In an IPv6 environment using DHCP, it may not be possible to configure a rendezvousIP that matches the actual address. This is because by default NetworkManager uses DUID-UUIDs for Client ID in the IPv6 DHCP Soliciation (see https://datatracker.ietf.org/doc/html/rfc6355) which are machine dependent. As a result, the DHCPv6 server cannot be configured with a pre-determined Client ID/IPv6 Address pair that matches the rendezvousIP and the nodes will be assigned random IPv6 addresses from the pool of DHCP addresses.

We can see the flow here (the DUID-UUID has a 00:04 prefix)

DHCPSOLICIT(ostestbm) 00:04:56:d2:b1:0b:ba:ef:8c:1a:00:58:3f:ed:e5:d3:5f:85

The DHCP server therefore assigns a new address from the pool, fd2e:6f44:5dd8:c956::32 in this case:
DHCPREPLY(ostestbm) fd2e:6f44:5dd8:c956::32 00:04:56:d2:b1:0b:ba:ef:8c:1a:00:58:3f:ed:e5:d3:5f:85

NetworkManager needs to be configured to use a deterministic Client ID so that a reliable Client ID/IPv6 address can be added to a DHCP server. The best way to do this is to configure NM for dhcp-duid=ll so that it uses a DUID-LL which based on the interface mac address. This is the approach taken by Baremetal IPI in   https://github.com/openshift/machine-config-operator/pull/1395

4.14.0

Every time

1. In an IPv6 environment set up agent-config.yaml with an expected IPv6 address and create the ISO
2. It's not possible to configure the DHCP server to assign this address since the Client ID that Node0 will use is unknown
3. Boot the nodes using the created ISO. The nodes will get IPv6 addresses from the DHCP server but its not possible to access the RendezvousIP