1. Distance-Vector Routing Protocols
The distance-vector routing algorithm passes complete routing table contents to neighboring routers
It’s possible to have a network that has multiple links to the same remote network
If that’s the case, the administrative distance is checked first
If the AD is the same, the protocol will have to use other metrics to determine the best path to use to that remote network

2. Routing Information Protocol (RIP)
RIP uses only hop count to determine the best path to a network
If RIP finds more than one link to the same remote network with the same hop count, it will automatically perform load balancing
RIP can perform load balancing for up to six equal-cost links

3. Routing Information Protocol (RIP)
In following Figure, the four routers start off with only their directly connected networks in the routing table
After a distance-vector routing protocol is started on each router, the routing tables are updated with all route information gathered from neighbor routers
Each router sends its complete routing table out to each active interface

4. Routing Information Protocol (RIP)
The routing table of each router includes
Network number, Exit interface, Hop count to the network

5. Routing Information Protocol (RIP)

6. Routing Information Protocol (RIP)
In Figure above, the routing tables are complete because they include information about all the networks in the internetwork
They are considered converged
When the routers are converging, it is possible that no data will be passed
That’s why fast convergence time is a serious issue
RIP has slow convergence time
The routing table in each router keeps information regarding
the remote network number
the interface to which the router will send packets to reach that network
the hop count or metric to the network

7. Routing Loops
Distance-vector routing protocols keep track of any changes to the internetwork by broadcasting periodic routing updates out all active interfaces
This broadcast includes the complete routing table
This works just fine, but it’s expensive in terms of CPU process and link bandwidth
And if a network outage happens, real problems can occur
Plus, the slow convergence of distance-vector routing protocols can result in inconsistent routing tables and routing loops

8. Routing Loops
Routing loops can occur because every router isn’t updated simultaneously, or even close to it
Here’s an example—let’s say that the interface to Network 5 in above Figure fails
All routers know about Network 5 from Router E
Router A, in its tables, has a path to Network 5 through Router B

9. Routing Loops When Network 5 fails, Router E tells Router C
This causes Router C to stop routing to Network 5 through Router E
But Routers A, B, and D don’t know about Network 5 yet, so they keep sending out update information
Router C will eventually send out its update and cause B to stop routing to Network 5, but Routers A and D are still not updated
To them, it appears that Network 5 is still available through Router B with a metric of 3

10. Routing Loops
The problem occurs when Router A sends out its regular 30-second “Hello, I’m still here— these are the links I know about” message, which includes the ability to reach Network 5
Now Routers B and D receive the news that Network 5 can be reached from Router A
So Routers B and D then send out the information that Network 5 is available
Any packet destined for Network 5 will go to Router A, to Router B, and then back to Router A
This is a routing loop—how do you stop it?

11. Maximum Hop Count
The routing loop problem just described is called counting to infinity, and it’s caused by gossip (broadcasts) and wrong information being communicated and propagated throughout the internetwork
Without some form of intervention, the hop count increases indefinitely each time a packet passes through a router
One way of solving this problem is to define a maximum hop count.

12. Maximum Hop Count
RIP permits a hop count of up to 15, so anything that requires 16 hops is deemed unreachable
After a loop of 15 hops, Network 5 will be considered down in previous example
Thus, the maximum hop count will control how long it takes for a routing table entry to become invalid or questionable
Though this is a workable solution, it won’t remove the routing loop itself
Packets will still go into the loop, but instead of traveling on unchecked, they’ll just whirl around for 16 bounces and die

13. Split Horizon
Another solution to the routing loop problem is called split horizon
This reduces incorrect routing information/overhead by enforcing the rule that routing information cannot be sent back in the direction from which it was received
The routing protocol differentiates which interface a network route was learned from, and once this is determined, it won’t advertise the route back out that same interface
This would have prevented Router A from sending the updated information it received from Router B back to Router B

14. Routing Loops Example

15. Route Poisoning Another way to avoid network loops is route poisoning
For example, when Network 5 goes down, Router E initiates route poisoning by advertising Network 5 as 16, or unreachable (sometimes referred to as infinite)
This poisoning of the route to Network 5 keeps Router C from being susceptible to incorrect updates about the route to Network 5
When Router C receives a route poisoning from Router E, it sends an update, called a poison reverse, back to Router E
This ensures all routes on the segment have received the poisoned route information

16. RIP Limitation
Network is a T1 link with a bandwidth of 1.544Mbps, Network is a 56K link
You’d want the router to choose the T1 over the 56K link
But hop count is the only metric used with RIP routing
So two links would be seen as being of equal cost
This little problem is called pinhole congestion.

17. Routing Information Protocol (RIP)

18. RIP Timers

19. RIP Summary

20. RIP Summary

21. RIP Summary A true distance-vector routing protocol
It sends the complete routing table out to all active interfaces every 30 seconds
RIP only uses hop count to determine the best way to a remote network
Maximum allowable hop count of 15 by default, meaning that 16 is deemed unreachable
RIP works well in small networks
Inefficient on large networks with slow WAN links or on networks with a large number of routers installed

22. RIP Summary
RIP version 1 uses only classful routing, which means that all devices in the network must use the same subnet mask
This is because RIP version 1 doesn’t send updates with subnet mask information
RIP version 2 provides something called prefix routing, and does send subnet mask information with the route updates
This is called classless routing