Re: Route fallback issue

Hi,

I /think/ Linux continues sending ARP requests and doesn't fall back to 
the other route because the route to the failed next hop still exists in 
the routing table with highest metric, so it continues looking for this 
next hop.  I get the same behaviour as you when labbing this up, I could 
not see a straightforward option to mark a route as invalid under 
changes in reachability, I'd also like to know if this feature is built 
in and exists.


However in the enterprise Cisco world, we can do what you are trying to 
do very easily using "route tracking" and "IP SLA" features.  Basically 
we define tests e.g. reachability via ping with appropriate frequency 
and threshholds, then attach these tests to one or more preferred 
routes.  If the test fails, the associated route is automatically 
uninstalled from the forwarding table, so any existing lower metric 
routes get exposed and are used instead.  When the test passes again, 
the preferred routes are reapplied.

The underlying logic of this can certainly be scripted under Linux to 
get very similar functionality, then put into a cron job or a while loop 
or similar.  Something along the lines of (pseudocode):
    if [the test such as ping fails] ; then
       if [preferred route exists] ; then ip route delete ... ; fi
    else  ## ping is successful
       if [preferred route doesn't exist] ; then ip route add ... ; fi
    fi


Hope that helps.  I'm also interested in any other solutions to do this 
under Linux.


On 2018-06-19 13:18, Akshat Kakkar wrote:
> I have 2 interfaces
> eno1 : 192.168.1.10/24
> eno2 : 192.168.2.10/24
>
> I added routes as
> 172.16.0.0/12 via 192.168.1.254 metric 1
> 172.16.0.0/12 via 192.168.2.254 metric 2
>
> My intention : All traffic to 172.16.0.0/12 should go thru eno1. If
> 192.168.1.254 is not reachable (no arp entry or link down), then it
> should fall back to eno2.
>
> But this is not working. My box keeps on looking for 192.168.1.254
> (i.e. sending arp requests) and never falls back.
>
> Can anyone help?
>