Re: [LARTC] Bridging two subnets selectively using routing

[Grant Taylor <gtaylor@riverviewtech.net>]

On 10/31/07 19:50, Joshua Kwan wrote:
> What am I doing wrong? Is this a job for iptables? I feel like I can 
> do this with just routing table magic.

As Corey said, make sure that IP Forwarding is enabled on A1.

You are setting your self up for a very weird scenario.  Probably what 
would be the easiest for long term would be to put a third network 
between your two routers, AR and BR.  This way you end up with the 
following type of scenario:

                +----+               +----+
                | AI |               | BI |
                +-+--+               +--+-+
                  |                     |
                +-+--+               +--+-+
---( Net A )---+ AR +---( Net C )---+ BR +---( Net B )---
        |       +----+               +----+       |
        |                                         |
        |       +----+                            |
        +-------+ A1 +----------------------------+
                +----+

With this type of set up, you can configure both AR and BR that each can 
reach the others network via the other router on network C.  I.e. AR can 
reach Net B via BR's IP on Net C.  Likewise BR can reach Net A via AR's 
IP on Net C.  This way, each and every computer on both networks A and B 
can reach any host they want to via their default gateway AR or BR 
respectively.  This may be a little more work to set up initially, but 
it is very simple routing that will be very easy to maintain for a long 
time.  In this scenario Net C can very simply be a cross over cable 
between routers AR and BR.

Another option is to enable routing on A1 and tell A2, A3, and A4 that 
they can reach B1 via A1 and likewise tell B1 that it can reach a2, A3, 
and A4 via A1.  Thus you have a bi-directional routing set up between 
just the four additional hosts.  The main disadvantage of this set up is 
that each and every host that wants to communicate across networks will 
have to have its own routing entries set up to pick the correct 
direction to route traffic to.  So if you only have two or three hosts 
on each network and you can not do the additional network (C) between 
the two routers, this will work.

There are some other things that you can do to allow the hosts to 
communicate across the networks like bridging and / or proxy ARP. 
However seeing as how you have different subnets on Net A and Net B, you 
will very likely be doing routing or multi-homing on all hosts to 
support bridging and / or proxy ARP.

I think your best bet is to do the additional network (C) between the 
two routers AR and BR.



Grant. . . .
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