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 Post subject: Strange ping replies.
PostPosted: Mon Apr 14, 2014 11:55 am 
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Joined: Mon Apr 14, 2014 11:32 am
Posts: 2
Hi everyone.

I will appreciate any help here. I am currently working on an EIGRP Lab. Image attached. I am witnessing the strangest things possible in a networking environment. In place of loopback interfaces in the lab, I have opted to use Virtual PCs (Virtual Box) instead.

I have interface f1/0 on R1 configured as 10.1.1.1/24 and PC1 as 10.1.1.2/24. The strange thing starts to happen when PC1 is able to ping R1's f0/0 interface on 10.1.100.1/24 with no routing of any kind configured on R1. I mean R1 literally has f0/0 and f1/0 IP address configuration as the only config on the router.

And the same happens throughout the topology. For instance, PC4 is able to ping all of R3's interfaces (f1/0, f2/0, f3/0) regardless of it not being in the same network/subnet as the other interfaces apart from it's own gateway. And I say again, despite have no routing configuration of any kind on the routers static or dynamic.

I have reloaded and reconfigured a dozen times. Dismantled and rebuilt. Used different modules. NM-16ESW and vlans instead of 3 NM-1FE-TX modules. Same result. I have put switches in between the routers and the PCs. Same results. I even changed the OS in the PCs(swapped between XP and Ubuntu), same results. I downloaded the latest version of gns3, same results.


Has anyone ever witnessed this? Is it a bug? If it isn't, how can this be solved? With PCs being able to ping everywhere even before routing is configured, it just destroys the whole purpose of the Lab.

Any help will be appreciated.




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EIGRP LAB.png
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 Post subject: Re: Strange ping replies.
PostPosted: Mon Apr 14, 2014 1:12 pm 
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Joined: Sun Sep 16, 2012 9:55 pm
Posts: 541
Location: England
nateevs wrote:
I have interface f1/0 on R1 configured as 10.1.1.1/24 and PC1 as 10.1.1.2/24. The strange thing starts to happen when PC1 is able to ping R1's f0/0 interface on 10.1.100.1/24 with no routing of any kind configured on R1. I mean R1 literally has f0/0 and f1/0 IP address configuration as the only config on the router.


This would be correct... A router knows about all of it's own interfaces. So in your topology you will be able to ping any interface on R1 from PC1.

So for pinging from the PC1 to R1 f0/0
  1. The packet leaves PC1 destined for the default gateway (as set on PC1)
  2. The default gateway (10.1.1.1 on R1) receives the packet and looks at where it needs to be sent
  3. R1 then looks up using its routing table (show ip route), to find out if it knows how to get to the destination
  4. In the routing table it will find an an entry for both the 10.1.100.0/24 (subnet) and 10.1.100.1/32 (directly connected interface)... So it can now say ... "I know where 10.1.100.1 is! That's on this router and is assigned to interface f0/0, lets send your packet there"
  5. The packet is received by f0/0, it replies and the same process happens to get back to PC1

Hope that helps

_________________
Daniel
Forum Moderator & Debian Package Maintainer for GNS3, Dynamips & VPCS.
Standalone DEB Packages are available from http://gns3.serverb.co.uk - To be updated!


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 Post subject: Re: Strange ping replies.
PostPosted: Mon Apr 14, 2014 2:08 pm 
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Joined: Mon Apr 14, 2014 11:32 am
Posts: 2
Oh my goodness! Of course. You are absolutely correct. I had no problem all along.
I am absolutely embarrassed. Thanks for your reply and sorry for the waste of time.


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 Post subject: Re: Strange ping replies.
PostPosted: Mon Apr 14, 2014 2:25 pm 
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Joined: Sun Sep 16, 2012 9:55 pm
Posts: 541
Location: England
nateevs wrote:
Oh my goodness! Of course. You are absolutely correct. I had no problem all along.
I am absolutely embarrassed. Thanks for your reply and sorry for the waste of time.

It's no problem whatsoever! Trust me we've all been there... with the answer staring us in the face!

Have fun with GNS3 and don't hesitate to ask if you have more questions



_________________
Daniel
Forum Moderator & Debian Package Maintainer for GNS3, Dynamips & VPCS.
Standalone DEB Packages are available from http://gns3.serverb.co.uk - To be updated!


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