Conf IPv6 - restart networking - Failed to start Raise network interfaces

Bonsoir
Nouveau Proxmox 5.3
Je configure IPv6 suivant le guide OVH.
https://docs.ovh.com/fr/dedicated/network-ipv6/

Je mets ceci dans /etc/network/interfaces

# The loopback network interface
auto lo
iface lo inet loopback

# vmbr0: Bridging. Make sure to use only MAC adresses that were assigned to you.
auto vmbr0
iface vmbr0 inet static
address 51.75.nnn.nnn/24
gateway 51.75.nnn.254
bridge_ports eno1
bridge_stp off
bridge_fd 0

iface vmbr0 inet6 static
address 2001:41d0:nnnn:42cd::
netmask 64
post-up /sbin/ip -f inet6 route add 2001:41d0:nnnn:42ff:ff:ff:ff:ff dev vmbr0
post-up /sbin/ip -f inet6 route add default via 2001:41d0:nnnn:42ff:ff:ff:ff:ff
pre-down /sbin/ip -f inet6 route del default via 2001:41d0:nnnn:42ff:ff:ff:ff:ff
pre-down /sbin/ip -f inet6 route del 2001:41d0:nnnn:42ff:ff:ff:ff:ff dev vmbr0


Je redémarre le réseau avec

service networking restart

**J'ai un msg d'erreur :**

Job for networking.service failed because the control process exited with error code.
See "systemctl status networking.service" and "journalctl -xe" for details.

**Je vais voir le statut :**

# service networking status
● networking.service - Raise network interfaces
Loaded: loaded (/lib/systemd/system/networking.service; enabled; vendor preset: enabled)
Active: failed (Result: exit-code) since Thu 2019-02-28 22:41:42 UTC; 4min 25s ago
Docs: man:interfaces(5)
Process: 7349 ExecStart=/sbin/ifup -a --read-environment (code=exited, status=1/FAILURE)
Process: 7272 ExecStartPre=/bin/sh -c [ "$CONFIGURE_INTERFACES" != "no" ] && [ -n "$(ifquery --read-environment
Main PID: 7349 (code=exited, status=1/FAILURE)
CPU: 74ms

Feb 28 22:41:42 rbx04 systemd[1]: Starting Raise network interfaces...
Feb 28 22:41:42 rbx04 ifup[7349]: Waiting for vmbr0 to get ready (MAXWAIT is 2 seconds).
Feb 28 22:41:42 rbx04 ifup[7349]: RTNETLINK answers: File exists
Feb 28 22:41:42 rbx04 ifup[7349]: ifup: failed to bring up vmbr0
Feb 28 22:41:42 rbx04 systemd[1]: networking.service: Main process exited, code=exited, status=1/FAILURE
Feb 28 22:41:42 rbx04 systemd[1]: Failed to start Raise network interfaces.
Feb 28 22:41:42 rbx04 systemd[1]: networking.service: Unit entered failed state.
Feb 28 22:41:42 rbx04 systemd[1]: networking.service: Failed with result 'exit-code'.

Je ne vois pas l'erreur dans la config des interfaces !

J'ai un message d'erreur "failed" , mais pourtant, le serveur répond toujours en IPv4
et maintenant, il répond au ping 6 en IPv6 !

Pourquoi cette erreur ?

Merci. Bonne fin de soirée.
Didier