Routing Information Protocol (RIP)

Disclaimer: 
===========
All the text written here is sometimes my interpretation of data and sometimes exact words
written in different websites, tutorials, white paper, books and other online and offline sources.
This text is mainly for my personal reference. Thanks.

	- RIP is a distance-vector routing protocol. RFC history: 1058, 2453, and 2080(RIPng).
	- It implements the Bellman-Ford algorithm. It employs the hop-count as the routing metric.
	- The maximum number of hops allowed is 15. 16 or more hops are treated as infinite distance
	  and hence unreachable.
	- RIP implements split horizon, route poisoning, and holddown mechanisms. A short description of these
	  terms is given at the end. Check out RIP-MTI.
	- RIP has been outdated by other routing protocols such as OSPF and IS-IS. But there is one advantage
	  of using RIP, that is, it is very easy to configure (we will see). RIP does not need any parameter
	  on a router unlike other protocols (Need to see this).
	- I did configure RIP on routers (in GNS3) and it is really very simple. Its a 2 step process.
		1) Enter the global config mode (by typing "enable" followed "config terminal") and
		   run "router rip". It starts the RIP routing protocol.
		2) Type "network network-number" for all the network this router is directly connected to.
		   For example: if a router is directly connected to 10.1.1.1/24 and 10.1.2.1/24, then
		   two commands to be fired are: "network 10.1.1.0" and "network 10.1.2.0". Followed by
		   "end" command. That's it. Do it on all the routers.
	- Two versions exists, v1 and v2.

RIP v1 and v2 differences:
==========================
- v1 was classful and v2 is CIDR.
- v1 used broadcast, v2 uses multicast (it reduces the load on routers/machines not participating in
  RIP.
- Check out route tags (in v2).


Split horizon:
==============
Split-horizon advertisement is a method of preventing routing loops in a distance-vector protocol by prohibiting
the router from advertising the route back onto the interface from where it was learnt.
There is a variant of Split-horizon called Split-horizon posion reverse. 
As the name suggests, it poisons the reverse path with a distance of infinite length 
and hence even if the actual path goes down, no routing loops would occur.

The main disadvantage of poison reverse is increased number of routing announcements.
 
Route Poisoning:
================
Route poisoning is a more general kind of poisoning the routes. What it means that whenever a router
discovers an invalid route, it advertises it as a route with infinite length so that the invalid route
is removed from the routing tables of all routers immediately.

Holddown Timer:
===============
Holddown timer works by having each router start a timer when they first receive an information about a network
that is unreachable. Now until the timer expires, these routers will discard any reachability information for
that network. It is helpful in cases of interface or route flapping or a better route is learned.

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