Is this Network work in RIP v1

Answered Question
May 15th, 2007
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hi every body ,

i have configured this network topology using boson software and RIP V1 routing protocol with following settings:


1) pc#1 has this subnet 192.168.1.16/28

2) pc#2 has this subnet 192.168.1.32/28

3) pc#3 has this subnet 192.168.1.48/28

4) pc#4 has this subnet 192.168.1.64/28


the network between router A and B is subnet 192.168.1.0/25


the network between router B and C is subnet 192.168.1.128/25


5) pc#11 has this subnet 192.168.1.144/28

6) pc#12 has this subnet 192.168.1.160/28

7) pc#13 has this subnet 192.168.1.176/28

8) pc#14 has this subnet 192.168.1.192/28


as I know this topology doesn't work with RIP v1 but in my lab it worked....



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Correct Answer by Richard Burts about 10 years 1 month ago

Ibrahim


I believe that we are finally describing things in the same way in this discussion and you are exactly correct.


In several of your posts you phrase the question in terms of the subnets on PCs. And I have said several times that it does not matter what is configured on PCs but matters what is on router interfaces. It was not clear to me in your original picture whether each PC was connected to a unique interface or whether the PCs shared a common interface. From the continuing discussion I recognize that you were using them as each PC connected to a unique interface with a corresponding subnet. Now that I understand this I completely agree with you that it would not work on real routers running RIPv1 and that if it works in the virtual lab then it is a problem in the virtual lab.


HTH


Rick

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Richard Burts Tue, 05/15/2007 - 14:14
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ibrahim


It would help us understand the situation better and perhaps to give you a correct answer if you would provide a few more details. Especially we need to know how many interfaces on each router and how the addresses on the interfaces are configured. We also need to know how you configured routing on the routers. Perhaps you could just post the configuration of the routers if they are not complicated. It might also help if you post the output of show ip route from routers A and C.


I guess that you believe that there is a problem because the routers use one mask and the PCs use a different mask for addresses in the same address space. However the routers do not know and do not care what mask is configured on the PCs. So this is not an issue.


HTH


Rick

alwahidi22 Wed, 05/16/2007 - 09:33
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hi Richard ,


unfortunately i didn't save my configuration , but i worked it again with router A first ...

first i assign this ip address to interface s1/0 of router A : 192.168.1.1/25 after that i'm trying to assign this ip address: 192.168.1.17/28 to interface e0/0 wich pc1 attach to it but my virtual simulator told me that you have an overlap with interface s1/0 but as i said that interface s1/0 have configured in previous virtual lab with the same ip address without any comments....


i think that this time my virtual lab is correct ???? is it ?

mohammedmahmoud Wed, 05/16/2007 - 10:58
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Hi Ibrahim,


Your scenario should work fine as it is not a discontigous network plus the subnet mask is identical all over the network, Rick has explained this issue perfectly in another thread, please do check it:


http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Network%20Infrastructure&topic=LAN%2C%20Switching%20and%20Routing&CommCmd=MB%3Fcmd%3Ddisplay_location%26location%3D.1dde7f5c




HTH, please do rate all helpful replies,

Mohammed Mahmoud.

alwahidi22 Wed, 05/16/2007 - 11:12
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Hi mohammad,


But i have different subnets masks over the network i have /25 and /28 subnets for 192.168.1.0 network.. so i think that it will not work ......


Regards

ibrahim alwahidi

mohammedmahmoud Wed, 05/16/2007 - 11:34
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Hi Ibrahim,


Ok i think it should work, and i hope that Rick can comment on this post:


Since the subnet masks are identically between the routers A, B and C then interface A IP subnet is sent correct to router C and vice versa.


Since the /28 has an unmatched masks with the interface that should advertise them while being part of the same major network, they won't be advertised between any routers.


But the routes exchanged between the 3 routers will do the reachability job.



http://www.cisco.com/en/US/tech/tk364/technologies_tech_note09186a0080093f1e.shtml

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080093fd6.shtml

http://www.cisco.com/en/US/tech/tk364/technologies_tech_note09186a0080093fd8.shtml


HTH,

Mohammed Mahmoud.

alwahidi22 Wed, 05/16/2007 - 11:59
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Hi mohammad ,


Thank you for replying immediately and thank you for the useful links you sent to me...

I know that routers don't have a problem with exchanging between each other but if pc1 ping pc11 it will not see it ... is it ???????!!!!!!!!

Richard Burts Wed, 05/16/2007 - 13:50
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Ibrahim


I am still not sure what you believe the issue is. Perhaps you can clarify?


As Mohammad says I believe that this will work. As far as the routers are concerned and as far as RIPv1 is concerned there is only one mask being used (unless you have configured the PCs to run RIP). The only real effect of putting a mask on the PC that is different than the mask on the router is to introduce ambiguity about what is in the local subnet and what is not. As far as the PC is concerned if the destination is in the local subnet it will ARP for the destination and if the destination is not in the local subnet (according to the PC) it will forward to its default gateway. So it should work.


HTH


Rick


alwahidi22 Thu, 05/17/2007 - 06:48
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Hi Rick,


I know that i'm annoying you ... but i have configured my router A and router B e0/0 through e0/4 which are the router interfaces to the subnet that are the same subnet of the pcs on both sides....


so as i know that subnets in both router A and B Ethernet interfaces will overlap with router A and B serial interfaces which were 192.168.1.0/25 and 192.168.1.128/25...


is my scenario correct???!!!


Just i want to know if it's correct to help me in my CCNA exam preparation

Richard Burts Thu, 05/17/2007 - 07:12
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Ibrahim


This is now a significantly different question than what you started with. If you have subnets on LAN interfaces with one subnet mask and have serial interfaces in the same network with a different mask is very different from having different masks on the PCs.


You ask if the scenario is correct. I would ask clarification: is it correct for what? If you are trying to build a "normal" router config then no it is not correct. If you are trying to build a config to explore the behavior of the routing protocol then perhaps it is.


In terms of CCNA preparation it is not correct to have overlapping subnets on serial interfaces and on LAN interfaces. It is also not correct to have one subnet on a serial interface and to have a different subnet on the interface on the other end of the serial interface.


If you want to explore the effect of different masks in RIPv1 I would suggest a somewhat different configuration to try:

on router A configure:

e0/0 192.168.1.16/29

e0/1 192.168.1.32/29

e0/2 192.168.1.48/29

e0/3 192.168.1.64/29

serial 192.168.1.128/30

on router B configure

e0/0 192.168.1.144/29

e0/1 192.168.1.160/29

e0/2 192.168.1.176/29

e0/3 192.168.1.192/29

serial 192.168.1.128/30

and configure RIPv1 as the routing protocol.

This avoids the issue of overlapping subnets between serial and ethernet interfaces and creates different subnet masks.


If you try this the results should show you something important about the behavior of RIPv1 when different subnet masks are used within the same network.


HTH


Rick

alwahidi22 Thu, 05/17/2007 - 08:34
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Hi Rick,


this is the output of RouterAA and RouterBB routing table and running config file ....


I think that it shouldn't work at all but in my virtual lab it worked...


so ... may be the problem with my virtual lab



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Richard Burts Thu, 05/17/2007 - 08:48
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Ibrahim


I agree that there is a problem with the virtual lab. You did set it up as I suggested. And what would have happened on a real router is that the ethernet subnets would not have been advertised over the serial interface because the subnet masks do not match. So the routing table should not have RIP routes for the ethernet subnets from the other router. The virtual lab does include those subnets and that is a flaw in the virtual lab.


HTH


Rick

alwahidi22 Thu, 05/17/2007 - 08:55
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Hi Rick ,


I appreciated your effort to make me understanding the situation.... so if i assume that my first post have the ip address of pcs into Ethernet interfaces of routers ... it will not work too???.....


Regards,

Ibrahim Alwahidi

Correct Answer
Richard Burts Thu, 05/17/2007 - 09:24
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Ibrahim


I believe that we are finally describing things in the same way in this discussion and you are exactly correct.


In several of your posts you phrase the question in terms of the subnets on PCs. And I have said several times that it does not matter what is configured on PCs but matters what is on router interfaces. It was not clear to me in your original picture whether each PC was connected to a unique interface or whether the PCs shared a common interface. From the continuing discussion I recognize that you were using them as each PC connected to a unique interface with a corresponding subnet. Now that I understand this I completely agree with you that it would not work on real routers running RIPv1 and that if it works in the virtual lab then it is a problem in the virtual lab.


HTH


Rick

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