CIDR

[P-vanVliet]

I'm new here, please be gentle

I'm confused..and i need some help please

CIDR - Classless inter-domain routing

Routing between network segments does not rely on the standard subnet mask...(/8, 16, 24)

RIP version 2 support routing on a 192.168.0.0/28 link                    so RIPv2 support CIDR

RIP version 2 is not allowing to route on a 192.168.0.0/16 link         so RIPv2 does not support CIDR

Is RIPv2 only supporting CIDR/subnetting and not CIDR/supernetting....

How do you called it then, only naming it CIDR doesn't cut it

Is there a difference in CIDR subnetting/supernetting in OSPF

Thanks,

[Peter Paluch]

Hi Peter,

The inability of running RIPv2 over 192.168.0.0/16 link is not a limitation of RIPv2 itself! You are facing a limitation of the RIPv2 implementation in Cisco IOS. Mostly because of backward compatibility reasons with RIPv1, you can not add networks to RIPv2 whose netmasks are shorter than the corresponding classful mask using the network command. However, RIPv2 itself has absolutely no problems with running over any network and carrying information about both subnets and supernets. It's just how the RIPv2 is programmed in Cisco's IOS that exhibits this limitation.

As a proof, when you manage to get the RIPv2 running over a 192.168.0.0/28 link, create a loopback interface on one of the routers configured with IP of 172.16.1.1 255.240.0.0. Then, in the RIPv2 configuration, use the following commands:

no auto-summary

redistribute connected

They will turn off automatic summarization and inject all directly connected networks, including the 172.16.0.0/12, into RIPv2. Note that the 172.16.0.0/12 network is a supernet (an aggregate), and yet, thanks to the redistribute command, this network will be injected into RIPv2 and carried to the other router.

Please feel welcome to ask further!

Best regards,

Peter

[P-vanVliet]

Peter

Thank, for the answer, but i'm still a little confused

When i try to configure this in a lab, RIPv2 is capable of routing supernets on a major class link even a subnet link but not over a supernet link..

a picture says more than a thousand words

So if CIDR means classless inter domain routing, why does it work on a major class or subnet link and not an a supernet link..

Maybe my defenition of CIDR incorrect..

[Peter Paluch · Accepted solution]

Hello Peter,

So if CIDR means classless inter domain routing, why does it work on a major class or subnet link and not an a supernet link..

It is because if the link is addressed using a supernet, the network command in RIPv2 configuration does not apply to it. You surely know that in order for a routing protocol to communicate over an interface, you must configure that interface's network using the network command. Sometimes we call this "adding an interface to a routing protocol". Cisco's RIPv2 implementation in IOS has a limitation that the network command looks only for interfaces whose networks are either exactly equal to the classful (major) network specified in the command or are subnets thereof. It does not apply to interfaces whose network is a supernet of the network specified in the network command. This means that you are unable to add a supernet-addressed interface to RIPv2 using the network command. That's why it does not work with supernet-addressed link - the interface on this link simply cannot be added to RIPv2.

What you experience in your setup is just a limitation of Cisco's RIPv2 implementation that does not allow you to add the supernet-addressed link to RIPv2 so that it can communicate over it. However, the RIPv2 alone in its basic specification is perfectly fine with running over any kind of link. This has nothing to do with CIDR whatsoever.

Please feel welcome to ask further!

Best regards,

Peter

[Joseph W. Doherty]

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Posting

Is there a difference in CIDR subnetting/supernetting in OSPF

Yes (laugh), Cisco's OSPF doesn't have the issues Peter notes with Cisco's RIPv2.

Note: as I understand it, CIDR and subnetting/supernetting are two different things; the latter subdividing/combining classful networks.

PS:

BTW, Peter, thanks.  I've bumped into these issues lab testing RIPv2 on Cisco routers and switches, but I had assumed they were generic to RIPv2 for backward compatibility with RIPv1.  Never investigated them further, so didn't realize they were Cisco implementation specific.