quagga_interface does not support adding IPv6 addresses to an interface

[sid3windr]

Only IPv4 is supported, as for v6 you need to use "ipv6 address" instead of "ip address" on the Quagga CLI.

I looked at the code but unfortunately it's currently out of my league: it's done efficiently with mapping and templates - I have no idea how to make this work for different address families without introducing an entirely new ip6_address parameter (it would be a lot more elegant if this would work transparently).