Considerations about multilink conection under the same IP subnet

From: ArcosCom Linux User
Date: Tue May 27 2008 - 10:11:10 EST


Hi, I'm having a problem and need some help to solve it.

I have 2 uplink ADSL routers that allow me to connect my lan to internet.
Call them ADSL0 and ADSL1. They have the same IP, call it IP_GW.

I have to configure 2 IP in my linux router under the same IP subnet as
IP_GW, call them PUB_IP0 and PUB_IP1.

The conection is (more or less):

LAN -- (eth0) LINUX ROUTER (eth1) -- ADSL0 --- INTERNET
(eth2) -- ADSL1 ----|

The problem is that ADSL0 and ADSL1 have the same IP and I have to say to
my linux router that balance between them.

I read LARTC and stand fine how to balance the conection when the ADSL0
and ADSL1 (and PUB_IP0 and PUB_IP1) are in diferent IP subnets (two route
tables, using marks to allow the feedback in the same interface and so
on), but I need some help to apply the same concepts in this case.

The questions I have are:
a) How can I configure the linux box to allow ADSL0 and ADSL1 don't
fight with ARP? (This question is because I think that 1 IP with 2 MACs
under diferent interfaces can have a serious conflict). I think that I
must change some /proc/sys/net/.../ files, but a don't know what files
I have to change.
b) What filters rules I have to put to disallow any type of confusion
in the routing table? I have the "nexthops" using the same $IP_GW with
with the "dev" explicitily writen in the default table as this:

=== REGLAS DE ENRUTAMIENTO ===
0: from all lookup local
50: from all lookup main
100: from all fwmark 0x8000/0xf000 lookup uno
101: from all fwmark 0x4000/0xf000 lookup dos
150: from PUB_SUBNET lookup uno
151: from PUB_SUBNET lookup dos
200: from all lookup defecto
32766: from all lookup main
32767: from all lookup default
=== TABLAS DE RUTAS ===
=== MAIN ===
PUB_SUBNET dev eth1 proto kernel scope link src PUB_IP0
PUB_SUBNET dev eth2 proto kernel scope link src PUB_IP1
192.168.3.0/24 dev zlan0 proto kernel scope link src 192.168.3.247
192.168.2.0/24 dev zlan0 proto kernel scope link src 192.168.2.247
192.168.1.0/24 dev zlan0 proto kernel scope link src 192.168.1.247
10.1.1.0/24 dev zlan0 proto kernel scope link src 10.1.1.6
169.254.0.0/16 dev zlan0 scope link
=== eth1 TABLA 150 ===
default via IP_GW dev eth1 proto static src PUB_IP0
prohibit default proto static metric 1
=== eth2 TABLA 151 ===
default via IP_GW dev eth1 proto static src PUB_IP1
prohibit default proto static metric 1
=== TABLA 200 (defecto) ===
default via IP_GW dev eth1 nexthop via IP_GW dev eth2

Any help/suggestion about this problem?

Thanks

P.D.: Sorry for my English.

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