1. Distance Vector Protocols
CCNA2 Chapter 7
Distance Vector Protocols

2. Dynamic Routing Avoids configuration of static routes
Routers react to changes in the network
Routers adjust their routing tables accordingly, without the intervention of the network administrator
There are problems associated with dynamic distance vector routing

3. Distance Vector Routing Protocols
RIP is a distance vector routing protocol that is used in thousands of networks throughout the world
RIP is based on open standards and is easy to implement makes it attractive to some network administrators
RIP is a good basic protocol for networking students
IGRP is another distance vector routing protocol.
Unlike RIP, IGRP is a Cisco-proprietary protocol rather than a standards-based protocol.
IGRP is simple to implement
IGRP is a more complex routing protocol than RIP and can use many factors to determine the best route to a destination network.
NOTE: for our PacketTracer labs, we’ll use EIGRP)

4. Distance vector routing protocols
Require routers to forward their entire routing table when passing along updates
Routing table information is forwarded to neighbor routers, which continue to forward the information to their neighbors..
These routing tables include information about the total cost of a route and the logical address of the first router on the path to each network contained in the table.
Routers need to update the information in their routing tables to make good path determination decisions.
Updates may be initiated when topology changes occur
Changes in a network affect the decisions made by a router.
A router may be taken off line for upgrades or repairs or an interface on a router may go down.
If not aware of the changes that have occurred in a network, routers may switch packets to interfaces that are no longer connected to the best route.
Distance vector routing protocols typically send out updates at certain time intervals
Every 30 seconds for RIP..
Every 90 seconds for IGRP

5. 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 by RIP.

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

7. Split Horizon Split Horizon –
Disables the router from sending information about a ‘failed’ route in the routing table. This is done by not sending the information through the same interface that it learned about the failed route
That is, it would prevent Router A from sending the updated information if received from Router B back to Router B
Network is down B A
Get to network via B
Is Down!

8. 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.

9. One Solution to Count to Infinitive
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 different neighbor with a better metric than the original network entry, the holddown is removed and data is passed
However, if an update is received from the same neighbor router before the holddown timer expires, and it has a lower metric than the previous route, the update is ignored and the holddown timer keeps ticking

10. Configure RIP s0 s1 e0 RouterA# config t RouterA(config)# router rip
net s0
net s1 e0
net
RouterA# config t
RouterA(config)# router rip
RouterA(config-router)# network
RouterA(config-router)# network
RouterA(config-router)# network
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.

11. 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

12. 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, do
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

13. RIP Configuration Issues
Router# config term
Router(config)# router rip
Router(config-router)#
timers basic
update
invalid
holddown
flush
Intervals between updates
route is invalid after receiving no updates in secs
holddown time
when route is flushed from table
update – 30 seconds
holddown seconds
Administrative Distance - 120

14. RIP Configuration Issues
NOTE for RIP:
Its metric to determine a route to a destination is the hop count.
As a packet goes from router to router, RIP increments a counter called hop count.

15. RIP Configuration Verification
Use the following commands to make RIP verifications:
show ip route
The routing table will have “R” by the routes determined by the RIP routing protocol
show ip protocols
This will verify:
RIP routing is configured (which protocol is configured)
Which interfaces are sending & receiving RIP updates
Which network it is sending information to

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

17. Classless Routing NOTE:
Router Rip cannot handle Classless Routing, but Rip ver2 can.
A supernet route (classless route) is a route that covers a greater range of subnets with a single entry. An example a supernet of /16 could be /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.

18. 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]

19. Administrative Distance
Route Source Default Distance
Connected interface 0
Static route 1
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

20. Floating Static Routes
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.
Example:
RouterA(config)#ip route

21. RIP – Redistribute Static Routes
For RIP, if a static route is assigned to an interface that is not one of the networks defined in a network command, no dynamic routing protocols advertise the route. Use redistribute static command.
To redistribute static default route, must use the default-information originate command.
Example:
RTA(config)# ip route s0
RTA(config)# router rip
RTA(config-router)# default-information originate

22. 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, more it is under load)
sends updates every 90 seconds
maximum hop count is 255 (default 100)

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

24. Setting IGRP Basic Timers
Router# config term
Router(config)# router igrp 100
Router(config-router)# timers basic
update
invalid
holddown
flush
Intervals between updates
route is invalid after receiving no updates in secs
holddown time
when route is flushed from table
Router(config-router)# timers basic [Default settings]

25. Configure IGRP s0 s1 e0 RouterA# config t
network address s0
network address s1 e0
network address
RouterA# config t
RouterA(config)# router igrp 101
RouterA(config-router)# network
RouterA(config-router)# network
RouterA(config-router)# network

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