RIP , DEFINITION & CONFIGURATION

Router(config)#router rip
Router(config-router)#distance 150

Classful vs. Classless Routing
Classful routing protocols do not send subnet mask information with their routing updates. A router running a classful routing protocol will react in one
of two ways when receiving a route: If the router has a directly connected interface belonging to the same major network, it will apply the same subnet mask as that interface. If the router does not have any interfaces belonging to the same major network, it will apply the classful subnet mask to the route.
Classless routing protocols do send the subnet mask with their updates.Thus, Variable Length Subnet Masks (VLSMs) are allowed when using
classless routing protocols. Examples of classful routing protocols include RIPv1 and IGRP.
Examples of classless routing protocols include RIPv2, EIGRP, OSPF, and IS-IS.

To configure your router in “classful” mode:
Router(config)# no ip classless

To configure your router in “classless” mode (this is default in IOS 12.0 and greater):
Router(config)# ip classless

ROUTING METRICS – Routing protocols use their own rules and metrics to build and update routing tables automatically. Routing metrics are measures of path desirability. Different protocols use different metrics. Some common metrics are as follows:
Bandwidth The link data capacity.
Delay The time required to move the packet from the current router to the destination. This depends on bandwidth, Port delays, Congestion, and distance.
Load The amount of activity on the interface.
Reliability The error rate of each network link.
Hop count The number of routers the packet must travel through before reaching the destination.
Cost An arbitrary value based on bandwidth, expense, and other metrics assigned by the administrator.

Techniques to eliminate Routing Loops.
split-horizon route advertisement is a method of preventing routing loops in distance-vector routing protocols by prohibiting a router from advertising a route back onto the interface from which it was learned.
In short, split horizon is to prohibit a router from advertising a route back out the interface from which it was learned. Why we need to introduce this approach? Let’s see the following example first.
Distance-Vector routing protocol like RIPv1 and RIPv2 have slow convergence and count-to-infinity issues. We assume the network is: A — B — C. A, B and C are three routers running RIP routing protocol. In the convergence status, B knows there is one hop to C and A knows there are two hops to C via B. If the link is broken between B and C, since B get advertisement from A that there are 2 hops can get to C from A. Then, B updates its routing table to set hop to 3. A get advertisement from B. Then A updates its routing table to set hop to 4, so on and so on until count to infinity. The network will never be convergence status.
Split horizon is useful in this situation. Following the definition, A will not send advertisement to B since A learned route to C from B. It effectively reduces the count-to-infinity problem. To speed up convergent in RIP network, it introduce the maximum hop number is 15. In the case above, when the link between B and C is broken, B sends advertisement to A that the hop to go to C is 16, which is called split horizon with poison reverse. Then, A knows that C is unreachable and updates its routing table.
However, split horizon with poison reverse does have disadvantages.
First, it increases the size of the routing messages. In a hub-spoke network, hub is as backbone router and each spoke is as gateway router. “If split horizon with poisoned reverse is used, the gateway must mention all routes that it learns from the backbone, with metrics of 16. If the system is large, this can result in a large update message, almost all of whose entries indicate unreachable networks.”
Second, it will prevent any routing loops that involve only two gateways engaged in mutual deception. It is highly possible that three or more gateways in this situation. So, RFC 1058 introduces “Triggered updates” approach. In short, it is required to send update messages almost immediately whenever a gateway changes the metric for a route. Split horizon processing is done when generating triggered updates as well as normal updates.
Since this is nature born characters of distance-vector routing protocol, more sophisticated routing protocols are applied in the industry such as OSPF.

Hold Down timers -

Route Poison - Route poisoning is a method of preventing a network from sending packets through a route that has become invalid. When the routing protocol detects an invalid route (such as can be caused by a severed cable or the failure of a network node), all of the routers in the network are informed that the bad route has a hop count of 16, which stands for infinity. This makes all nodes on the invalid route appear infinitely distant, thereby preventing any of the routers from sending packets over the invalid route. When the path between two routers in a network goes bad, all the routers in the network are informed immediately. However, it is possible for this information to be lost, causing some routers to once again attempt to send packets over the bad route. This requires that they be informed again that the route is invalid, and again, this information can be lost. The resulting problem is known as a routing loop. Route poisoning is used in conjunction with hold downs. A hold down keeps update messages from falsely reinstating the validity of a bad route. This prevents routing loops, improving the overall efficiency of the network.
Poison reverse - In a computer network that uses the Routing Information Protocol (RIP) or other distance vector routing protocols, a poison reverse
is a way in which a gateway node tells its neighbor gateways that one of the gateways is no longer connected. To do this, the notifying gateway sets the number of
hops to the unconnected gateway to a number that indicates "infinite" (meaning "You can't get there"). Since RIP allows up to 15 hops to another gateway, setting
the hop count to 16 would mean "infinite."

Triggered Updates -

RIP PROTOCOL

Routing Information Protocol (RIP) is a true distance-vector routing protocol. RIP sends the complete routing table to all active interfaces every 30 seconds. RIP uses the hop count only to determine the best way to a remote network, but it has a maximum allowable hop count
of 15 by default, meaning that 16 is deemed unreachable. RIP works well in small networks, but it’s inefficient on large networks with slow WAN links or on networks with a large number of routers installed. There are two version of RIP. V1 & V2

RIP v1 AND v2 COMPARISONS

RIP v1 is a classful protocol, meaning that it does not send its subnet mask in routing updates. As a result , RIP v1 does not support VLSM (Variable Length Subnet Mask)
RIP v2 is a classless protocol that supports VLSM and sends its subnet mask in routing updates.
RIP v2 also send routing updates through multicast. RIP v1 broadcasts updates. RIP v2 supports manual route summarization and authentication. RIP v1 does not.

RIP TIMERS - RIP uses four timers to regulate performance and route updates.

Route update timer : The time between router updates. Default is 30 seconds.
Route Invalid Timer : The time that must expire before a route becomes invalid. Default is 180 seconds.
Route Hold Down Timer :If RIP receives an update with a hop count higher than the metric recording in the routing table, RIP goes into a hold down for 180 seconds.
Route flush Timer : The time from when a route becomes invalid to when it is removed from the routing table. Default is 240 seconds.

CONFIGURATION OF RIP

Router#config t
Router(config)#router rip
Router(config-router)#network 10.0.0.0 advertise own network
Router(config-router)#version 2 Convert to version 2
Router(config-router)#no auto-summary Turns off auto-summarization

OTHER COMMANDS
Router#show ip route Displays the routers routing table
Router#show ip protocols Displays the routing protocols and interfaces used with all routing protocols
Router#debug ip rip Show rip updates being sent and received on your router