Spreading subnets accross routers.

Hello there, I dont know if this question has an obvious answer but Im slightly confused so Ill ask anyway.

For my CCNA coursework Ive been asked to set up a LAN with 4 routers using OSPF, one for each site, and 7 VLANs spread across the whole LAN.
Now as far as I can understand you need to have each VLAN in the same subnet for it to work correctly.
The part I dont understand is that if the VLANs are spread accross the LAN, then there could be one computer on RouterA in VLAN1 and one computer on RouterB on VLAN1 both in the same subnet but on different routers. How would the routers deal with this? Say I want to send a packet from 192.168.1.1 to 192.168.1.2 but they are on switches attached to seperate routers, how will the routers know where to send the packet when that subnet could be on either routers interface.

Thanks in advance for any replies, I need all the help I can get!

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Netstudent

You should not use the same subnet in multiple VLANS. I think you might be misinterpreting the assignment. Also if you have 4 routers for 4 separate sites, then that would probably be considered a WAN unless it is a campus network with multiple facilities.

Secondly you should not use the same subnet over multiple routers. That will cause overlap and loops.

Grogger4130

Thanks for the reply, I dont think I explained clearly. Maybe I have misinterpreted the assignment though lol. I dont want to spread the subnet across VLANs, I want each VLAN to have the same subnet. BUT what you said about not spreading the subnet across routers is what got me confused in the first place. We have only skimmed over VLANs, but I remember we were told to keep each VLAN in its own subnet. This is basically impossible if each subnet is attached to a different router. Is it just a case of forgetting about each VLAN being in its own subnet?

EDIT: Im sorry I should have pointed out we were told to set up all 4 routers and that the 7 VLANs spread across all 4 sites.
Hopefully this image will clear things up:

Netstudent

How well do you know subnetting? I'm wondering if you are getting confused between a major network and a subnet within that network.

You can have 192.168.1.0/24 and then break that down into multiple subnets. like 192.168.1.0/27 would give you

192.168.1.0

192.168.1.32

192.168.1.64

102.168.1.96

192.168.1.128

160

192

Then you could use those 7 subnets within the same 192.168.1.0/24
for your LAN's or VLANS.

But you can't use the same subnet over all 7 vlans. AS much as you want to, it will not work. You will never do that in the real world and you will never do that for the CCNA exam.

shednik

That sounds most logical in this situation that the major network has been subnetted as netstudent described above....Looking at your diagram I would also be willing to guess the same.

mikej412

Grogger4130 wrote:

Say I want to send a packet from 192.168.1.1 to 192.168.1.2 but they are on switches attached to seperate routers, how will the routers know where to send the packet when that subnet could be on either routers interface.

The only way would be if you were bridging across the router -- and I don't think that's a CCNA topic.

That's also bad design.

Grogger4130

But you can't use the same subnet over all 7 vlans. AS much as you want to, it will not work. You will never do that in the real world and you will never do that for the CCNA exam.

I actually want to do the opposite, I want to have several subnets in one VLAN. I think thats all I needed to say really but I got carried away.

My first thought was this:

7 VLANs = 7 Subnets, 1 per VLAN to keep things simple.

But whats stopping me is that there are 4 routers invloved in the diagram, which makes me assume that there has to be at least 1 subnet for each routers FA interface, or why would you use a router and not just some switches?

Bear in mind that these VLANS have to be spread across all 4 sites. Im not talking about 7 VLANS per site, Im saying I need inter-VLAN communication across 4 sites.

To make things extra simple lets just say there are 2 routers linked by serial. each of these routers then has a switch attached, with 2 hosts per switch. Between these 4 hosts there are only 2 VLANS, but host 1 on the switch attached to router 1, needs to be in the same VLAN as host 3 on the switch attached to router 2. Ive tried to put this into a diagram which isnt my strong point im afraid so here goes

gojericho0

You will be able to do the SAME VLAN on a DIFFERENT SUBNETS because each VLAN will terminate on their respected routers. Each VLAN interface must belong to a different network though in order to router correctly

Grogger4130

Lovely thank you

I wish Id started out a bit more clearly and with a better ropic title or this could have been much quicker. Cheers to all who replied though.