Features
Holddowns, Split horizons, & Poison-reverse updates
With IGRP, poison reverse updates are sent only if a route metric has increased by a factor of 1.1 or greater.

38. Show IP Protocols

39. Show IP Protocols IGRP default timer values Router# show ip protocols
Routing protocol is “IGRP 101”
Sending updates every 90 seconds, next due in 51 seconds
Invalid after 270 seconds, holddown 280 seconds, flushed
after 630 seconds
< output omitted >
How frequently routing update messages should be sent
Time before a route is flushed from the routing table (Default: 7 x U)
How long to wait in the absence of specific updates before declaring a route invalid (Default: 3 x U)
Amount of time for which information about poorer routes is ignored (Default: 3 x U + 10)

40. Configuring IGRP

41. Migrating RIP to IGRP

42. Migrating RIP to IGRP
Consider this network on which RIP is already running:
/24
/24 A B
/24
Next update
A# show ip route
< output omitted >
C /24 is directly connected, FastEthernet0/0
C /24 is directly connected, Serial0/0
R /24 [120/1] via , 00:00:29, Serial0/0
IGRP is then configured on both routers, example:
A(config)#
router igrp 101
A(config-router)#
network
A(config-router)#
network
Once A has received an IGRP update from B:
A# show ip route
< output omitted >
C /24 is directly connected, FastEthernet0/0
C /24 is directly connected, Serial0/0
I /24 [100/80135] via , 00:00:69, Serial0/0
AD and metric

43. Verifying IGRP Configuration
Some commands for checking IGRP configuration are as follows:
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

44. Troubleshooting IGRP
The following commands are useful when troubleshooting IGRP:
show ip protocols {summary}
show ip route
debug ip igrp events IGRP protocol events
debug ip igrp transactions IGRP protocol transactions
ping
traceroute

45. Summary