netdev - Kernel creates 2nd default route, why can't iproute2?

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Message-ID: <SJ0PR84MB2088C0CC554432A482DBDE98D8DE2@SJ0PR84MB2088.NAMPRD84.PROD.OUTLOOK.COM>
Date: Thu, 4 Jul 2024 00:42:24 +0000
From: "Muggeridge, Matt" <matt.muggeridge2@....com>
To: "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Kernel creates 2nd default route, why can't iproute2?

When the kernel receives two RAs, each from a different router, it creates two default routes, like so:

$ ip -6 r
<snip>
default via fe80::200:10ff:fe10:1060 dev enp0s9 proto ra metric 1024 expires 594sec hoplimit 64 pref medium
default via fe80::200:10ff:fe10:1061 dev enp0s9 proto ra metric 1024 expires 599sec hoplimit 64 pref medium

Why can't I manually create the same default routes?

E.g., I tried the following sequence, but the 2nd default route returns an error: File Exists.

$ ip -6 r
<no routes>

$ ip -6 r add default via fe80::200:10ff:fe10:1060 dev enp0s9 proto ra metric 1024 expires 600 hoplimit 64 pref medium

$ ip -6 r
<snip>
default via fe80::200:10ff:fe10:1060 dev enp0s9 proto ra metric 1024 expires 594sec hoplimit 64 pref medium

$ ip -6 r add default via fe80::200:10ff:fe10:1061 dev enp0s9 proto ra metric 1024 expires 600 hoplimit 64 pref medium
RTNETLINK answers: File exists

Thanks,
Matt.