Redundancy using double interface & gateway

Hi,
I have the following configuration with 2 PC :

+------+ eth0 eth0 +------+
| PC1 +---------/--------+ PC2 | eth2
| | | +---------------(Device, IP3)
| +---------/--------+ |
+------+ eth1 eth1 +------+

I want to send IP packets to the device (with IP3 address) from PC1 and
using PC2 as a gateway. I have configured PC2 as a gateway for eth0 ->
eth2 and eth1 -> eth2.

But how can I set routes on PC1 ?
What I want is in fact redundancy between eth0 and eth1 :
Connecting to IP3 from PC1 use the link eth0<->eth0 if this link is up
and use the link eth1<->eth1 if eth0<->eth0 is down ?

I think I have to use ip route + nexthop but it's not very clear.
Someone can help me ?

cwpbl a écrit :

> I have the following configuration with 2 PC :
>
> +------+ eth0 eth0 +------+
> | PC1 +---------/--------+ PC2 | eth2
> | | | +---------------(Device, IP3)
> | +---------/--------+ |
> +------+ eth1 eth1 +------+
>
> I want to send IP packets to the device (with IP3 address) from PC1 and
> using PC2 as a gateway. I have configured PC2 as a gateway for eth0 ->
> eth2 and eth1 -> eth2.
> But how can I set routes on PC1 ?
> What I want is in fact redundancy between eth0 and eth1 :
> Connecting to IP3 from PC1 use the link eth0<->eth0 if this link is up
> and use the link eth1<->eth1 if eth0<->eth0 is down ?
> I think I have to use ip route + nexthop but it's not very clear.


Sorry, I forgot : I work under Linux 2.6 kernel based (Red Hat 4)

On Mar 18, 2:03 pm, cwpbl <cw...@rf.oohay> wrote:
> Hi,
> I have the following configuration with 2 PC :
>
> +------+ eth0 eth0 +------+
> | PC1 +---------/--------+ PC2 | eth2
> | | | +---------------(Device, IP3)
> | +---------/--------+ |
> +------+ eth1 eth1 +------+
>
> I want to send IP packets to the device (with IP3 address) from PC1 and
> using PC2 as a gateway. I have configured PC2 as a gateway for eth0 ->
> eth2 and eth1 -> eth2.
>
> But how can I set routes on PC1 ?

Simply set two routes on PC1. They should both be to the address
assigned to the device. The first one should have a gateway of PC2/
eth0 address and the other should have a gateway of PC2/eth1 address.

This is the quick/kludgy way. There are more complex but more elegant
ways to do it. For example, you could assign both PC1 and PC2
'internal' addresses not assigned with any interface, add two routes
to PC2's 'internal' address to PC1, and then route IP3 to PC2's
internal address.

You can also use a routing protocol (such as OSPF) and have PC2
advertise its route to IP3 through both interfaces.

DS

Hello,

cwpbl a écrit :
> I have the following configuration with 2 PC :
>
> +------+ eth0 eth0 +------+
> | PC1 +---------/--------+ PC2 | eth2
> | | | +---------------(Device, IP3)
> | +---------/--------+ |
> +------+ eth1 eth1 +------+
>
> I want to send IP packets to the device (with IP3 address) from PC1 and
> using PC2 as a gateway. I have configured PC2 as a gateway for eth0 ->
> eth2 and eth1 -> eth2.
>
> But how can I set routes on PC1 ?
> What I want is in fact redundancy between eth0 and eth1 :
> Connecting to IP3 from PC1 use the link eth0<->eth0 if this link is up
> and use the link eth1<->eth1 if eth0<->eth0 is down ?
>
> I think I have to use ip route + nexthop but it's not very clear.
> Someone can help me ?

A multipath route (ip route + nexthop) only provides load balancing, not
redundancy. Multiple routes to the same destination provide neither
redundancy nor load balancing, because only the same one route will
always be used regardless of the link state. AFAIK Linux does not
natively support dead gateway detection.

Have you considered ethernet bonding in active-backup mode ?

On Mar 19, 2:27 am, David Schwartz <dav...@webmaster.com> wrote:

> Simply set two routes on PC1. They should both be to the address
> assigned to the device. The first one should have a gateway of PC2/
> eth0 address and the other should have a gateway of PC2/eth1 address.
>
> This is the quick/kludgy way. There are more complex but more elegant
> ways to do it. For example, you could assign both PC1 and PC2
> 'internal' addresses not assigned with any interface, add two routes
> to PC2's 'internal' address to PC1, and then route IP3 to PC2's
> internal address.

Pascal is right, this won't work.

> You can also use a routing protocol (such as OSPF) and have PC2
> advertise its route to IP3 through both interfaces.

This is right.

DS