In our last post we discussed the basics of Router Information Protocol (RIP) such as the metric and the timers. We even saw a few examples of debug and show commands to understand some of the processes that occur behind the curtain with RIP. However, for the CCNA exam you must also have good grasp on understanding RIP version 1.
First, RIP version 1 is a classful routing protocol. Primarily, classful routing protocols do not advertise the subnet mask with network addresses inside of routing updates. As shown in example 1, the debug ip rip command is running on two routers, R1 and R2. We can see here that R1 is sending a version 1 update without the subnet mask for any network entry. We also see this being received on R2. Also, to be a classful routing protocol you will always summarize to the classful boundary between routers that have a different assortment of networks. Example 1 displays that the update has class A, class B, and class C networks and not its subnets.
Additionally, you also can see that RIP version 1 updates are sent out as broadcast packets. These unsolicited packets are sent containing the metric. When the updates were the received, the process sees that network entries are away in hops.
Classful routing protocols also assume the subnet mask based on the interface the updates are received on. As show in example 2, R1 has been configured with a loopback address with five IP addresses. (The use of the secondary address command allows a router to have more than one IP address on its interfaces. This is most helpful with migration from one subnet to another.)
All of these subnets are using the subnet mask 255.255.255.0. When updates are sent between R1 and R2 you can see the subnet sent, but not the mask. When these are received on R2 (as shown in Example 4), RIP version 1 assumes the subnet mask is the same for the 10.X.X.X subnets, and lists these with the show ip route command as the network 10.0.0.0 subnetted with 255.255.255.224 (/27)
These are the basics to a classful routing protocol. We can see the disadvantages to RIP version 1, however, understanding how this protocol works is necessary for understanding the advantages of a classless routing protocol. Next time we will discuss RIP version 2.
Author: Jason Wyatte