02-19-2012 09:09 PM - edited 03-04-2019 03:20 PM
Hello
suppose we have the following diagram
using RIPv2.
RIPv2 will auto-summaries by default. i thought it will make summary to 192.168.1.0/25 since this includes 192.168.1.0/26 and 192.168.1.64/26. however i discoverd that it summaries to 192.168.1.0/24 !!
any explanaition why it is doing so?
note that according to the example in ICND2 (volume1 , page 3-58) it should be 192.168.1.0/25.
the config file looks like this:
interface FastEthernet0/0
ip address 192.168.1.1 255.255.255.192
duplex auto
speed auto
!
interface FastEthernet0/1
ip address 192.168.1.65 255.255.255.192
duplex auto
speed auto
!
interface Ethernet1/0
ip address 192.168.0.1 255.255.255.252
half-duplex
!
router rip
version 2
network 192.168.0.0
network 192.168.1.0
!
no ip http server
ip classless
Solved! Go to Solution.
02-19-2012 11:48 PM
Hello,
RIPv2 will auto-summaries by default. i thought it will make summary to 192.168.1.0/25 since this includes 192.168.1.0/26 and 192.168.1.64/26. however i discoverd that it summaries to 192.168.1.0/24 !!
Yes, your observation is correct - and your confusion is shared by many people that have tried to look more closely at the auto-summarization process
This behavior is correct. Automatic summarization does not try to find the best fitting summary network. Instead, it always performs summarization to the classful network into which the subnets belong. As your subnetworks 192.168.1.0/26 and 192.168.1.64/26 are both allocated from the C-class classful network 192.168.1.0/24, this is what the RIPv2 will advertise.
The reason for this behavior lies in the backward compatibility with classful routing protocols. As you know, classful routing protocols did not include netmasks in their updates, and instead, they relied on deducing the correct netmask. If a router wanted to advertise a subnet of a particular classful network through an interface that was itself located in a different classful network, this subnet would be automatically summarized to its own classful network and advertised in this form. The receiving router would receive this update through its interface, discover that the network inside the update is from a different classful network than the receiving interface, and in turn assign a classful netmask to it. This was the original automatic summarization in classful routing protocols that could not be deactivated because it formed a key component of their operation. With classless routing protocols, automatic summarization is just a legacy to keep backward compatibility but as you see yourself, it is not really usable, save for special environments.
Feel welcome to ask further!
Best regards,
Peter
02-19-2012 11:48 PM
Hello,
RIPv2 will auto-summaries by default. i thought it will make summary to 192.168.1.0/25 since this includes 192.168.1.0/26 and 192.168.1.64/26. however i discoverd that it summaries to 192.168.1.0/24 !!
Yes, your observation is correct - and your confusion is shared by many people that have tried to look more closely at the auto-summarization process
This behavior is correct. Automatic summarization does not try to find the best fitting summary network. Instead, it always performs summarization to the classful network into which the subnets belong. As your subnetworks 192.168.1.0/26 and 192.168.1.64/26 are both allocated from the C-class classful network 192.168.1.0/24, this is what the RIPv2 will advertise.
The reason for this behavior lies in the backward compatibility with classful routing protocols. As you know, classful routing protocols did not include netmasks in their updates, and instead, they relied on deducing the correct netmask. If a router wanted to advertise a subnet of a particular classful network through an interface that was itself located in a different classful network, this subnet would be automatically summarized to its own classful network and advertised in this form. The receiving router would receive this update through its interface, discover that the network inside the update is from a different classful network than the receiving interface, and in turn assign a classful netmask to it. This was the original automatic summarization in classful routing protocols that could not be deactivated because it formed a key component of their operation. With classless routing protocols, automatic summarization is just a legacy to keep backward compatibility but as you see yourself, it is not really usable, save for special environments.
Feel welcome to ask further!
Best regards,
Peter
02-20-2012 02:30 AM
thanks Peter
so the example in ICND2 (volume1 , page 3-58) is incorrect
it said router A will summarize :
172.16.168.0/24
172.16.169.0/24
172.16.170.0/24
172.16.171.0/24
172.16.172.0/22
in
172.16.168.0/21.
according to you it will summarize to 172.16.0.0. isn't it
02-20-2012 03:01 AM
Hello,
It seems that the ICND2 book is wrong on this topic. If these 172.16.x.x subnets are to be advertised out an interface that is in a different network than 172.16.x.x, the automatic summarization will produce just the entire network 172.16.0.0/16, exactly as you pointed out.
If more precise summaries are to be advertised, they have to be configured manually. Automatic summarization is not going to help you here.
Best regards,
Peter
02-20-2012 09:10 PM
thanks peter for your help
Find answers to your questions by entering keywords or phrases in the Search bar above. New here? Use these resources to familiarize yourself with the community: