IPv6 Address on L2TP VPN

We have a corporate L2TP/IPSEC VPN which issues IPv4 addresses via PPP and has Stateless Automatic Address Configuration with radvd for IPv6 addresses.

Under Lion (10.7.4) this worked, the client machines required an ip-up script that called rtsol and added the interface default route, but none the less worked.

Since the upgrade to Mountain Lion rtsol no longer adds the IPv6 address to the interface, despite receiving the RA (radvdump compiled from macports confirms this). The client is also receiving the periodic RAs from the VPN concentrator correctly.

rtsol reports the following debug information (which it didn't under Lion): -

# rtsol -d ppp0

link-layer address option has null length on ppp0. Treat as not included.

ioctl(SIOCGIFMEDIA) on ppp0: Operation not supported on socket

checking if ppp0 is ready...

ppp0 is ready

send RS on ppp0, whose state is 2

received RA from fe80::3dbf:9a93:5543:6e47 on ppp0, state is 2

stop timer for ppp0

there is no timer

The link local address as the from is the correct LL negotiated by IP6CP in the VPN setup. But the address is not added: -

# ifconfig ppp0

ppp0: flags=8051<UP,POINTOPOINT,RUNNING,MULTICAST> mtu 1280

inet6 fe80::3e07:54ff:fe55:c171%ppp0 prefixlen 64 scopeid 0xa

inet 95.x.x.41 --> 95.x.x.40 netmask 0xff000000

Both ends agree on the Link Local addresses, but ping6 from the client end can't ping the concentrator - insists No route to host.

Does anyone have any ideas?