how to effectively use multiple gateways?

Consider this simplified network topology:

(to world)
|
+-o-+
| | firewall
| |
+-o-+
| 192.168.0/22
------o-------o-------o-------o-------o------gbe
| | | | |
+---+ +---+ +---+ +---+ +---+
|a1 | |a2 | |a3 | |a4 | |a5 |
| | | | | | | | | |
+---+ +---+ +---+ +---+ +---+
| | |
------o-------o-------o----------------------gbe
| | | 192.168.4/22
+---+ +---+ +---+
|b1 | |b2 | |b3 |
| | | | | |
+---+ +---+ +---+

Systems a1, a2 and a3 are three systems have interfaces on both
192.168.0/22 and 192.168.4/22 (not the actual values). Systems a4 and a5
(actually many systems) are connected only to 192.168.0/22. Systems b1, b2
and b3 (several dozens of these) are connected only to 192.168.4/22 via a
single interface each.

How would you establish the default gateway for b1, b2, ..., given that
a1, a2 and a3 can do NAT on behalf of the b's. I'd like to take advantage
of the fact that there are three possible default gateways for the b's to
give some redundancy if a1 (say) is shut down (no manual intervention).
three default gateways? routed? gated?

Steve
----------------------------------------------------------------------------
Steve Thompson E-mail: smt AT vgersoft DOT com
Voyager Software LLC Web: http://www DOT vgersoft DOT com
39 Smugglers Path VSW Support: support AT vgersoft DOT com
Ithaca, NY 14850
"186,300 miles per second: it's not just a good idea, it's the law"
----------------------------------------------------------------------------

On 2007-02-19, Steve Thompson <(E-Mail Removed)> wrote:
> Consider this simplified network topology:
<zap>
> How would you establish the default gateway for b1, b2

The gateway should be the firewall, that in this case will also act
as a router routing 'back' the connection for the aX machines.

Davide

--
Windows NT -- it'll drive you buggy!
-- Gareth Barnard

On Mon, 19 Feb 2007, in the Usenet newsgroup comp.os.linux.networking, in
article <(E-Mail Removed). com>,
Steve Thompson wrote:

>Consider this simplified network topology:

OK

>How would you establish the default gateway for b1, b2, ..., given that
>a1, a2 and a3 can do NAT on behalf of the b's. I'd like to take advantage
>of the fact that there are three possible default gateways for the b's to
>give some redundancy if a1 (say) is shut down (no manual intervention).
>three default gateways? routed? gated?

'gated' is probably the better solution, but unless a1..a3 are acting
like yo-yos (up/down/up/down) I'm not sure it's worth the extra hassle.
The kernel is going to ignore multiple gateways unless you are using
iproute2 and have set up policies.

Old guy

On Tue, 20 Feb 2007, in the Usenet newsgroup comp.os.linux.networking, in
article <(E-Mail Removed) >, Davide Bianchi
wrote:

>On 2007-02-19, Steve Thompson <(E-Mail Removed)> wrote:
>> Consider this simplified network topology:
><zap>
>> How would you establish the default gateway for b1, b2
>The gateway should be the firewall, that in this case will also act
>as a router routing 'back' the connection for the aX machines.

Look again at his picture and description - the router is on 192.168.0/22
while his "b1, b2" and such is on 192.168.4/22. The route command would
return "SIOCADDRT: Invalid argument" because the gateway isn't on the
same network.

Old guy

Steve Thompson wrote:

> Systems a1, a2 and a3 are three systems have interfaces on both
> 192.168.0/22 and 192.168.4/22 (not the actual values). Systems a4 and a5
> (actually many systems) are connected only to 192.168.0/22. Systems b1,
> b2 and b3 (several dozens of these) are connected only to 192.168.4/22
> via a single interface each.
>
> How would you establish the default gateway for b1, b2, ..., given that
> a1, a2 and a3 can do NAT on behalf of the b's. I'd like to take
> advantage of the fact that there are three possible default gateways for
> the b's to give some redundancy if a1 (say) is shut down (no manual
> intervention).
> three default gateways? routed? gated?

Redundant routers with Linux and Keepalived
http://tips.linux.com/tips/05/05/10/....shtml?tid=100