osmo-ggsn requires a link-local IPv6 address to be added to the
tun interface, otherwise the apn will not be configured correctly and it
won't be able to allocate addresses from the ipv6 pool later on.
Some OS don't support autoconfiguring link-local IPv6 addresses when the
interface is brought up (some linux versions are known to fail at it).
This is the case for our Prod osmo-gsm-tester setup (running debian8
with kernel 3.16.51).
Make sure we configure correctly the interface by forcing osmo-ggsn to
set on the interface and use a specific IPv6 link-local address. This is
done by using the "ipv6 link-local" vty cmd in osmo-ggsn.
After this modification, we can re-enable ipv6 gprs context creation as
it will work in Prod setup.
Related: OS#2746
Change-Id: Ib291c02a3c57a4189f9c4b1b856109be97ad2a34
Pau Espin