Hi Naohiro,

I'm not convinced yet.
I don't think it's correct that kernel or ping6 arbitrarily 
decides outgoing interface.

Is this really an expected behaviour?
Could you explain to me the reason of it, too?

I think the behavior should be similar to IPv4.

It's due to routing.

Your routing table shows this:
---
    # route -A inet6
    Kernel IPv6 routing table
    Destination            Next Hop Flags Metric Ref    Use Iface
    2001:2c0:418:1::/64    *        U     256    0        0 eth0
    2001:2c0:418:2::/64    *        U     256    0        0 eth1
    fe80::/64              *        U     256    0        0 eth0
    fe80::/64              *        U     256    0        0 eth1
    ::::::
    2001:2c0:418:1::1/128  *        U     0      8        1 lo
    2001:2c0:418:2::2/128  *        U     0      5        1 lo
    ::::::
    ff00::/8               *        U     256    0        0 eth0
    ff00::/8               *        U     256    0        0 eth1
    #
---

...with eth0 being preferred over eth1 for multicast packets.

You could add a dedicated route for ff02::/16 to make the kernel output the packet via eth1.

Bye,

Marcus
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