netdev - IPv6 Router Advertisement Router Preference (RFC 4191) behavior issue

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Message-ID: <CACS3ZpA=QDLqXE6RyCox8sCX753B=8+JC3jSxpv+vkbKAOwkYQ@mail.gmail.com>
Date:   Thu, 18 Nov 2021 12:35:36 +0200
From:   Juhamatti Kuusisaari <juhamatk@...il.com>
To:     netdev@...r.kernel.org
Subject: IPv6 Router Advertisement Router Preference (RFC 4191) behavior issue

Hello,

I have been testing IPv6 Router Advertisement Default Router
Preference on 5.1X and it seems it is not honoured by the Linux
networking stack. Whenever a new default router preference with a
higher or lower preference value is received, a new default gateway is
added as an ECMP route in the routing table with equal weight. This is
a bit surprising as RFC 4191 Sec. 3.2 mentions that the higher
preference value should be preferred. This part seems to be missing
from the Linux implementation.

The problem has existed apparently for a while, see below discussion
for reference:
https://serverfault.com/questions/768932/can-linux-be-made-to-honour-ipv6-route-advertisement-preferences

I am happy to test any improvements to this, in case someone takes a look.

Thanks!
--
 Juhamatti