Classful vs Classless Routing Protocols

Routing protocols can be classful or classless:

  • Classful routing protocols DO NOT send the subnet mask along with their updates.
  • Classless routing protocols DO send the subnet mask along with their updates.

Let’s take a look at an example to emphasize this:

three cisco routers

We have three routers and a bunch of networks. Take a close look at the networks that we have here:

  • /24
  • /24
  • /24
  • /24

Remember the class A,B and C ranges?

  • and fall within the class B range.
  • and fall within the class C range.

What subnet mask do class B and class C networks have by default?

  • Class B:
  • Class C:

A classful routing protocol will not send the subnet mask along with the routing update so this is what will happen:

three routers configured for classful routing

R1 and R3 don’t send the subnet mask along with the routing update so it will advertise the classful network which is in this case. So what happens with R2? It thinks it can reach the network by sending packets either left or right and if the metric is equal it will try to load-balance. Obviously this is going to cause problems.

Classless routing protocols advertise the subnet mask along with their updates:

classless routing

As you can see R1 is now advertising its subnet with a subnet mask. R3 is advertising its subnet with a subnet mask as well.

Which routing protocols are classful or classless? I’m going to give you an overview:

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Forum Replies

  1. 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 /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 .


  2. 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 this will be installed in the routing table as since 172.16.X.X is a class B network. This works fine as long as all destinations of 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:


    ... Continue reading in our forum

  3. Thank you Laz!!

  4. Great explanation, I really get this now, thanks!

  5. Hi,
    The below from Eigrp module
    EIGRP will check if you have any subnets that fall within the range of your network command(s) and if so, it will advertise the classful A, B or C network to its neighbors. It will only summarize routes from subnets on its own interfaces, not the ones you learn from other routers.

    In this article you are saying Eigrp is classless

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