RIPv2 and Supernetting (CIDR)

[alihassan1618]

Hi Guys, have a nice day. Now learning RIPv2 and RIPng along with IPv6. Found a interesting question while searching topic of RIPng and RIPv2. 

A network engineer is going to deploy RIPv2 on a network that uses supernet advertisement. Which difficulty could he face? 

A. Supernet is not supported in a RIPv2.
B. Only supernet component network that use VLSM are supported by RIPv2
C. Only classful supernet networks are supported by RIPv2
D. Only classless supernet networks are supported by RIPv2

as per my knowledge CIDR is supported by RIPv2. if a protocol is supporting VLSM then definitely it will also support classful. right..??  So option "C" and "D" Fails and i'm pretty sure "A" is not the correct answer here so i will go with last remaining option "B" but if this is the correct answer then please explain.

Please correct me if my understanding is wrong. 

Thanks in Advance

[Martin L]

CIDR stands for Classless Inter-Domain Routing and, true, it is based on variable length subnet masking (VLSM),

I like D and B. Problem is with word "only"

[Giuseppe Larosa · Accepted solution]

Hello AliHassan1618,

RIPv2 is classless in sense that it supports carrying the subnet mask and VLSM, but when it comes to creating a summary address like 192.168.0.0/16 it is not able to create it.

RIPv2 is bounded to Classful limits in configuring a summary address out of an interface.

In this specific context, RIPv2 should satisfy the following answers:

>> A. Supernet is not supported in a RIPv2.

>> C. Only classful supernet networks are supported by RIPv2

To be added when creating an aggregate out of an interface RIPv2 can create only one for each Major Network (Class A,B,C).

EIGRP summary-address has not these limitations: multiple summary routes  for the same classful Major network can be created.

Supernetting is supported like 192.168.0.0/16 and even a default route 0.0.0.0/0 can be created in this way (with some care for the fact the default AD of a EIGRP summary route is 5 and a route to null0 is created with same AD...)

Hope to help

Giuseppe

[alihassan1618]

Thanks for your reply but i have argument with you on this, 

Since it's clearly mentioned in features of RIPv2 that it's classless(supports VLSM). So i'm confused that you're saying RIPv2 only supports classful supernets...!!! if there is any reference or RFC where it's mentioned that "RIPv2 supports only classful supernet networks" ??? i think option "B" could be the correct answer because as i understand that RIPv2 partially supports (some components) of VLSM, as you say it is having some limitations and your gave reference of EIGRP. 

Please Correct me if i'm wrong. Also share any solid reference or RFC where they prove that RIPv2 supports only classful supernet networks to support your answer. Thanks in advance.

[Joseph W. Doherty]

At least on the Cisco equipment I've tried using RIPv2, I can confirm it's not total classless, like OSPFv2. Unsure whether what I bumped into was specific to Cisco or aligns with the RFCs, but I do recall I found it unsuitable.

[Giuseppe Larosa · Accepted solution]

Hello Alihassan1618,

I appreciate your approach because I didn't provide any reference.

I realize  I have actually  reported limits of Cisco implementation of RIPv2 manual summarization.

as a reference see the following document about IOS XE

https://content.cisco.com/chapter.sjs?uri=/searchable/chapter/content/en/us/td/docs/ios-xml/ios/iproute_rip/configuration/xe-3e/irr-xe-3e-book/irr-ipsum-adr-rip2.html.xml&searchurl=https%3A%2F%2Fsearch.cisco.com%2Fsearch%3Fquery%3DRIP%2520summary%252...

a) supernet manual summary beyond major network mask is not supported

Supernet advertisement (advertising any network prefix less than its classful major network) is not allowed in RIP route summarization, other than advertising a supernet learned in the routing tables. Supernets learned on any interface that is subject to configuration are still learned.

For example, the following supernet summarization is invalid:

Router(config)# interface gigabitEthernet 0/0/0
Router(config-if)# ip summary-address rip 10.0.0.0 252.0.0.0 

Router(config)# interface gigabitEthernet 0/0/0
Router(config-if)# ip summary-address rip 10.0.0.0 252.0.0.0

b) multiple summary routes for the same Major network  out the same interface are not supported

Each route summarization on an interface must have a unique major network, even if the subnet mask is unique. For example, the following configuration is not permitted:

Router(config)# interface gigabitEthernet 0/0/0

Router(config)# ip summary-address rip 10.1.0.0 255.255.0.0

Router(config)# ip summary-address rip 10.2.2.0 255.255.255.0

I agree that these limitations may be Cisco specific as RFC 2453 refers to avoid sending supernets to RIP v1 only neighbors.

see

https://tools.ietf.org/html/rfc2453

Section 4.3 Subnet mask

3) supernet routes (routes with a netmask less specific than the
      "natural" network mask) must not be advertised where they could be
      misinterpreted by RIP-1 routers.

Again classless routing protocol = a protocol that carries subnet mask and RIPv2 it is classless.

Supporting CIDR supernetting is not the same concept = going beyond the Major Network mask,  and Cisco implementation of RIPv2 is an example of this.

Going back to your initial post I have chosen two answers compatible with the Cisco implementation of RIPv2.

given the four available answers I do not see two other answers that can be correct at the same time (according to RFC ).

Hope to help

Giuseppe

[alihassan1618]

Thanks for your help brother. After studying your shared references and RFC i conclude that "Only classful supernet networks are supported by RIPv2". This is the best suitable answer for this question on this topic RIPv2 and Supertnet Networks. 

I appreciate the healthy discussion with you. Thanks a lot for clarification and help in this regard. 

Ali Hassan Manzoor