netdev - Re: Route fallback issue

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Date:   Wed, 20 Jun 2018 09:38:35 -0600
From:   Grant Taylor <gtaylor@...tconsulting.net>
To:     netdev <netdev@...r.kernel.org>
Cc:     lartc <lartc@...r.kernel.org>
Subject: Re: Route fallback issue

On 06/20/2018 09:18 AM, Grant Taylor wrote:
> Where can I find more information on ignore_routes_with_linkdown?  I 
> don't see it listed in $Kernel/Documentation/networking/ip-sysctl.txt. 
> (I do see fib_multipath_use_neigh documented there in.)

I'm specifically interested in if ignore_routes_with_linkdown and / or 
fib_multipath_use_neigh will cause Linux to fall back to an alternate 
(higher metric) route if the link is still up but the neighbor is not 
accessible across it.

           +-------+
           | Linux |
           +---+---+
               |
+-----+   +---+----+   +-----+
| R 1 +---+ Switch +---+ R 2 |
+-----+   +--------+   +-----+

A typical scenario is where Linux is connected to a DSL or Cable modem 
where the physical link stays up even if the neighbor R {1,2} goes 
offline.  It's not possible to rely on the local link (MII) status to 
determine that a neighbor is not reachable.  I.e. R 1 going away like below.

           +-------+
           | Linux |
           +---+---+
               |
+-----+   +---+----+   +-----+
| R 1 X   X Switch +---+ R 2 |
+-----+   +--------+   +-----+

-- 
Grant. . . .
unix || die

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