[RndTbl] Enabling Linux failover routing

Bill Reid billreid at shaw.ca
Wed May 11 14:08:43 CDT 2005

John Lange wrote:
> Sean,
> What do you base this on? Google turns up plenty of references to the
> gc_timeout method of failover but none are specific on what they mean by
> "when a route dies".

Yes, I see that there are a ton of obscure parameters that can be set for routing.

> I can't think of any reason why the kernel would care if the route timed
> out because the link was down or because the route was down for some
> other reason.

It is the definition of what is meant by "a route dies". In further reading it 
seems to imply problems with the NIC.  On the otherhand it needs the Ethernet 
MAC address of the gateway so when the ARP times out it will not be able to 
reach the gateway so I assume that this is another case of the route dieing.
gc_stale_time sets how long to wait for an ARP entry to become stale.

If you think about it how does the routing algorithm determine that a gateway is 
dead. It just forwards packets to it. If the gateway responds to the ARP then 
there is no way for the routing algorithms to determine that the default route 
is dead.

I never thought of the ARP timeout before but that would be a good way to 
determine if the default route is at least up. No idea if this mechanism is 
actually used.

-- Bill

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