The Routing Information Protocol (RIP) is a simple distance-vector protocol. In order to avoid confusion, you should be aware that two versions of RIP exist – RIP version 1 (RIP), and RIP version 2 (RIPv2). For the purpose of the CCNA exam, you need to be familiar with RIP version 1, which is what we’ll look at in this section. RIPv2 will be looked at a little later in the chapter.
The first thing that needs to be made clear is that RIP is a classful protocol. What that means is that RIP will always assume the class of an address according to the standard Class A, B, and C designations. Ultimately, this means that RIP is expecting that every single host in a network is using the same subnet mask. This does not mean that RIP won’t work on a subnetted network; only that all of the subnet masks are assumed to be the same. Techniques like VLSM won’t work with RIP, mainly because when RIP sends out its routing table updates, it doesn’t include any subnet mask information.
Recall that RIP uses hop count as its metric in determining the best route to a network. Remember that RIP will always take the path with the fewest hops, regardless of factors like bandwidth, delay, or otherwise.
The maximum diameter of a RIP network is 15 hops. That means that a maximum of 15 routers can be traversed between a source and destination network before a network is considered unreachable. At the 16th hop (which RIP considers to be “infinity”), the TTL reaches 0, and a packet is discarded. Obviously this makes RIP a poor choice for very large internetworks.
RIP is also incredibly chatty. By default, a RIP router will broadcast out its complete routing table every 30 seconds, regardless of whether anything has changed. That’s not terribly efficient, and causes a great deal of unnecessary traffic. Remember that broadcasts go to all hosts in a broadcast domain, meaning that even regular computers will have to process RIP packets to some degree before discarding them. RIP may not be pretty, but it is simple.
There are a few different timers that you should be familiar with on a RIP network. There include:
Route Update Timer. The route update timer controls how often a router will broadcast routing table updates. As mentioned, a RIP router will broadcast its complete routing table every 30 seconds by default.
Route Timeout Timer. The route timeout timer specifies the amount of time that will pass before a router will mark a network as unavailable, and is set to three times the update interval (180 seconds) by default. For example, if a router doesn’t hear about network 172.16.0.0 from any other router for 180 seconds, it will mark the route as invalid. After marking a route as invalid, the information will be sent to other routers via a triggered update.
Route Holddown Timer. The route holddown timer specifies the length of the holddown timer that will be used when RIP receives information about an unreachable routing table entry. The default holddown timer is 180 seconds.
Route Flush Timer. After marking a route as invalid, a RIP router will not immediately remove the route from its routing table. Instead, it will wait until the flush timer (sometimes called the garbage collection interval) has expired. This gives the router time to let other routers know about the invalid route before removing the entry from its routing table. The default flush timer is 240 seconds.
While RIP may not be the most efficient or effective routing protocol for use on large networks, it still does the job. A big reason why RIP is so popular is because of how easy it is to set up. With as few as two commands, you can have your router fully configured for RIP.