Re: [LARTC] Redundant internet connections.

[Grant Taylor <gtaylor@riverviewtech.net>]

On 06/21/07 17:35, Grant Taylor wrote:
> The problem with this method is that I have yet to get it to start 
> re-using the primary route when it becomes available again.

After doing some more testing and investigation, I think I know why the 
system appears to not be using the primary route.  My test / lab setup 
consists of a Linux router with two subnets bound to one interface (eth0 
and eth0:1) and my (VMWare) test Linux system with two ethernet 
interfaces bridged the the local LAN with one subnet on each interface. 
  I have two (as far as Linux is concerned) physical interfaces so that 
I can have TX / RX counters for each interface to see which way the 
traffic is going out.  This worked fine to have the system fall from the 
primary down to the secondary route when the primary route went away.

However I never saw the traffic from the test Linux system back to the 
interface for the primary route.  After doing some investigation I think 
this is because the same MAC address is used for both the primary and 
secondary routes, seeing as how both addresses are on the same physical 
interface on my Linux router.

So, to test this, I took down the primary route, let the test Linux box 
fall back to the backup route, which it did.  Then I brought the primary 
route back on line and waited.  As expected the traffic did not start 
using the primary route, presumably because of MAC addresses for routes 
being cached with an association to a device.  So, while the system was 
pinging out to the world with the primary route brought back up, I 
cleared entries from the local test Linux boxes ARP cache and all of the 
sudden, traffic started going out the correct interface.

So, now I think that the method of having two equal cost (metric) routes 
on the box will work.  I'm now going to test where the two routes are 
different MAC addresses to see if the traffic does indeed start using 
the proper rout again (Seeing as how there should not be any confusion 
with MAC addresses.)



Grant. . . .
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