Configuring pppd to accept link-local IPv6 address from remote peer

I have two Raspberry Pis with their serial ports connected to each other. I have established a PPP link between the two of them and successfully ICMPV6 pinged and opened TCP sockets between them. But I can't work out how to get the 'client' pppd to accept the link-local IPv6 address provided by the 'server' pppd. I am trying to use static addresses so I know the link-local IP address of the remote peer. On the 'server' I am running:

pppd file ./ppp-options ipv6 ::1,::2 /dev/ttyAMA0 115200

And on the 'client', I am running:

pppd file ./ppp-options ipv6cp-accept-local /dev/ttyAMA0 115200

However the ipv6cp-accept-local option doesn't seem to work as the [man page](https://man.cx/pppd(1)) describes: > With this option, pppd will accept the peer's idea of our local IPv6 interface > identifier, even if the local IPv6 interface identifier was specified in an option. The 'client' machine is instead using a randomly assigned link local address:

Using interface ppp0
Connect: ppp0  /dev/ttyAMA0
Deflate (15) compression enabled
local  LL address fe80::fd28:565e:1186:02ff
remote LL address fe80::0000:0000:0000:0001

The full output with debug turned on the client is here: https://gist.github.com/njh/ab3282f43c72dcf6932b3693eb7dfca4 And this is my configuration file (used by both):

nodetach
noauth
persist
local
noip
+ipv6

I am running Raspberry Pi OS, which has pppd version *2.4.7* installed on both devices.