[RndTbl] Enabling Linux failover routing

John Lange john.lange at open-it.ca
Wed May 11 14:56:35 CDT 2005

On Wed, 2005-05-11 at 14:38 -0500, Bill Reid wrote:
> 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.

So bottom line is, altering gc_timeout would only be effective if the
cable modem itself died or the cable was pulled. Since that type of
failure happens only rarely compared to other networking problems this
technique will have minimal impact.

Ultimately you end up resorting to a monitoring program/script of some

So I wonder how the linksys and sonicwall devices do their failover?

John Lange
President OpenIT ltd. www.Open-IT.ca (204) 885 0872
VoIP, Web services, Linux Consulting, Server Co-Location

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