Comment # 3 on bug 931595 from Marius Tomaschewski

(In reply to Freek de Kruijf from comment #0)
> eiktum:~ # more /etc/sysconfig/network/ifroute-enp1s0 
> default 192.168.178.1 - enp1s0 
          ^^^^^^^^^^^^^
OK, as gateway in same network as one of the IPADDRs.

> default fd00::9ec7:fe0a:1f48 - enp1s0 
          ^^^^^^^^^^^^^^^^^^^^

This will work only for a gateway from fe80::/64 (link-local) network.
To use a gateway from another networks, you need either an address in
the fd00::/64 network or an explicit direct route for this gateway:

fd00::9ec7:fe0a:1f48 -                    - enp1s0
default              fd00::9ec7:fe0a:1f48 - enp1s0

The following will work:

# ip a s dev nic1
11: nic1: <BROADCAST,MULTICAST> mtu 1500 qdisc pfifo_fast state DOWN group
default qlen 1000
    link/ether 68:05:ca:0a:39:e7 brd ff:ff:ff:ff:ff:ff

# ip a a 2001:980:aaaa::1:37/64 dev nic1
# ip r r default via 2001:980:aaaa::1 dev nic1

# ip a s dev nic1
11: nic1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP
group default qlen 1000
    link/ether 68:05:ca:0a:39:e7 brd ff:ff:ff:ff:ff:ff
    inet6 2001:980:aaaa::1:37/64 scope global 
       valid_lft forever preferred_lft forever
    inet6 fe80::6a05:caff:fe0a:39e7/64 scope link 
       valid_lft forever preferred_lft forever
# ip -6 r s dev nic1
2001:980:aaaa::/64  proto kernel  metric 256 
fe80::/64  proto kernel  metric 256 
default via 2001:980:aaaa::1  metric 1024 

also this:

# ip r r default via fe80::9ec7:a6ff:fe0a:1f48 dev nic1
# ip -6 r s dev nic1
2001:980:aaaa::/64  proto kernel  metric 256 
fe80::/64  proto kernel  metric 256 
default via fe80::9ec7:a6ff:fe0a:1f48  metric 1024 

but not a route via a not directly connected gateway:

# ip r r default via fd00::9ec7:a6ff:fe0a:1f48 dev nic1
RTNETLINK answers: No route to host

until you declare the gateway (network) as directly connected:

# ip r r fd00::9ec7:a6ff:fe0a:1f48 dev nic1
# ip r r default via fd00::9ec7:a6ff:fe0a:1f48 dev nic1

# ip -6 r s dev nic1
2001:980:aaaa::/64  proto kernel  metric 256 
fd00::9ec7:a6ff:fe0a:1f48  metric 1024 
fe80::/64  proto kernel  metric 256 
default via fd00::9ec7:a6ff:fe0a:1f48  metric 1024 

Please note also, that IPv6 strictly inhibits using not link-local
addresses as nexthop / gateway address. Otherwise, router will not
be able to send redirects.

> It looks like the problem is not so much YaST, but wicked, because
> these configuration files look OK.

No, they are incomplete.