[RndTbl] Enabling Linux failover routing

Bill Reid billreid at shaw.ca
Wed May 11 14:38:51 CDT 2005

Sean A. Walberg wrote:

> That said, Bill brought up the idea of ARP tables.  This has two problems:
> 1 - If the link is idle for a while, the arp cache will clear, and be seen 
> as a failure
> 2 - There are lots of layer 2 networks that don't use ARP.  The ethernet 
> code is separate from the ipv4 code.
> In addition, this is a common problems on hardware routers.  Even a box 
> that's specifically designed to be a router can't do it without resorting 
> to ICMP tests and policy routing. 

Good comments Sean. I agree with your comments and it agrees with what I saw 
when I was doing testing of default route failover a few years back. The only 
way I could get it to work efectively was for a program ping the gateways to see 
if they are up and change the default routes when it noticed a failure.

On the otherhand when the NIC dies (layer 1 ?) layer 3 finds out about it but 
routing does need to be aware of the interface. I think it tries to send packets 
to a host out the same interface that it received packets from the host.

-- Bill

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