sub-net routing question

[marthter]

Hi Lennart,

I was replying to your other one before I saw that you'd sent this one.  
This one mostly answers my question, thanks.  (with some follow ups)....

On 11-08-17 06:04 PM, Lennart Sorensen wrote:
> On Wed, Aug 17, 2011 at 05:29:10PM -0400, marthter wrote:
>> Yes I realize the three routers in my example network could be
>> replaced with three switches, and thus left computer could access
>> right computer and vice versa.  But I'm trying to experiment with
>> and understand the three-routers example so that later when some
>> links are VPN links, I can apply it there.
>>
>> My thinking (in the LAN and WAN addresses/netmasks I proposed above)
>> is that the middle router could be set so its LAN is set up as the
>> big subnet (i.e. with a /16 netmask), and the left and right router
>> have their WAN ports as clients of the middle router, and have their
>> LAN ports to clients on smaller subnets (i.e. with /24 netmask).
> That's NOT a good solution.  It generally doesn't work.  The problem
> is that every device in the large subnet connected to the middle router
> thinks everything is local for that large subnet, and don't know that some
> parts of the large subnet are in fact not local but rather behind some
> other routers.
Okay, that was my misunderstanding then, in thinking that was enough for 
the router that could deliver it to be local, not that the final 
destination needed to be local.

>    Then to solve that every client in the middle large subnet
> needs a route entry for each of the smaller subnets.  That's a mess.
>> And I'm thinking (and perhaps this is the wrong part), that it is
>> easier/better/cleaner/properer that the two smaller subnets be
>> actual SUBNETS OF the big subnet of the middle router.
> Only if no devices are directly part of the larger subnet.  So it would
> be OK to have:
>
>                       Main router
>         +-------------192.168.0.254/24-----------+
>         |                   |                    |
>         |                   |                    |
> 192.168.0.1/24        192.168.0.2/24       192.168.0.3/24
> Router 1              Router 2             Router 3
> 192.168.1.254/24      192.168.2.254/24     192.168.3.254/24
>    |   |   |              |   |   |            |   |   |
>    A1  A2  A3             B1  B2  B3           C1  C2  C3
>
> The default route on Router 0, 1 and 2 is the main router (192.168.0.254)
> The default route of clients A1 through A3 would be 192.168.1.254 since
> that is their local router.  The main router would have route entries
> saying 192.168.1.0/24 is through 192.168.0.1, and similar for the other
> two networks.
okay the above diagram basically looks like what I want.  Thanks!  I 
will try it.

> This way anything from C1 for another subnet will be forwarded by
> router 3 to its default gateway (since it doesn't know where A1 is for
> example), and then the main router will know that it is through router 1,
> so it forwards it there, and router 1 knows where A1 is and sends the
> data there.
> Now if the main router had a link to someone above it, it would be fine
> for the one above it to have a route for 192.168.0.0/16 through main
> router, since main router knows where all the 192.168.x.0/24 networks
> are and can forward traffic to all of them.
I see, so the 192.168.0.0/16 network/netmask might be used in routing 
RULES somewhere higher up, but no devices would actually have it in 
their interface definition?

>> I'm just saying I don't know if it is valid for the left router's
>> LAN address to be 192.168.1.1 with /24 netmask, and its WAN address
>> to be 192.168.1.2 with a /16 netmask.
> No it is not.  After all if it had to send something to 192.168.1.3, would
> that be through the LAN or the WAN interface?  It has no way to know.
>
Yes that's why it didn't seem right but I couldn't picture anything that 
seemed better (although I was kind of creeping towards the answer in my 
"on the third hand" paragraph).

Thanks again.

Martin

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