Friday, December 31, 2010

Lesson 35 - Routing Information Protocol Part1

Now, that you have learned the principles regarding Distance Vector algorithm, it's time to look at the operation of the oldest distance vector routing protocol: RIP.

Routing Information Protocol is seldom used these days. It has been superseded by more sophisticated protocols (OSPF, EIGRP, IS-IS). However, Cisco still keeps it in the curriculum. All professionals up to the CCIE level (as of writing this post) need to know how it works. In order to enable RIP you must choose which version you want to run. By default, if you configure RIP the version used is version 1. You can change it to version 2 which is more preferred if one want to RIP to begin with. The below table summarizes the main feature of both versions.

Table 1- RIPv1 and RIPv2 Features.

Classful, Classless, and Summarization
Routers share their information (routing table) with their neighbors using advertisements. It is important to know that the shorter the routing table is, the less time a router needs to process the incoming packets in order to find the outbound (egress) interface and expedite them.

If your network design uses proper, consecutive IP scheme, a router can advertise fewer prefixes (routing entries) to their neighbors represented by so called summary routes. A summary route represents multiple more specific destinations. But the router which receives this summary route can still send packets towards the subnets that are being summarized. Consider the following picture (pic. 1).

Pic. 1 – Route Summarization.

Icons designed by: Andrzej Szoblik -

In the pic. 1, there are four subnets located behind R1. Router R1 can advertise them ‘as they are’ towards R2. This way, R2 will populate its routing table with all four of them (,,, with R1 as the gateway to these subnets. However, the subnet addresses behind R1 are designed to be represented by one summary route. This route is going to represent exactly these four subnets. So R1, instead of advertising the four of them, will announce only one address:

So the question is: how to summarize subnets/networks?

Assuming that IP addresses subnets/networks to be summarized are consecutive numbers when converted to binary, the rule to create the summary route presented in the pic.1 can be applied using three steps.

Step 1
Convert all consecutive summary candidates into binary. List them from the lowest number to the highest like presented in pic.1. = 00001010.00000001.00000000.00000000 = 00001010.00000001.00000001.00000000 = 00001010.00000001.00000010.00000000 = 00001010.00000001.00000011.00000000

Step 2
Find the number of bits that is identical and draw the line to see how many bits do not change. = 00001010.00000001.00000000.00000000 = 00001010.00000001.00000001.00000000 = 00001010.00000001.00000010.00000000 = 00001010.00000001.00000011.00000000

22 bits do not change = the length of the network mask representing all four entries.

Step 3
Advertise the address with the lowest number in the range using network mask length achieved in the step 2 (bits that never change give you the length of the network mask). The actual commands will be different depending which protocols you use. I will explain them in due time. = 00001010.00000001.00000000.00000000
Netmask     = 11111111.11111111.11111100.00000000 =

Optionally, you can check if the summary route covers exactly the prefixes. Notice that the two of the bits that change in the third octet will tell you how many addresses have been represented by the summary route (aka aggregate). Look at the bits that do change (in red). = 00001010.00000001.00000000.00000000 = 00001010.00000001.00000001.00000000 = 00001010.00000001.00000010.00000000 = 00001010.00000001.00000011.00000000

There are two bits that change in this range in the third byte. This number of bits with the exponent of 2, tells you how many subnets/networks are being summarized. Here: 2 to the power of 2 = 4. Exactly, four subnets have been summarized using prefix So this prefix represents exactly the subnets in question.

Now, that you know how to summarize consecutive subnets/networks, let’s see what automatic summarization to the class boundary will look like if you use RIPv1 and have different classes of IP addresses in your desing. Consider the picture presented below (pic. 2).

Pic. 2 – RIPv1 Automatic Summarization to the Class Boundary.

Icons designed by: Andrzej Szoblik -

The routers R1 and R3 are boundary routers between two different classes. They will advertise class A address ( since RIPv1 does it by default if the outbound interface (the one to send the advertisement) belongs to different class than the subnet being advertised. This will inevitably create confusion as far as R2 is concerned. It will think that both R1 and R3 are gateways to the same class A network As a result of that, the packets destined to the subnets and will not be delivered properly.

In RIPv2 you can disable this automatic summarization by using ‘no auto-summary’ keyword in the ‘config-router’ configuration context. This option makes RIPv2 classless routing protocol (it can advertise subnets of major classes). Classless routing protocols can advertise full prefix (IP address and the network mask) without summarizing it to the class like shown in the pic. 2.

RIPv1 Configuration
In order to configure RIP in our design, we need to perform two steps.

Step 1
Enable RIPv1 protocol process in the global configuration context.

Step 2
Using ‘network’ statement in ‘config-router’ context, instruct the router which interfaces should participate in the RIP process. RIP will run on these interfaces. In RIP protocol the ‘network’ statement has to be followed by the Class A, B, or C address which instructs the router which interfaces should be RIP-enabled.

The ‘network’ statement is one of the most misinterpreted keywords in Cisco IOS. RIP, OSPF, EIGRP use the ‘network’ statement to inform the router which interfaces should be enabled for the routing protocol. The ‘network’ statement does NOT advertise anything. Only when BGP protocol is used, the ‘network’ statement actually advertises the prefixes since BGP is an application using TCP (port 179) and is not run on the interfaces like all IGP protocols are.

Having said that, let’s enable RIPv1 in my design (pic. 3).

Pic. 3 – Topology Diagram.

Icons designed by: Andrzej Szoblik -

R1 Configuration:
R1#configurer terminal
R1(config)#router rip

R2 Configuration:
R2#configurer terminal
R2(config)#router rip

R3 Configuration:
R3#configurer terminal
R3(config)#router rip

R4 Configuration:
R4#configurer terminal
R4(config)#router rip

R5 Configuration:
R5#configurer terminal
R5(config)#router rip

I suggest that you analyze the topology, configuration and information in this post. There will be few issues we will come across here using RIPv1 (one of them I have not discussed yet but it will rear its ugly head on R3).

I will explain these problems in my next post. I will also show you how to check the configuration of RIP ('show' and 'debug' commands). Also, I will enable RIPv2 to show you the differences in their operation, and how to optimize and verify its operation.

Since, today is New Year's Eve, let me wish everyone:

Happy and Prosperous New Year 2011.
God bless you!


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