Classful vs Classless Routing Protocols

This topic is to discuss the following lesson:

https://networklessons.com/cisco/ccna-routing-switching-icnd1-100-105/classful-vs-classless-routing-protocols/

Very clear and easy to understand thank you.

Crystal clear. You have explained it very nicely.

Agree with the comments above. Clear definition. Thanks.

Thanks…

Great explanation. Cheers dude

great help !!! thanks :slight_smile:

Hi Rene,

i have configured rip version 2, i could not see the advertisement of “172.16.2.0 subnet with a subnet mask” and 172.16.1.0 subnet with a subnet mask.

Configure “no auto-summary” under router RIP. By default RIP version 2 behaves classfull.

Hi Rene,
In what scenarios we can use classful and classless routing protocols?
Is it beneficial to use RIPv1 in small networks??

Hi Sham Rao,

There is no good reason to use a classful routing protocol nowadays. Keep in mind that some of these concepts are really old. The RFC for RIP for example is from 1988…

We didn’t have things like VLSM back then :slight_smile:

Rene

So simple and beautifully explained. Thanks Rene.

Very simple and easy to understand…Thank you

Rene,

Since you mentioned in the video that RIP is no longer on the CCNA exam, is there anywhere else that classful routing might come up in the exam?

Thanks again, and nicely done,

Chris

Hi Chris,

Yes there is, EIGRP also supports classful and classless routing.

Rene

If I remember well, the same thing if we put 172.17.1.0/24 and 172.17.2.0/24 insteadof 192.168… Because 172.17 … Is another major network… Right??

Hi Francesco,

Those two networks are from the class B range so yes, they would be summarized to 172.17.0.0/16.

192.168.X.0/24 is no problem btw…this is a class C network.

Rene

Hi Rene ,

Is “Discontiguous Network” issue is due mainly classfull routing lookup ? , because it will do look up only classfull manner , like router has to do a look up for destination network 10.11.1.0 /24 , as the 1st octate is belongs to class A range it wont check further the the mask part , if network not existing it will drop it ?

Can you guide on “Discontiguous Network” and its solution aprroach .

Regards,
Sameer.

Hello Sameer

For the explanation, I will use the subnets used in the lesson.

The problem with classful networks is that if there is a destination of 172.16.1.0/24 this will be installed in the routing table as 172.16.0.0/16 since 172.16.X.X is a class B network. This works fine as long as all destinations of 172.16.0.0/16 are to be routed via the same interface.

However, if you have a case such as this, discontiguous networks with classful routing presents a problem:
image
Assuming a classful routing protocol is being used, R3 will inform R2 that it has a destination to 172.16.2.0/24 and R2 will put the following into its routing table:

to reach 172.16.0.0/16, route via Fa0/1 with a next hop IP of 192.168.23.3

But R1 also sends a routing update to R2 stating that it has a route to 172.16.1.0/24 so R2 will put the following into its routing table:

to reach 172.16.0.0/16, route via Fa0/0 with a next hop IP of 192.168.12.1

During the next routing updates, R3 and R1 will alternately inform R2 that they have routes to these destinations, while all the time R2 is installing the classful network into the routing table, causing incorrect routing to occur.

So, this is why discontiguous networks can cause problems with classful routing.

In order to solve such issues, there are two solutions:

  1. The first involves adjusting your addressing scheme so that networks are not discontiguous, that is, all class A networks can be reached via a single interface, and similarly for class B and class C. This is not always possible and can be somewhat restrictive.
  2. The ideal solution, and the one that is considered best practice, is to use a classless routing protocol so that you can define specific subnets within classful networks and route in a classless manner.

The second choice should always be the selected solution in all modern IP networks.

I hope this has been helpful!

Laz

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Thank you Laz!!
Regards,
Sameer.

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