[Bug 1869961] [NEW] systemd-networkd ignores RA preference and creates multipath routes anyway

[Joseph Marsden]

Public bug reported:

When IPv6 ND RAs are sent from two different routers, one with Medium
and one with High preferences, systemd-networkd will ignore the fact
they have different preferences and create two multipath routes with the
same weight.

I ran a tcpdump of the RAs and confirmed that both have different
preferences sent.

i.e.

nms-mon1:~$ ip -6 route
2a0d:1a40:7551:6010::/64 dev ens18 proto ra metric 100 pref medium
fe80::/64 dev ens18 proto kernel metric 256 pref medium
default proto ra metric 100 
	nexthop via fe80::c030:9cff:fe10:3492 dev ens18 weight 1 
	nexthop via fe80::215:5dff:fe83:c81c dev ens18 weight 1

Ubuntu version: Ubuntu 18.04.4 LTS
systemd version: 237-3ubuntu10.39

** Affects: systemd (Ubuntu)
     Importance: Undecided
         Status: New

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Title:
  systemd-networkd ignores RA preference and creates multipath routes
  anyway

Status in systemd package in Ubuntu:
  New

Bug description:
  When IPv6 ND RAs are sent from two different routers, one with Medium
  and one with High preferences, systemd-networkd will ignore the fact
  they have different preferences and create two multipath routes with
  the same weight.

  I ran a tcpdump of the RAs and confirmed that both have different
  preferences sent.

  i.e.

  nms-mon1:~$ ip -6 route
  2a0d:1a40:7551:6010::/64 dev ens18 proto ra metric 100 pref medium
  fe80::/64 dev ens18 proto kernel metric 256 pref medium
  default proto ra metric 100 
  	nexthop via fe80::c030:9cff:fe10:3492 dev ens18 weight 1 
  	nexthop via fe80::215:5dff:fe83:c81c dev ens18 weight 1

  Ubuntu version: Ubuntu 18.04.4 LTS
  systemd version: 237-3ubuntu10.39

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