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New Member

Dual 3560s with Two host NICs/Two Subnets

Attached is a picture of my network.


RedHat Linux Host 1 has a Eth0 with 172.21.0.17/19.  Eth1 has 172.21.32.17/19.

Since Linux (and all operating systems) can have only a single default gateway, we need to figure out how to have some routes for the Eth1 interface.

My local switches are in Layer 2 and in VLAN1.  They are connected up to a router with an inside interface on the same subnet with Eth0 (gateway) and Eth1.

Because I have a default gateway on Eth0, I have no problem pinging the 10.0.3.1 NTP server that you see beyond my router.  When I unplug the cable on Eth0 I lose my ability to ping the NTP server since there is no route via Eth1.

So, I know this is a more of a Linux question but I thought maybe someone here has come across this before.

Thanks in advance for any help you can provide.  My fingers are practically bloody from all the Googling I have been doing on this topic.

Oh, and the first suggestion I usually get is to just bond the NICs...unfortunately, our application does not support the bonding.                   

5 REPLIES
Purple

Dual 3560s with Two host NICs/Two Subnets

what do you want exactly ? fallover on eth1  for this 10 network ? then why not just put 2 routes to this network, one with a longest match through eth0 and the other with  a shorter match through eth 1 ? the default will then be used for other remote subnets other than the 10 network.

Regards.

Alain

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New Member

Dual 3560s with Two host NICs/Two Subnets

What I need to do is, when my Eth0 switch goes down (and my NIC with it), I need my host to send packetsto the 10.0.3.1 via 172.21.32.3 router since my default gateway router is unreachable.

James

Purple

Dual 3560s with Two host NICs/Two Subnets

Hi,

Wouldn't it be what you are searching for ?

http://kindlund.wordpress.com/2007/11/19/configuring-multiple-default-routes-in-linux/

Regards.

Alain

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New Member

Dual 3560s with Two host NICs/Two Subnets

Unfortunately, that is not it.

The issue here is with the Linux kernel in that it doesn't appear to remove the default route out Eth0 when a cable is unplugged or the switch goes down.

The ping packets to the 10.x.x.x network still try to go out Eth0.  When I run an ifdown eth0 then the packets will see the route on Eth1 since the default route finally gets taken away.

But that is a manual intervention I cannot implement.

Purple

Dual 3560s with Two host NICs/Two Subnets

Hi,

maybe you could do a script which flushes the primary default route when a ping to next-hop is failing( a little bit like implementing static routing with EOT on Cisco devices)

Regards.

Alain.

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