Comment 1 for bug 1899008

Root cause is no default route for IPv6 on the undercloud. There is also no route's specific for the leafs fd12:3456:789a:2::/64 and fd12:3456:789a:3::/64

(undercloud) [centos@undercloud ~]$ ip -6 route
::1 dev lo proto kernel metric 256 pref medium
unreachable ::/96 dev lo metric 1024 pref medium
unreachable ::ffff:0.0.0.0/96 dev lo metric 1024 pref medium
unreachable 2002:a00::/24 dev lo metric 1024 pref medium
unreachable 2002:7f00::/24 dev lo metric 1024 pref medium
unreachable 2002:a9fe::/32 dev lo metric 1024 pref medium
unreachable 2002:ac10::/28 dev lo metric 1024 pref medium
unreachable 2002:c0a8::/32 dev lo metric 1024 pref medium
unreachable 2002:e000::/19 dev lo metric 1024 pref medium
unreachable 3ffe:ffff::/32 dev lo metric 1024 pref medium
fd12:3456:789a:1::2 dev br-ctlplane proto kernel metric 256 pref medium
fd12:3456:789a:1::3 dev br-ctlplane proto kernel metric 256 pref medium
fd12:3456:789a:1::/64 dev br-ctlplane proto kernel metric 256 pref medium
fe80::/64 dev eth0 proto kernel metric 256 pref medium
fe80::/64 dev eth2 proto kernel metric 256 pref medium
fe80::/64 dev br-ctlplane proto kernel metric 256 pref medium
fe80::/64 dev eth1 proto kernel metric 256 pref medium

Adding a default route for IPv6 using gateway on leaf1 fixes the problem:
sudo ip -6 route add default via fd12:3456:789a:1::fffe dev br-ctlplane